JavaScript engine feature support

Compare ECMAScript features between the ECMAScript 2021 (ES12) and ES5 Standards JavaScript modes in Xanadu. Both modes support a subset of ECMAScript features.

For more information about these features, see the ECMAScript language specifications (ECMA-262) on the Ecma International website.

Support definitions

Supported

The feature has been tested and validated.

Not Supported

The feature has not been validated in the current release.

Disallowed

The feature does not align with the ServiceNow AI Platform programming model or poses a security or performance risk. Disallowed features result in an error.

ECMAScript 2022 (ES13) features

Important:

Prior to deploying code to production, you should test scripts using supported ECMAScript 2022 (ES13) features thoroughly due to the newly added and partial support of features across this ECMAScript version.

Table 1. Instance class fields

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
public instance class fields	Supported	Not Supported
private instance class fields basic support	Not Supported	Not Supported
private instance class fields initializers	Not Supported	Not Supported
optional private instance class fields access	Not Supported	Not Supported
optional deep private instance class fields access	Not Supported	Not Supported
computed instance class fields	Supported	Not Supported

Table 2. Static class fields

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
public static class fields	Supported	Not Supported
static class fields use [[Define]]	Supported	Not Supported
private static class fields	Supported	Not Supported
computed static class fields	Supported	Not Supported

Table 3. Private class methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
private instance methods	Not Supported	Not Supported
private static methods	Supported	Not Supported
private accessor properties	Not Supported	Not Supported
private static accessor properties	Supported	Not Supported

Table 4. .at() method on the built-in indexables

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Array.prototype.at()	Not Supported	Not Supported
String.prototype.at()	Supported	Not Supported
%TypedArray%.prototype.at()	Disallowed	Disallowed

Table 5. Object.hasOwn

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Basic functionality	Supported	Not Supported
ToObject called before ToPropertyKey	Supported	Not Supported

Table 6. Error.cause property

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Error has cause	Supported	Not Supported
Error.prototype lacks cause	Supported	Not Supported
EvalError has cause	Supported	Not Supported
EvalError.prototype lacks cause	Supported	Not Supported
RangeError has cause	Supported	Not Supported
RangeError.prototype lacks cause	Supported	Not Supported
ReferenceError has cause	Supported	Not Supported
ReferenceError.prototype lacks cause	Supported	Not Supported
SyntaxError has cause	Supported	Not Supported
SyntaxError.prototype lacks cause	Supported	Not Supported
TypeError has cause	Supported	Not Supported
TypeError.prototype lacks cause	Supported	Not Supported
URIError has cause	Supported	Not Supported
URIError.prototype lacks cause	Supported	Not Supported
AggregateError has cause	Supported	Not Supported
AggregateError.prototype lacks cause	Supported	Not Supported

Table 7. RegExp Match Indices (`hasIndices` / `d` flag)

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
constructor supports it	Not Supported	Not Supported
shows up in flags	Not Supported	Not Supported

Table 8. Ergonomic brand checks for private fields

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Ergonomic brand checks for private fields	Not Supported	Not Supported

Table 9. Class static initialization blocks

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Class static initialization blocks	Supported	Not Supported

ECMAScript 2021 (ES12) features

Table 10. Promise.any

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
fulfillment	Disallowed	Disallowed
AggregateError	Disallowed	Disallowed

Table 11. WeakReferences

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
WeakRef minimal support	Disallowed	Disallowed
FinalizationRegistry minimal support	Disallowed	Disallowed

Table 12. Logical assignment

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
||= basic support	Supported	Not Supported
||= short-circuiting behavior	Supported	Not Supported
||= setter not unnecessarily invoked	Supported	Not Supported
&&= basic support	Supported	Not Supported
&&= short-circuiting behavior	Supported	Not Supported
&&= setter not unnecessarily invoked	Supported	Not Supported
??= basic support	Supported	Not Supported
??= short-circuiting behavior	Supported	Not Supported
??= setter not unnecessarily invoked	Supported	Not Supported

Table 13. Numeric separators

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
numeric separators	Supported	Not Supported

Table 14. String.prototype.replaceAll

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
String.prototype.replaceAll	Supported	Supported

ECMAScript 2020 (ES11) features

Table 15. String.prototype.matchAll

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Supported	Not Supported
throws on non-global regex	Supported	Not Supported

Table 16. BigInt

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Not Supported	Not Supported
constructor	Not Supported	Not Supported
BigInt.asUintN	Not Supported	Not Supported
BigInt.asIntN	Not Supported	Not Supported
BigInt64Array	Not Supported	Not Supported
BigUint64Array	Not Supported	Not Supported
DataView.prototype.getBigInt64	Not Supported	Not Supported
DataView.prototype.getBigUint64	Not Supported	Not Supported

Table 17. globalThis

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
"globalThis" global property is global object	Disallowed	Disallowed
"globalThis" global property has correct property descriptor	Disallowed	Disallowed

Table 18. Optional chaining operator (?.)

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
optional property access	Supported	Not Supported
optional bracket access	Supported	Not Supported
optional method call	Supported	Not Supported
optional function call	Supported	Not Supported
spread parameters after optional chaining	Supported	Not Supported

Table 19. Promise.allSettled

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Promise.allSettled	Disallowed	Disallowed

Table 20. Nullish coalescing operator (??)

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
nullish coalescing operator (??)	Supported	Not Supported

ECMAScript 2019 (ES10) features

Table 21. Symbol.prototype.description

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic	Supported	Not Supported
empty description	Supported	Not Supported
undefined description	Supported	Not Supported

Table 22. String trimming

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
String.prototype.trimLeft	Supported	Supported
String.prototype.trimRight	Supported	Supported
String.prototype.trimStart	Supported	Not Supported
String.prototype.trimEnd	Supported	Not Supported

Table 23. Array.prototype.{flat, flatMap}

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Array.prototype.flat	Supported	Not Supported
Array.prototype.flatMap	Supported	Not Supported
flat and flatMap in Array.prototype[@@unscopables]	Supported	Not Supported

Table 24. Object.fromEntries

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Object.fromEntries	Supported	Not Supported

Table 25. Optional catch binding

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic	Disallowed	Disallowed
await	Disallowed	Disallowed
yield	Disallowed	Disallowed

Table 26. Function.prototype.toString revision

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
functions created with the Function constructor	Disallowed	Disallowed
arrows	Disallowed	Disallowed
[native code]	Disallowed	Disallowed
class expression with implicit constructor	Disallowed	Disallowed
class expression with explicit constructor	Disallowed	Disallowed
unicode escape sequences in identifiers	Disallowed	Disallowed
methods and computed property names	Disallowed	Disallowed

Table 27. JSON superset

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Line separator can appear in string literals	Disallowed	Disallowed
Paragraph separator can appear in string literals	Disallowed	Disallowed

