ECMA-262 (12th Edition) ECMAScript 2021 Language Specification - page 24

is used in all syntactic grammar

contexts where a

is permitted but neither a

TemplateMiddle

, nor a

TemplateTail

is permitted. The

InputElementTemplateTail

goal is used in all syntactic grammar contexts where a

is not permitted. In all other contexts,

InputElementDiv

is used as the lexical

goal symbol

NOTE

InputElementDiv

The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect
automatic semicolon insertion. For example, there are no syntactic grammar contexts where both
a leading division or division-assignment, and a leading

); in examples such as the following:

are permitted.

This is not affected by semicolon insertion (see

12.9

where the first non-whitespace, non-comment code point after a

is U+002F

(SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is
inserted at the

WhiteSpace
LineTerminator
Comment
CommonToken
DivPunctuator
RightBracePunctuator

. That is, the above example is interpreted in the same way as:

a = b
/hi/g.exec(c).map(d);

a = b / hi / g.exec(c).map(d);

11.2.3 Non-ECMAScript Functions

12 ECMAScript Language: Lexical Grammar

Syntax

276

InputElementRegExp

WhiteSpace
LineTerminator
Comment
CommonToken
RightBracePunctuator
RegularExpressionLiteral

InputElementRegExpOrTemplateTail

WhiteSpace
LineTerminator
Comment
CommonToken
RegularExpressionLiteral
TemplateSubstitutionTail

InputElementTemplateTail

WhiteSpace
LineTerminator
Comment
CommonToken
DivPunctuator
TemplateSubstitutionTail

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as
LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of
text in the absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control
characters may be used within comments, and within string literals, template literals, and regular expression literals.

U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control characters that are
used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text
these code points may also be used in an

after the first character.

U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark
it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this

12.1 Unicode Format-Control Characters

277

purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In
ECMAScript source text <ZWNBSP> code points are treated as white space characters (see

12.2

The special treatment of certain format-control characters outside of comments, string literals, and regular expression
literals is summarized in

Table 33

Table 33: Format-Control Code Point Usage

Code Point

Name

Abbreviation

Usage

U+200C

ZERO WIDTH NON-JOINER

<ZWNJ>

U+200D

ZERO WIDTH JOINER

<ZWJ>

U+FEFF

ZERO WIDTH NO-BREAK SPACE <ZWNBSP>

White space code points are used to improve source text readability and to separate tokens (indivisible lexical units)
from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at
the start or end of input. White space code points may occur within a

, a

TemplateSubstitutionTail

, a

Template

, or a

where they are considered significant code points forming part of a literal value.

They may also occur within a

Comment

, but cannot appear within any other kind of token.

The ECMAScript white space code points are listed in

Table 34

Table 34: White Space Code Points

Code Point

Name

Abbreviation

U+0009

CHARACTER TABULATION

<TAB>

U+000B

LINE TABULATION

<VT>

U+000C

FORM FEED (FF)

<FF>

U+0020

SPACE

<SP>

U+00A0

NO-BREAK SPACE

<NBSP>

U+FEFF

ZERO WIDTH NO-BREAK SPACE

Other category “Zs” Any other Unicode “Space_Separator” code point <USP>

ECMAScript implementations must recognize as

code points listed in the “Space_Separator” (“Zs”)

category.

NOTE

Other than for the code points listed in

Table 34

, ECMAScript

intentionally excludes

all code points that have the Unicode “White_Space” property but which are not classified in
category “Space_Separator” (“Zs”).

12.2 White Space

278

). A line terminator cannot occur within any token except a

<TAB>

<VT>

<FF>

<SP>

<NBSP>

<USP>

Like white space code points, line terminator code points are used to improve source text readability and to separate
tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have
some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two
tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the
process of automatic semicolon insertion (

12.9

TemplateSubstitutionTail

Template

, or

. <LF> and <CR> line terminators cannot occur within a

but cannot occur within a

token except

as part of a

LineContinuation

A line terminator can occur within a

MultiLineComment

SingleLineComment

Line terminators are included in the set of white space code points that are matched by the

class in regular

expressions.