Table 28. Well-formed JSON.stringify

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Well-formed JSON.stringify	Disallowed	Disallowed

ECMAScript 2018 (ES9) features

Table 29. Object rest/spread properties

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
object rest properties	Supported	Not Supported
object spread properties	Supported	Not Supported

Table 30. Promise.prototype.finally

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic support	Disallowed	Disallowed
don't change resolution value	Disallowed	Disallowed
change rejection value	Disallowed	Disallowed

Table 31. Asynchronous iterators

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
async generators	Disallowed	Disallowed
for-await-of loops	Disallowed	Disallowed

Table 32. s (dotAll) flag for regular expressions

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
s (dotAll) flag for regular expressions	Supported	Not Supported

Table 33. RegExp named capture groups

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
RegExp named capture groups	Supported	Not Supported

Table 34. RegExp Lookbehind Assertions

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
RegExp Lookbehind Assertions	Not Supported	Not Supported

Table 35. RegExp Unicode Property Escapes

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
RegExp Unicode Property Escapes	Not Supported	Not Supported

Table 36. Template literal revision

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
template literal revision	Disallowed	Disallowed

ECMAScript 2017 (ES8) features

Table 37. Object static methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Object.values	Supported	Not Supported
Object.entries	Supported	Not Supported
Object.getOwnPropertyDescriptors	Supported	Not Supported
Object.getOwnPropertyDescriptors doesn't provide undefined descriptors	Not Supported	Not Supported

Table 38. String padding

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
String.prototype.padStart	Supported	Not Supported
String.prototype.padEnd	Supported	Not Supported

Table 39. Trailing commas in function syntax

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
in parameter lists	Supported	Not Supported
in argument lists	Supported	Not Supported

Table 40. Async functions

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
return	Disallowed	Disallowed
throw	Disallowed	Disallowed
no line break between async and function	Disallowed	Disallowed
no "prototype" property	Disallowed	Disallowed
await	Disallowed	Disallowed
await, rejection	Disallowed	Disallowed
must await a value	Disallowed	Disallowed
can await non-Promise values	Disallowed	Disallowed
cannot await in parameters	Disallowed	Disallowed
async methods, object literals	Disallowed	Disallowed
async methods, classes	Disallowed	Disallowed
async arrow functions in methods, classes	Disallowed	Disallowed
async arrow functions	Disallowed	Disallowed
correct prototype chain	Disallowed	Disallowed
async function prototype, Symbol.toStringTag	Disallowed	Disallowed
async function constructor	Disallowed	Disallowed

Table 41. Shared memory and atomics

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
SharedArrayBuffer	Disallowed	Disallowed
SharedArrayBuffer[Symbol.species]	Disallowed	Disallowed
SharedArrayBuffer.prototype.byteLength	Disallowed	Disallowed
SharedArrayBuffer.prototype.slice	Disallowed	Disallowed
SharedArrayBuffer.prototype[Symbol.toStringTag]	Disallowed	Disallowed
Atomics.add	Disallowed	Disallowed
Atomics.and	Disallowed	Disallowed
Atomics.compareExchange	Disallowed	Disallowed
Atomics.exchange	Disallowed	Disallowed
Atomics.wait	Disallowed	Disallowed
Atomics.wake	Disallowed	Disallowed
Atomics.isLockFree	Disallowed	Disallowed
Atomics.load	Disallowed	Disallowed
Atomics.or	Disallowed	Disallowed
Atomics.store	Disallowed	Disallowed
Atomics.sub	Disallowed	Disallowed
Atomics.xor	Disallowed	Disallowed

Table 42. Object.prototype getter/setter methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
__defineGetter__	Disallowed	Disallowed
__defineGetter__, symbols	Disallowed	Disallowed
__defineGetter__, ToObject(this)	Disallowed	Disallowed
__defineSetter__	Disallowed	Disallowed
__defineSetter__, symbols	Disallowed	Disallowed
__defineSetter__, ToObject(this)	Disallowed	Disallowed
__lookupGetter__	Disallowed	Disallowed
__lookupGetter__, prototype chain	Disallowed	Disallowed
__lookupGetter__, symbols	Disallowed	Disallowed
__lookupGetter__, ToObject(this)	Disallowed	Disallowed
__lookupGetter__, data properties can shadow accessors	Disallowed	Disallowed
__lookupSetter__	Disallowed	Disallowed
__lookupSetter__, prototype chain	Disallowed	Disallowed
__lookupSetter__, symbols	Disallowed	Disallowed
__lookupSetter__, ToObject(this)	Disallowed	Disallowed
__lookupSetter__, data properties can shadow accessors	Disallowed	Disallowed

Table 43. Proxy internal calls, getter/setter methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
__defineGetter__	Disallowed	Disallowed
__defineSetter__	Disallowed	Disallowed
__lookupGetter__	Disallowed	Disallowed
__lookupSetter__	Disallowed	Disallowed

ECMAScript 2016 (ES7) features

Table 44. Exponentiation (**) operator

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic support	Supported	Not Supported
assignment	Supported	Not Supported
early syntax error for unary negation without parentheses	Disallowed	Disallowed

Table 45. Array.prototype.includes

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Array.prototype.includes	Supported	Not Supported
Array.prototype.includes is generic	Not Supported	Not Supported
%TypedArray%.prototype.includes	Disallowed	Disallowed

ECMAScript 2015 (ES6) features

Table 46. Proper tail calls (tail call optimization)

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
direct recursion	Disallowed	Disallowed
mutual recursion	Disallowed	Disallowed

Table 47. Default function parameters

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Supported	Not Supported
explicit undefined defers to the default	Supported	Not Supported
defaults can refer to previous parameters	Supported	Not Supported
arguments object interaction	Supported	Not Supported
temporal dead zone	Disallowed	Disallowed
separate scope	Supported	Not Supported
new Function() support	Disallowed	Disallowed

Table 48. Rest parameters

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Supported	Not Supported
function 'length' property	Supported	Not Supported
arguments object interaction	Not Supported	Not Supported
can't be used in setters	Disallowed	Disallowed
new Function() support	Disallowed	Disallowed

Table 49. Spread syntax for iterable objects

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
with arrays, in function calls	Supported	Not Supported
with arrays, in array literals	Supported	Not Supported
with sparse arrays, in function calls	Supported	Not Supported
with sparse arrays, in array literals	Supported	Not Supported
with strings, in function calls	Supported	Not Supported
with strings, in array literals	Supported	Not Supported
with astral plane strings, in function calls	Supported	Not Supported
with astral plane strings, in array literals	Supported	Not Supported
with generator instances, in calls	Disallowed	Disallowed
with generator instances, in arrays	Disallowed	Disallowed
with generic iterables, in calls	Supported	Not Supported
with generic iterables, in arrays	Supported	Not Supported
with instances of iterables, in calls	Supported	Not Supported
with instances of iterables, in arrays	Supported	Not Supported
spreading non-iterables is a runtime error	Supported	Not Supported

Table 50. Object literal extensions

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
computed properties	Supported	Not Supported
shorthand properties	Supported	Not Supported
shorthand methods	Supported	Not Supported
string-keyed shorthand methods	Supported	Not Supported
computed shorthand methods	Supported	Not Supported
computed accessors	Supported	Not Supported

Table 51. For-of loops

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
with arrays	Supported	Not Supported
with sparse arrays	Supported	Not Supported
with strings	Supported	Not Supported
with astral plane strings	Supported	Not Supported
with generator instances	Disallowed	Disallowed
with generic iterables	Supported	Not Supported
with instances of generic iterables	Supported	Not Supported
iterator closing, break	Supported	Not Supported
iterator closing, throw	Supported	Not Supported

Table 52. Octal and binary literals

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
octal literals	Supported	Not Supported
binary literals	Supported	Not Supported
octal supported by Number()	Not Supported	Not Supported
binary supported by Number()	Not Supported	Not Supported

Table 53. Template literals

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Supported	Not Supported
toString conversion	Supported	Not Supported
tagged template literals	Supported	Not Supported
passed array is frozen	Supported	Not Supported
line break normalization	Disallowed	Disallowed
TemplateStrings call site caching	Supported	Not Supported
TemplateStrings permanent caching	Supported	Not Supported

Table 54. RegExp "y" and "u" flags

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
"y" flag	Supported	Not Supported
"y" flag, lastIndex	Supported	Not Supported
"u" flag	Not Supported	Not Supported
"u" flag, non-BMP Unicode characters	Not Supported	Not Supported
"u" flag, Unicode code point escapes	Not Supported	Not Supported
"u" flag, case folding	Not Supported	Not Supported

Table 55. Destructuring, declarations

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
with arrays	Supported	Not Supported
with sparse arrays	Supported	Not Supported
with strings	Supported	Not Supported
with astral plane strings	Supported	Not Supported
with generator instances	Disallowed	Disallowed
with generic iterables	Supported	Not Supported
with instances of generic iterables	Supported	Not Supported
iterator closing	Supported	Not Supported
trailing commas in iterable patterns	Supported	Not Supported
with objects	Supported	Not Supported
object destructuring with primitives	Supported	Not Supported
trailing commas in object patterns	Supported	Not Supported
throws on null and undefined	Supported	Not Supported
computed properties	Supported	Not Supported
multiples in a single var statement	Supported	Not Supported
nested	Supported	Not Supported
in for-in loop heads	Supported	Not Supported
in for-of loop heads	Supported	Not Supported
in catch heads	Supported	Not Supported
rest	Supported	Not Supported
defaults	Supported	Not Supported
defaults, let temporal dead zone	Disallowed	Disallowed

Table 56. Destructuring, assignment

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
with arrays	Supported	Not Supported
with sparse arrays	Supported	Not Supported
with strings	Supported	Not Supported
with astral plane strings	Supported	Not Supported
with generator instances	Disallowed	Disallowed
with generic iterables	Supported	Not Supported
with instances of generic iterables	Supported	Not Supported
iterator closing	Supported	Not Supported
iterable destructuring expression	Supported	Not Supported
chained iterable destructuring	Supported	Not Supported
trailing commas in iterable patterns	Supported	Not Supported
with objects	Supported	Not Supported
object destructuring with primitives	Supported	Not Supported
trailing commas in object patterns	Supported	Not Supported
object destructuring expression	Supported	Not Supported
parenthesized left-hand-side is a syntax error	Disallowed	Disallowed
chained object destructuring	Supported	Not Supported
throws on null and undefined	Supported	Not Supported
computed properties	Supported	Not Supported
nested	Supported	Not Supported
rest	Supported	Not Supported
nested rest	Supported	Not Supported
empty patterns	Supported	Not Supported
defaults	Supported	Not Supported

Table 57. Destructuring, parameters

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
with arrays	Supported	Not Supported
with sparse arrays	Supported	Not Supported
with strings	Supported	Not Supported
with astral plane strings	Supported	Not Supported
with generator instances	Disallowed	Disallowed
with generic iterables	Supported	Not Supported
with instances of generic iterables	Supported	Not Supported
iterator closing	Supported	Not Supported
trailing commas in iterable patterns	Supported	Not Supported
with objects	Supported	Not Supported
object destructuring with primitives	Supported	Not Supported
trailing commas in object patterns	Supported	Not Supported
throws on null and undefined	Supported	Not Supported
computed properties	Supported	Not Supported
nested	Supported	Not Supported
'arguments' interaction	Supported	Not Supported
new Function() support	Disallowed	Disallowed
in parameters, function 'length' property	Supported	Not Supported
rest	Supported	Not Supported
empty patterns	Supported	Not Supported
defaults	Supported	Not Supported
defaults, separate scope	Supported	Not Supported
defaults, new Function() support	Disallowed	Disallowed
aliased defaults, arrow function	Supported	Not Supported
shorthand defaults, arrow function	Supported	Not Supported
duplicate identifier	Disallowed	Disallowed

Table 58. Unicode code point escapes

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
in strings	Supported	Not Supported
in identifiers	Not Supported	Not Supported
in property key definitions	Not Supported	Not Supported
in property key accesses	Not Supported	Not Supported

Table 59. New.target

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
in constructors	Not Supported	Not Supported
assignment is an early error	Disallowed	Disallowed

Table 60. Const

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic support	Supported	Supported
is block-scoped	Supported	Not Supported
scope shadow resolution	Supported	Not Supported
cannot be in statements	Disallowed	Disallowed
redefining a const is an error	Disallowed	Disallowed
for loop statement scope	Supported	Not Supported
for-in loop iteration scope	Supported	Not Supported
for-of loop iteration scope	Supported	Not Supported
temporal dead zone	Not Supported	Not Supported
basic support (strict mode)	Supported	Supported
is block-scoped (strict mode)	Supported	Not Supported
scope shadow resolution (strict mode)	Supported	Not Supported
cannot be in statements (strict mode)	Disallowed	Disallowed
redefining a const (strict mode)	Disallowed	Disallowed
for loop statement scope (strict mode)	Supported	Not Supported
for-in loop iteration scope (strict mode)	Supported	Not Supported
for-of loop iteration scope (strict mode)	Supported	Not Supported
temporal dead zone (strict mode)	Not Supported	Not Supported