The ECMAScript line terminator code points are listed in

Table 35

Table 35: Line Terminator Code Points

Code Point

Unicode Name

Abbreviation

U+000A

LINE FEED (LF)

<LF>

U+000D

CARRIAGE RETURN (CR)

<CR>

U+2028

LINE SEPARATOR

<LS>

U+2029

PARAGRAPH SEPARATOR <PS>

Only the Unicode code points in

Table 35

are treated as line terminators. Other new line or line breaking Unicode code

points are not treated as line terminators but are treated as white space if they meet the requirements listed in

Table

. The sequence <CR><LF> is commonly used as a line terminator. It should be consider ed a single

for the purpose of reporting line numbers.

Because a single-line comment can contain any Unicode code point except a

<LF>

Syntax

12.3 Line Terminators

Syntax

279

<CR>

<LS>

<PS>

LineTerminatorSequence

<LF>

<CR>

[lookahead

≠

<LF>

]

<LS>

<PS>

<CR>

<LF>

Comments can be either single or multi-line. Multi-line comments cannot nest.

code point, and because of

the general rule that a token is always as long as possible, a single-line comment always consists of all code points
from the

marker to the end of the line. However, the

contains a line terminator

at the end of the line is not considered to be

part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of
input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence
of single-line comments does not affect the process of automatic semicolon insertion (see

12.9

Comments behave like white space and are discarded except that, if a

MultiLineComment

code point, then the entire comment is considered to be a

for purposes of parsing by the syntactic

grammar.

Comment

MultiLineComment
SingleLineComment

MultiLineComment

opt

MultiLineNotAsteriskChar

PostAsteriskCommentChars

PostAsteriskCommentChars

MultiLineNotForwardSlashOrAsteriskChar

PostAsteriskCommentChars

MultiLineNotAsteriskChar

MultiLineNotForwardSlashOrAsteriskChar

but not

but not one of

12.4 Comments

Syntax

280

SingleLineComment

SingleLineCommentChars

opt

SingleLineCommentChars

SingleLineCommentChar

SingleLineCommentChars

opt

SingleLineCommentChar

but not

IdentifierName
Punctuator
NumericLiteral
StringLiteral
Template

A number of productions in this section are given alternative definitions in section

B.1.3

CommonToken

NOTE

and

are tokens that are interpreted according to the Default Identifier Syntax given in

Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications.

is an

enumerated subset of

. The syntactic grammar defines

. The Unicode identifier grammar is based on character properties specified by the Unicode Standard.

The Unicode code points in the specified categories in the latest version of the Unicode standard must be treated as in
those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize
identifier code points defined in later editions of the Unicode Standard.

NOTE 1

Unicode escape sequences are permitted in an

, where they contribute a single Unicode code point to the

. The code point is expressed by the

CodePoint

of the

(see

12.8.4

). The

preceding

the

and the

and

{ }

code units, if they appear, do not contribute code points to the

. A

cannot be used to put a code point into an

that would otherwise

be illegal. In other words, if a

sequence were replaced by the

it contributes,

The

DivPunctuator

TemplateSubstitutionTail

RightBracePunctuator

, and

productions derive additional tokens that are not included in the

CommonToken

production.

This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F
(LOW LINE) are permitted anywhere in an

, and the code points U+200C (ZERO

WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are permitted anywhere after the
first code point of an

12.5 Tokens

Syntax

12.6 Names and Keywords

281

the result must still be a valid

that has the exact same sequence of

elements as the

original

. All interpretations of

within this specification are based upon their actual code

points regardless of whether or not an escape sequence was used to contribute any particular code point.

Two

s that are canonically equivalent according to the Unicode standard are

not

equal unless, after

replacement of each

, they are represented by the exact same sequence of code points.