Table 61. Let

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic support	Supported	Not Supported
is block-scoped	Supported	Not Supported
scope shadow resolution	Supported	Not Supported
cannot be in statements	Disallowed	Disallowed
for loop statement scope	Supported	Not Supported
temporal dead zone	Not Supported	Not Supported
for/for-in loop iteration scope	Supported	Not Supported
for-in loop binding shadowing parameter	Disallowed	Disallowed
basic support (strict mode)	Supported	Not Supported
is block-scoped (strict mode)	Supported	Not Supported
scope shadow resolution (strict mode)	Supported	Not Supported
cannot be in statements (strict mode)	Disallowed	Disallowed
for loop statement scope (strict mode)	Supported	Not Supported
temporal dead zone (strict mode)	Not Supported	Not Supported
for/for-in loop iteration scope (strict mode)	Supported	Not Supported
for-in loop binding shadowing parameter (strict mode)	Disallowed	Disallowed

Table 62. Block-level function declaration

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
block-level function declaration	Supported	Not Supported

Table 63. Arrow functions

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
0 parameters	Supported	Not Supported
1 parameter, no brackets	Supported	Not Supported
multiple parameters	Supported	Not Supported
lexical "this" binding	Supported	Not Supported
"this" unchanged by call or apply	Supported	Not Supported
can't be bound, can be curried	Supported	Not Supported
lexical "arguments" binding	Supported	Not Supported
no line break between parameters and =>	Disallowed	Disallowed
correct precedence	Disallowed	Disallowed
no "prototype" property	Not Supported	Not Supported
lexical "super" binding in constructors	Supported	Not Supported
lexical "super" binding in methods	Supported	Not Supported
lexical "new.target" binding	Not Supported	Not Supported

Table 64. Class

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
class statement	Supported	Not Supported
is block-scoped	Supported	Not Supported
class expression	Supported	Not Supported
anonymous class	Supported	Not Supported
constructor	Supported	Not Supported
prototype methods	Supported	Not Supported
string-keyed methods	Supported	Not Supported
computed prototype methods	Supported	Not Supported
optional semicolons	Supported	Not Supported
static methods	Supported	Not Supported
computed static methods	Supported	Not Supported
accessor properties	Supported	Not Supported
computed accessor properties	Supported	Not Supported
static accessor properties	Supported	Not Supported
computed static accessor properties	Supported	Not Supported
class name is lexically scoped	Supported	Not Supported
computed names, temporal dead zone	Not Supported	Not Supported
methods aren't enumerable	Supported	Not Supported
implicit strict mode	Not Supported	Not Supported
constructor requires new	Supported	Not Supported
extends	Supported	Not Supported
extends expressions	Supported	Not Supported
extends null	Supported	Not Supported
new.target	Supported	Not Supported

Table 65. Super

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
statement in constructors	Supported	Not Supported
expression in constructors	Supported	Not Supported
in methods, property access	Supported	Not Supported
in methods, method calls	Supported	Not Supported
method calls use correct "this" binding	Supported	Not Supported
constructor calls use correct "new.target" binding	Supported	Not Supported
is statically bound	Supported	Not Supported
super() invokes the correct constructor	Supported	Not Supported

Table 66. Generators

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Disallowed	Disallowed
generator function expressions	Disallowed	Disallowed
correct "this" binding	Disallowed	Disallowed
can't use "this" with new	Disallowed	Disallowed
sending	Disallowed	Disallowed
%GeneratorPrototype%	Disallowed	Disallowed
%GeneratorPrototype% prototype chain	Disallowed	Disallowed
%GeneratorPrototype%.constructor	Disallowed	Disallowed
%GeneratorPrototype%.throw	Disallowed	Disallowed
%GeneratorPrototype%.return	Disallowed	Disallowed
yield operator precedence	Disallowed	Disallowed
yield *, arrays	Disallowed	Disallowed
yield *, sparse arrays	Disallowed	Disallowed
yield *, strings	Disallowed	Disallowed
yield *, astral plane strings	Disallowed	Disallowed
yield *, generator instances	Disallowed	Disallowed
yield *, generic iterables	Disallowed	Disallowed
yield *, instances of iterables	Disallowed	Disallowed
yield * on non-iterables is a runtime error	Disallowed	Disallowed
yield *, iterator closing	Disallowed	Disallowed
yield *, iterator closing via throw()	Disallowed	Disallowed
shorthand generator methods	Disallowed	Disallowed
string-keyed shorthand generator methods	Disallowed	Disallowed
computed shorthand generators	Disallowed	Disallowed
shorthand generator methods, classes	Disallowed	Disallowed
computed shorthand generators, classes	Disallowed	Disallowed
shorthand generators can't be constructors	Disallowed	Disallowed

Table 67. Typed arrays

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Int8Array	Disallowed	Disallowed
Uint8Array	Disallowed	Disallowed
Uint8ClampedArray	Disallowed	Disallowed
Int16Array	Disallowed	Disallowed
Uint16Array	Disallowed	Disallowed
Int32Array	Disallowed	Disallowed
Uint32Array	Disallowed	Disallowed
Float32Array	Disallowed	Disallowed
Float64Array	Disallowed	Disallowed
DataView (Int8)	Disallowed	Disallowed
DataView (Uint8)	Disallowed	Disallowed
DataView (Int16)	Disallowed	Disallowed
DataView (Uint16)	Disallowed	Disallowed
DataView (Int32)	Disallowed	Disallowed
DataView (Uint32)	Disallowed	Disallowed
DataView (Float32)	Disallowed	Disallowed
DataView (Float64)	Disallowed	Disallowed
ArrayBuffer[Symbol.species]	Disallowed	Disallowed
constructors require new	Disallowed	Disallowed
constructors accept generic iterables	Disallowed	Disallowed
correct prototype chains	Disallowed	Disallowed
%TypedArray%.from	Disallowed	Disallowed
%TypedArray%.of	Disallowed	Disallowed
%TypedArray%.prototype.subarray	Disallowed	Disallowed
%TypedArray%.prototype.join	Disallowed	Disallowed
%TypedArray%.prototype.indexOf	Disallowed	Disallowed
%TypedArray%.prototype.lastIndexOf	Disallowed	Disallowed
%TypedArray%.prototype.slice	Disallowed	Disallowed
%TypedArray%.prototype.every	Disallowed	Disallowed
%TypedArray%.prototype.filter	Disallowed	Disallowed
%TypedArray%.prototype.forEach	Disallowed	Disallowed
%TypedArray%.prototype.map	Disallowed	Disallowed
%TypedArray%.prototype.reduce	Disallowed	Disallowed
%TypedArray%.prototype.reduceRight	Disallowed	Disallowed
%TypedArray%.prototype.reverse	Disallowed	Disallowed
%TypedArray%.prototype.some	Disallowed	Disallowed
%TypedArray%.prototype.sort	Disallowed	Disallowed
%TypedArray%.prototype.copyWithin	Disallowed	Disallowed
%TypedArray%.prototype.find	Disallowed	Disallowed
%TypedArray%.prototype.findIndex	Disallowed	Disallowed
%TypedArray%.prototype.fill	Disallowed	Disallowed
%TypedArray%.prototype.keys	Disallowed	Disallowed
%TypedArray%.prototype.values	Disallowed	Disallowed
%TypedArray%.prototype.entries	Disallowed	Disallowed
%TypedArray%.prototype[Symbol.iterator]	Disallowed	Disallowed
%TypedArray%[Symbol.species]	Disallowed	Disallowed

Table 68. Map

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Supported	Not Supported
constructor arguments	Supported	Not Supported
constructor requires new	Supported	Not Supported
constructor accepts null	Supported	Not Supported
constructor invokes set	Supported	Not Supported
iterator closing	Supported	Not Supported
Map.prototype.set returns this	Supported	Not Supported
-0 key converts to +0	Supported	Not Supported
Map.prototype.size	Supported	Not Supported
Map.prototype.delete	Supported	Not Supported
Map.prototype.clear	Supported	Not Supported
Map.prototype.forEach	Supported	Not Supported
Map.prototype.keys	Supported	Not Supported
Map.prototype.values	Supported	Not Supported
Map.prototype.entries	Supported	Not Supported
Map.prototype[Symbol.iterator]	Supported	Not Supported
Map.prototype isn't an instance	Supported	Not Supported
Map iterator prototype chain	Supported	Not Supported
Map[Symbol.species]	Supported	Not Supported

Table 69. Set

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Supported	Not Supported
constructor arguments	Supported	Not Supported
constructor requires new	Supported	Not Supported
constructor accepts null	Supported	Not Supported
constructor invokes add	Supported	Not Supported
iterator closing	Supported	Not Supported
Set.prototype.add returns this	Supported	Not Supported
-0 key converts to +0	Supported	Not Supported
Set.prototype.size	Supported	Not Supported
Set.prototype.delete	Supported	Not Supported
Set.prototype.clear	Supported	Not Supported
Set.prototype.forEach	Supported	Not Supported
Set.prototype.keys	Supported	Not Supported
Set.prototype.values	Supported	Not Supported
Set.prototype.entries	Supported	Not Supported
Set.prototype[Symbol.iterator]	Supported	Not Supported
Set.prototype isn't an instance	Supported	Not Supported
Set iterator prototype chain	Supported	Not Supported
Set[Symbol.species]	Supported	Not Supported

Table 70. WeakMap

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Disallowed	Disallowed
constructor arguments	Disallowed	Disallowed
constructor requires new	Disallowed	Disallowed
constructor accepts null	Disallowed	Disallowed
constructor invokes set	Disallowed	Disallowed
frozen objects as keys	Disallowed	Disallowed
iterator closing	Disallowed	Disallowed
WeakMap.prototype.set returns this	Disallowed	Disallowed
WeakMap.prototype.delete	Disallowed	Disallowed
no WeakMap.prototype.clear method	Disallowed	Disallowed
.has, .get and .delete methods accept primitives	Disallowed	Disallowed
WeakMap.prototype isn't an instance	Disallowed	Disallowed

Table 71. WeakSet

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Disallowed	Disallowed
constructor arguments	Disallowed	Disallowed
constructor requires new	Disallowed	Disallowed
constructor accepts null	Disallowed	Disallowed
constructor invokes add	Disallowed	Disallowed
iterator closing	Disallowed	Disallowed
WeakSet.prototype.add returns this	Disallowed	Disallowed
WeakSet.prototype.delete	Disallowed	Disallowed
no WeakSet.prototype.clear method	Disallowed	Disallowed
.has and .delete methods accept primitives	Disallowed	Disallowed
WeakSet.prototype isn't an instance	Disallowed	Disallowed

Table 72. Proxy

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
constructor requires new	Disallowed	Disallowed
no "prototype" property	Disallowed	Disallowed
"get" handler	Disallowed	Disallowed
"get" handler, instances of proxies	Disallowed	Disallowed
"get" handler invariants	Disallowed	Disallowed
"set" handler	Disallowed	Disallowed
"set" handler, instances of proxies	Disallowed	Disallowed
"set" handler invariants	Disallowed	Disallowed
"has" handler	Disallowed	Disallowed
"has" handler, instances of proxies	Disallowed	Disallowed
"has" handler invariants	Disallowed	Disallowed
"deleteProperty" handler	Disallowed	Disallowed
"deleteProperty" handler invariant	Disallowed	Disallowed
"getOwnPropertyDescriptor" handler	Disallowed	Disallowed
"getOwnPropertyDescriptor" handler invariants	Disallowed	Disallowed
"defineProperty" handler	Disallowed	Disallowed
"defineProperty" handler invariants	Disallowed	Disallowed
"getPrototypeOf" handler	Disallowed	Disallowed
"getPrototypeOf" handler invariant	Disallowed	Disallowed
"setPrototypeOf" handler	Disallowed	Disallowed
"setPrototypeOf" handler invariant	Disallowed	Disallowed
"isExtensible" handler	Disallowed	Disallowed
"isExtensible" handler invariant	Disallowed	Disallowed
"preventExtensions" handler	Disallowed	Disallowed
"preventExtensions" handler invariant	Disallowed	Disallowed
"ownKeys" handler	Disallowed	Disallowed
"ownKeys" handler invariant	Disallowed	Disallowed
"apply" handler	Disallowed	Disallowed
"apply" handler invariant	Disallowed	Disallowed
"construct" handler	Disallowed	Disallowed
"construct" handler invariants	Disallowed	Disallowed
Proxy.revocable	Disallowed	Disallowed
Array.isArray support	Disallowed	Disallowed
JSON.stringify support	Disallowed	Disallowed

Table 73. Reflect

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Reflect.get	Disallowed	Disallowed
Reflect.set	Disallowed	Disallowed
Reflect.has	Disallowed	Disallowed
Reflect.deleteProperty	Disallowed	Disallowed
Reflect.getOwnPropertyDescriptor	Disallowed	Disallowed
Reflect.defineProperty	Disallowed	Disallowed
Reflect.getPrototypeOf	Disallowed	Disallowed
Reflect.setPrototypeOf	Disallowed	Disallowed
Reflect.isExtensible	Disallowed	Disallowed
Reflect.preventExtensions	Disallowed	Disallowed
Reflect.ownKeys, string keys	Disallowed	Disallowed
Reflect.ownKeys, symbol keys	Disallowed	Disallowed
Reflect.apply	Disallowed	Disallowed
Reflect.construct	Disallowed	Disallowed
Reflect.construct sets new.target meta-property	Disallowed	Disallowed
Reflect.construct creates instances from third argument	Disallowed	Disallowed
Reflect.construct, Array subclassing	Disallowed	Disallowed
Reflect.construct, RegExp subclassing	Disallowed	Disallowed
Reflect.construct, Function subclassing	Disallowed	Disallowed
Reflect.construct, Promise subclassing	Disallowed	Disallowed