IdentifierStart
IdentifierName

IdentifierStart

UnicodeIDStart

UnicodeIDContinue

<ZWNJ>

<ZWJ>

UnicodeIDStart

any Unicode code point with the Unicode property “ID_Start”

UnicodeIDContinue

any Unicode code point with the Unicode property “ID_Continue”

The definitions of the nonterminal

is given in

12.8.4

NOTE 2

NOTE 3

IdentifierStart

It is a Syntax Error if the

is none of

"$"

, or

"_"

, or !

UTF16EncodeCodePoint

(

) for

some Unicode code point

matched by the

UnicodeIDStart

lexical grammar production.

derives

The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include,
respectively, the code points with Unicode properties “Other_ID_Start” and
“Other_ID_Continue”.

Syntax

12.6.1 Identifier Names

12.6.1.1 Static Semantics: Early Errors

282

It is a Syntax Error if the

is none of

"$"

"_"

(<ZWNJ>), !

) for some Unicode code point

that would

be matched by the

UnicodeIDContinue

lexical grammar production.

keyword

is a token that matches

, but also has a syntactic use; that is, it appears literally, in a

fixed width

font, in some syntactic production. The keywords of ECMAScript include

while

async

await

, and many others.

reserved word

is an

that cannot be used as an identifier. Many keywords are reserved words, but some

are not, and some are reserved only in certain contexts.

and

while

are reserved words.

await

is reserved only

inside async functions and modules.

async

is not reserved; it can be used as a variable name or statement label

without restriction.

This specification uses a combination of grammatical productions and

early error

rules to specify which names are

valid identifiers and which are reserved words. All tokens in the

list below, except for

await

and

yield

, are unconditionally reserved. Exceptions for

await

and

yield

are specified in

13.1

, using parameterized

syntactic productions. Lastly, several

early error

rules restrict the set of valid identifiers. See

. In summary, there are five categories of identifier names:

Those that are always allowed as identifiers, and are not keywords, such as

Math

window

toString

, and

;

Those that are never allowed as identifiers, namely the

s listed below except

await

and

yield

;

Those that are contextually allowed as identifiers, namely

await

and

yield

;

Those that are contextually disallowed as identifiers, in

strict mode code

let

static

implements

interface

package

private

protected

, and

public

;

Those that are always allowed as identifiers, but also appear as keywords within certain syntactic productions,
at places where

Identifier

is not allowed:

async

from

get

set

, and

target

The term

conditional keyword

, or

contextual keyword

, is sometimes used to refer to the keywords that fall in the last three

categories, and thus can be used as identifiers in some contexts and as keywords in others.

OptionalChainingPunctuator
OtherPunctuator

one of

await

break

case

catch

class

const

continue

debugger

default

delete

else

enum

export

extends

false

finally

for

function

import

instanceof

new

null

return

super

switch

this

throw

true

try

typeof

var

void

while

with

yield

12.6.2 Keywords and Reserved Words

Syntax

283

NOTE 1

NOTE 2

NOTE 3

Punctuator

OptionalChainingPunctuator

[lookahead

∉

]

OtherPunctuator

one of

{

(

)

[

]

...

;

===

!==

>>>

**=

<<=

>>=

>>>=

&&=

||=

??=

DivPunctuator

RightBracePunctuator

}

Per

5.1.5

, keywords in the grammar match literal sequences of specific

elements.

A code point in a keyword cannot be expressed by a

can contain

s, but it is not possible to declare a variable

named "else" by spelling it

els\u{65}

. The

early error

rules in

13.1.1

rule out identifiers with

the same

StringValue

as a reserved word.

enum

is not currently used as a keyword in this specification. It is a

future reserved word

, set aside

for use as a keyword in future language extensions.