Table 74. Promise

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Disallowed	Disallowed
constructor requires new	Disallowed	Disallowed
Promise.prototype isn't an instance	Disallowed	Disallowed
Promise.all	Disallowed	Disallowed
Promise.all, generic iterables	Disallowed	Disallowed
Promise.race	Disallowed	Disallowed
Promise.race, generic iterables	Disallowed	Disallowed
Promise[Symbol.species]	Disallowed	Disallowed

Table 75. Symbol

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Supported	Not Supported
typeof support	Supported	Not Supported
symbol keys are hidden to pre-ES6 code	Supported	Not Supported
Object.defineProperty support	Supported	Not Supported
symbols inherit from Symbol.prototype	Supported	Not Supported
cannot coerce to string or number	Not Supported	Not Supported
can convert with String()	Not Supported	Not Supported
new Symbol() throws	Supported	Not Supported
Object(symbol)	Not Supported	Not Supported
JSON.stringify ignores symbol primitives	Supported	Not Supported
JSON.stringify ignores symbol objects	Not Supported	Not Supported
global symbol registry	Supported	Not Supported

Table 76. Well-known symbols

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Symbol.hasInstance	Disallowed	Disallowed
Symbol.isConcatSpreadable	Disallowed	Disallowed
Symbol.iterator, existence	Disallowed	Disallowed
Symbol.iterator, arguments object	Disallowed	Disallowed
Symbol.species, existence	Disallowed	Disallowed
Symbol.species, Array.prototype.concat	Disallowed	Disallowed
Symbol.species, Array.prototype.filter	Disallowed	Disallowed
Symbol.species, Array.prototype.map	Disallowed	Disallowed
Symbol.species, Array.prototype.slice	Disallowed	Disallowed
Symbol.species, Array.prototype.splice	Disallowed	Disallowed
Symbol.species, RegExp.prototype[Symbol.split]	Disallowed	Disallowed
Symbol.species, Promise.prototype.then	Disallowed	Disallowed
Symbol.replace	Disallowed	Disallowed
Symbol.search	Disallowed	Disallowed
Symbol.split	Disallowed	Disallowed
Symbol.match	Disallowed	Disallowed
Symbol.match, RegExp constructor	Disallowed	Disallowed
Symbol.match, String.prototype.startsWith	Disallowed	Disallowed
Symbol.match, String.prototype.endsWith	Disallowed	Disallowed
Symbol.match, String.prototype.includes	Disallowed	Disallowed
Symbol.toPrimitive	Disallowed	Disallowed
Symbol.toStringTag	Disallowed	Disallowed
Symbol.toStringTag affects existing built-ins	Disallowed	Disallowed
Symbol.toStringTag, new built-ins	Disallowed	Disallowed
Symbol.toStringTag, misc. built-ins	Disallowed	Disallowed
Symbol.unscopables	Disallowed	Disallowed

Table 77. Object static methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Object.assign	Supported	Not Supported
Object.is	Supported	Not Supported
Object.getOwnPropertySymbols	Supported	Not Supported
Object.setPrototypeOf	Not Supported	Not Supported

Table 78. Function "name" property

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
function statements	Supported	Supported
function expressions	Supported	Supported
new Function	Not Supported	Not Supported
bound functions	Not Supported	Not Supported
variables (function)	Supported	Not Supported
object methods (function)	Supported	Not Supported
accessor properties	Not Supported	Not Supported
shorthand methods	Supported	Not Supported
shorthand methods (no lexical binding)	Supported	Not Supported
symbol-keyed methods	Not Supported	Not Supported
class statements	Supported	Not Supported
class expressions	Supported	Not Supported
variables (class)	Supported	Not Supported
object methods (class)	Not Supported	Not Supported
class prototype methods	Supported	Not Supported
class static methods	Supported	Not Supported
isn't writable, is configurable	Not Supported	Not Supported

Table 79. String static methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
String.raw	Supported	Not Supported
String.fromCodePoint	Supported	Not Supported

Table 80. String.prototype methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
String.prototype.codePointAt	Supported	Supported
String.prototype.normalize	Supported	Supported
String.prototype.repeat	Supported	Supported
String.prototype.startsWith	Supported	Supported
String.prototype.startsWith throws on RegExp	Not Supported	Not Supported
String.prototype.endsWith	Supported	Supported
String.prototype.endsWith throws on RegExp	Not Supported	Not Supported
String.prototype.includes	Supported	Supported
String.prototype[Symbol.iterator]	Supported	Not Supported
String iterator prototype chain	Supported	Not Supported

Table 81. RegExp.prototype properties

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
RegExp.prototype.flags	Supported	Not Supported
RegExp.prototype[Symbol.match]	Not Supported	Not Supported
RegExp.prototype[Symbol.replace]	Supported	Not Supported
RegExp.prototype[Symbol.split]	Supported	Not Supported
RegExp.prototype[Symbol.search]	Not Supported	Not Supported
RegExp[Symbol.species]	Supported	Not Supported

Table 82. Array static methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Array.from, array-like objects	Supported	Not Supported
Array.from, generator instances	Disallowed	Disallowed
Array.from, generic iterables	Supported	Not Supported
Array.from, instances of generic iterables	Supported	Not Supported
Array.from map function, array-like objects	Supported	Not Supported
Array.from map function, generator instances	Disallowed	Disallowed
Array.from map function, generic iterables	Supported	Not Supported
Array.from map function, instances of iterables	Supported	Not Supported
Array.from, iterator closing	Supported	Not Supported
Array.of	Supported	Not Supported
Array[Symbol.species]	Supported	Not Supported

Table 83. Array.prototype methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Array.prototype.copyWithin	Supported	Not Supported
Array.prototype.find	Supported	Not Supported
Array.prototype.findIndex	Supported	Not Supported
Array.prototype.fill	Supported	Not Supported
Array.prototype.keys	Supported	Not Supported
Array.prototype.values	Supported	Not Supported
Array.prototype.entries	Supported	Not Supported
Array.prototype[Symbol.iterator]	Supported	Not Supported
Array iterator prototype chain	Supported	Not Supported
Array.prototype[Symbol.unscopables]	Supported	Not Supported

Table 84. Number properties

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Number.isFinite	Supported	Not Supported
Number.isInteger	Supported	Not Supported
Number.isSafeInteger	Supported	Not Supported
Number.isNaN	Supported	Not Supported
Number.parseFloat	Disallowed	Disallowed
Number.parseInt	Disallowed	Disallowed
Number.EPSILON	Supported	Not Supported
Number.MIN_SAFE_INTEGER	Supported	Not Supported
Number.MAX_SAFE_INTEGER	Supported	Not Supported

Table 85. Math methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Math.clz32	Supported	Not Supported
Math.imul	Supported	Not Supported
Math.sign	Supported	Not Supported
Math.log10	Supported	Not Supported
Math.log2	Supported	Not Supported
Math.log1p	Supported	Not Supported
Math.expm1	Supported	Not Supported
Math.cosh	Supported	Not Supported
Math.sinh	Supported	Not Supported
Math.tanh	Supported	Not Supported
Math.acosh	Supported	Not Supported
Math.asinh	Supported	Not Supported
Math.atanh	Supported	Not Supported
Math.trunc	Supported	Not Supported
Math.fround	Supported	Not Supported
Math.cbrt	Supported	Not Supported
Math.hypot	Supported	Not Supported

Table 86. Date.prototype[Symbol.toPrimitive]

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Date.prototype[Symbol.toPrimitive]	Supported	Not Supported

Table 87. Array is subclassable

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
length property (accessing)	Disallowed	Disallowed
length property (setting)	Disallowed	Disallowed
correct prototype chain	Disallowed	Disallowed
Array.isArray support	Disallowed	Disallowed
Array.prototype.concat	Disallowed	Disallowed
Array.prototype.filter	Disallowed	Disallowed
Array.prototype.map	Disallowed	Disallowed
Array.prototype.slice	Disallowed	Disallowed
Array.prototype.splice	Disallowed	Disallowed
Array.from	Disallowed	Disallowed
Array.of	Disallowed	Disallowed

Table 88. RegExp is subclassable

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Disallowed	Disallowed
correct prototype chain	Disallowed	Disallowed
RegExp.prototype.exec	Disallowed	Disallowed
RegExp.prototype.test	Disallowed	Disallowed

Table 89. Function is subclassable

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
can be called	Disallowed	Disallowed
correct prototype chain	Disallowed	Disallowed
can be used with "new"	Disallowed	Disallowed
Function.prototype.call	Disallowed	Disallowed
Function.prototype.apply	Disallowed	Disallowed
Function.prototype.bind	Disallowed	Disallowed

Table 90. Promise is subclassable

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Disallowed	Disallowed
correct prototype chain	Disallowed	Disallowed
Promise.all	Disallowed	Disallowed
Promise.race	Disallowed	Disallowed

Table 91. Miscellaneous subclassables

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Boolean is subclassable	Disallowed	Disallowed
Number is subclassable	Disallowed	Disallowed
String is subclassable	Disallowed	Disallowed
Error is subclassable	Disallowed	Disallowed
Map is subclassable	Disallowed	Disallowed
Set is subclassable	Disallowed	Disallowed

Table 92. Prototype of bound functions

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functions	Disallowed	Disallowed
generator functions	Disallowed	Disallowed
arrow functions	Disallowed	Disallowed
classes	Disallowed	Disallowed
subclasses	Disallowed	Disallowed

Table 93. Proxy, internal 'get' calls

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
ToPrimitive	Disallowed	Disallowed
CreateListFromArrayLike	Disallowed	Disallowed
instanceof operator	Supported	Disallowed
HasBinding	Disallowed	Disallowed
CreateDynamicFunction	Disallowed	Disallowed
ClassDefinitionEvaluation	Disallowed	Disallowed
IteratorComplete, IteratorValue	Disallowed	Disallowed
ToPropertyDescriptor	Disallowed	Disallowed
Object.assign	Disallowed	Disallowed
Object.defineProperties	Disallowed	Disallowed
Function.prototype.bind	Disallowed	Disallowed
Error.prototype.toString	Disallowed	Disallowed
String.raw	Disallowed	Disallowed
RegExp constructor	Disallowed	Disallowed
RegExp.prototype.flags	Disallowed	Disallowed
RegExp.prototype.test	Disallowed	Disallowed
RegExp.prototype.toString	Disallowed	Disallowed
RegExp.prototype[Symbol.match]	Disallowed	Disallowed
RegExp.prototype[Symbol.replace]	Disallowed	Disallowed
RegExp.prototype[Symbol.search]	Disallowed	Disallowed
RegExp.prototype[Symbol.split]	Disallowed	Disallowed
Array.from	Disallowed	Disallowed
Array.prototype.concat	Disallowed	Disallowed
Array.prototype iteration methods	Disallowed	Disallowed
Array.prototype.pop	Disallowed	Disallowed
Array.prototype.reverse	Disallowed	Disallowed
Array.prototype.shift	Disallowed	Disallowed
Array.prototype.splice	Disallowed	Disallowed
Array.prototype.toString	Disallowed	Disallowed
JSON.stringify	Disallowed	Disallowed
Promise resolve functions	Disallowed	Disallowed
String.prototype.match	Disallowed	Disallowed
String.prototype.replace	Disallowed	Disallowed
String.prototype.search	Disallowed	Disallowed
String.prototype.split	Disallowed	Disallowed
Date.prototype.toJSON	Disallowed	Disallowed

Table 94. Proxy, internal 'set' calls

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Object.assign	Disallowed	Disallowed
Array.from	Disallowed	Disallowed
Array.of	Disallowed	Disallowed
Array.prototype.copyWithin	Disallowed	Disallowed
Array.prototype.fill	Disallowed	Disallowed
Array.prototype.pop	Disallowed	Disallowed
Array.prototype.push	Disallowed	Disallowed
Array.prototype.reverse	Disallowed	Disallowed
Array.prototype.shift	Disallowed	Disallowed
Array.prototype.splice	Disallowed	Disallowed
Array.prototype.unshift	Disallowed	Disallowed

Table 95. Proxy, internal 'defineProperty' calls

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
[[Set]]	Disallowed	Disallowed
SetIntegrityLevel	Disallowed	Disallowed

Table 96. Proxy, internal 'deleteProperty' calls

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Array.prototype.copyWithin	Disallowed	Disallowed
Array.prototype.pop	Disallowed	Disallowed
Array.prototype.reverse	Disallowed	Disallowed
Array.prototype.shift	Disallowed	Disallowed
Array.prototype.splice	Disallowed	Disallowed
Array.prototype.unshift	Disallowed	Disallowed

Table 97. Proxy, internal 'getOwnPropertyDescriptor' calls

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
[[Set]]	Disallowed	Disallowed
Object.assign	Disallowed	Disallowed
Object.prototype.hasOwnProperty	Disallowed	Disallowed
Function.prototype.bind	Disallowed	Disallowed

Table 98. Proxy, internal 'ownKeys' calls

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
SetIntegrityLevel	Disallowed	Disallowed
TestIntegrityLevel	Disallowed	Disallowed
SerializeJSONObject	Disallowed	Disallowed

Table 99. Object static methods accept primitives

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Object.getPrototypeOf	Disallowed	Disallowed
Object.getOwnPropertyDescriptor	Disallowed	Disallowed
Object.getOwnPropertyNames	Disallowed	Disallowed
Object.seal	Disallowed	Disallowed
Object.freeze	Disallowed	Disallowed
Object.preventExtensions	Disallowed	Disallowed
Object.isSealed	Disallowed	Disallowed
Object.isFrozen	Disallowed	Disallowed
Object.isExtensible	Disallowed	Disallowed
Object.keys	Disallowed	Disallowed

Table 100. Own property order

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Object.keys	Disallowed	Disallowed
Object.getOwnPropertyNames	Disallowed	Disallowed
Object.assign	Disallowed	Disallowed
JSON.stringify	Disallowed	Disallowed
JSON.parse	Disallowed	Disallowed
Reflect.ownKeys, string key order	Disallowed	Disallowed
Reflect.ownKeys, symbol key order	Disallowed	Disallowed

Table 101. Updated identifier syntax

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
var ⸯ;	Disallowed	Disallowed
var ð ‹€;	Disallowed	Disallowed
no escaped reserved words as identifiers	Disallowed	Disallowed

Table 102. Non-strict function semantics

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
hoisted block-level function declaration	Disallowed	Disallowed
labeled function statements	Disallowed	Disallowed
function statements in if-statement clauses	Disallowed	Disallowed

Table 103. __proto__ in object literals

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic support	Disallowed	Disallowed
multiple __proto__ is an error	Disallowed	Disallowed
not a computed property	Disallowed	Disallowed
not a shorthand property	Disallowed	Disallowed
not a shorthand method	Disallowed	Disallowed

Table 104. Object.prototype.__proto__

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
get prototype	Disallowed	Disallowed
set prototype	Disallowed	Disallowed
absent from Object.create(null)	Disallowed	Disallowed
present in hasOwnProperty()	Disallowed	Disallowed
correct property descriptor	Disallowed	Disallowed
present in Object.getOwnPropertyNames()	Disallowed	Disallowed

Table 105. String.prototype HTML methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
existence	Disallowed	Disallowed
tags' names are lowercase	Disallowed	Disallowed
quotes in arguments are escaped	Disallowed	Disallowed

Table 106. RegExp.prototype.compile

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
basic functionality	Disallowed	Disallowed
returns this	Disallowed	Disallowed

Table 107. RegExp syntax extensions

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
hyphens in character sets	Disallowed	Disallowed
invalid character escapes	Disallowed	Disallowed
invalid control-character escapes	Disallowed	Disallowed
invalid Unicode escapes	Disallowed	Disallowed
invalid hexadecimal escapes	Disallowed	Disallowed
incomplete patterns and quantifiers	Disallowed	Disallowed
octal escape sequences	Disallowed	Disallowed
invalid backreferences become octal escapes	Disallowed	Disallowed

ECMAScript 2009 (ES5) features

Table 108. Object/array literal extensions

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Getter accessors	Supported	Supported
Setter accessors	Supported	Supported
Trailing commas in object literals	Supported	Supported
Trailing commas in array literals	Supported	Supported
Reserved words as property names	Supported	Supported

Table 109. Object static methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Object.create	Supported	Supported
Object.defineProperty	Supported	Supported
Object.defineProperties	Supported	Supported
Object.getPrototypeOf	Supported	Supported
Object.keys	Supported	Supported
Object.seal	Supported	Supported
Object.freeze	Supported	Supported
Object.preventExtensions	Supported	Supported
Object.isSealed	Supported	Supported
Object.isFrozen	Supported	Supported
Object.isExtensible	Supported	Supported
Object.getOwnPropertyDescriptor	Supported	Supported
Object.getOwnPropertyNames	Supported	Supported

Table 110. Array methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Array.isArray	Supported	Supported
Array.prototype.indexOf	Supported	Supported
Array.prototype.lastIndexOf	Supported	Supported
Array.prototype.every	Supported	Supported
Array.prototype.some	Supported	Supported
Array.prototype.forEach	Supported	Supported
Array.prototype.map	Supported	Supported
Array.prototype.filter	Supported	Supported
Array.prototype.reduce	Supported	Supported
Array.prototype.reduceRight	Supported	Supported
Array.prototype.sort: compareFn must be function or undefined	Not Supported	Not Supported
Array.prototype.sort: compareFn may be explicit undefined	Supported	Supported

Table 111. String properties and methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Property access on strings	Supported	Supported
String.prototype.split	Supported	Not Supported
String.prototype.trim	Supported	Supported

Table 112. Date methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Date.prototype.toISOString	Supported	Supported
Date.now	Supported	Supported
Date.prototype.toJSON	Not Supported	Not Supported

Table 113. Immutable globals

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
undefined	Supported	Supported
NaN	Supported	Supported
Infinity	Supported	Supported

Table 114. Number methods

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Number.prototype.toExponential rounds properly	Supported	Supported
Number.prototype.toExponential throws on ±Infinity fractionDigits	Supported	Supported
Number.prototype.toExponential does not throw on edge cases	Supported	Supported

Table 115. Strict mode

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
reserved words	Disallowed	Disallowed
"this" is undefined in functions	Disallowed	Disallowed
"this" is not coerced to object in primitive methods	Disallowed	Disallowed
"this" is not coerced to object in primitive accessors	Disallowed	Disallowed
legacy octal is a SyntaxError	Disallowed	Disallowed
assignment to unresolvable identifiers is a ReferenceError	Disallowed	Disallowed
assignment to eval or arguments is a SyntaxError	Disallowed	Disallowed
assignment to non-writable properties is a TypeError	Disallowed	Disallowed
eval or arguments bindings is a SyntaxError	Disallowed	Disallowed
arguments.caller removed or is a TypeError	Disallowed	Disallowed
arguments.callee is a TypeError	Disallowed	Disallowed
(function(){}).caller and (function(){}).arguments is a TypeError	Disallowed	Disallowed
arguments is unmapped	Disallowed	Disallowed
eval() can't create bindings	Disallowed	Disallowed
deleting bindings is a SyntaxError	Disallowed	Disallowed
deleting non-configurable properties is a TypeError	Disallowed	Disallowed
"with" is a SyntaxError	Disallowed	Disallowed
repeated parameter names is a SyntaxError	Disallowed	Disallowed
function expressions with matching name and argument are valid	Disallowed	Disallowed

Table 116. Function.prototype.bind

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
Function.prototype.bind	Supported	Supported

Table 117. JSON

Feature	ECMAScript 2021 (ES12) mode	ES5 Standards mode
JSON	Supported	Supported