Similarly,

implements

interface

package

private

protected

, and

public

are

future reserved words in

strict mode code

The names

arguments

and

eval

are not keywords, but they are subject to some restrictions in

12.7 Punctuators

Syntax

12.8 Literals

12.8.1 Null Literals

Syntax

284

NullLiteral

null

BooleanLiteral

true

false

DecimalLiteral
DecimalBigIntegerLiteral
NonDecimalIntegerLiteral

[+Sep]

opt

[+Sep]

[Sep]

[?Sep]

[?Sep]

[?Sep]

[+Sep]

opt

[+Sep]

opt

[+Sep]

opt

opt

NonZeroDigit
NonZeroDigit

opt

[+Sep]

12.8.2 Boolean Literals

Syntax

12.8.3 Numeric Literals

Syntax

285

DecimalDigit
DecimalDigits

[Sep]

[?Sep]

[+Sep]

[+Sep]

one of

one of

[Sep]

[?Sep]

one of

[Sep]

[?Sep]

BinaryDigit
BinaryDigits

[?Sep]

[+Sep]

[+Sep]

one of

[Sep]

[?Sep]

[?Sep]

OctalDigit
OctalDigits

[+Sep]

OctalDigit

one of

[Sep]

286

[?Sep]

[?Sep]

[+Sep]

[+Sep]

one of

The

, must not extend, as described in

immediately following a

NOTE

A conforming implementation, when processing

strict mode code

LegacyOctalIntegerLiteral

, nor extend the syntax of

NonOctalDecimalIntegerLiteral

to include

A numeric literal stands for a value of the Number type or the BigInt type.

The MV of

is the MV of

The MV of

The MV of

is the MV of

The MV of

is the MV of

The MV of

is the MV of

The MV of

is the MV of

plus (the

MV of

is the number of code points in

), where

, excluding all occurrences of

The MV of

is the MV of

where

is the MV of

The MV of

is (the MV of

plus (the MV of

))

, where

is the number of code points in

, excluding all occurrences of

The MV of

, where

is the number of code

points in

, excluding all occurr ences of

The MV of

, where

is the

number of code points in

, excluding all occurrences of

, and

is the MV of

The MV of

The MV of

, where

The MV of

is 0.

The MV of

The MV of

opt

is (the MV of

For example:

3in

is an error and not the two input elements

and

12.8.3.1 Static Semantics: MV

287

) plus the MV of

, where

is the number of code points in

excluding all occurrences of

The MV of

is the MV of

The MV of

is (the MV of

10) plus the MV of

The MV of

10) plus the MV of

The MV of

The MV of

The MV of

is the MV of

The MV of

is the negative of the MV of

The MV of

or of

or of

or of

is 0.

The MV of

or of

or of

or of

or of

is 1.

The MV of

or of

or of

or of

is 2.

The MV of

or of

or of

or of

is 3.

The MV of

or of

or of

or of

is 4.

The MV of

or of

or of

or of

is 5.

The MV of

or of

or of

or of

is 6.

The MV of

or of

or of

or of

is 7.

The MV of

or of

or of

is 8.

The MV of

or of

or of

is 9.

The MV of

or of

is 10.

The MV of

or of

is 11.

The MV of

or of

is 12.

The MV of

or of

is 13.

The MV of

or of

is 14.

The MV of

or of

is 15.

The MV of

The MV of

The MV of

is the MV of

The MV of

is (the MV of

The MV of

is (the MV of

plus the MV of

The MV of

OctalDigits

is the MV of

OctalDigits

The MV of

is the MV of

The MV of

The MV of

is (the MV of

8) plus the MV of

The MV of

OctalDigit

is (the MV of

OctalDigits

8) plus the

MV of

The MV of

is the MV of

The MV of

is the MV of

The MV of

The MV of

is (the MV of

16) plus the MV of

The MV of

is (the MV of

16) plus the MV

288

1. Return the

that results from rounding the MV of

as described below.

1. Return the

Number value

that results from rounding the MV of

), unless the literal is a

as described below.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. If the
MV is 0, then the rounded value is

𝔽

; otherwise, the rounded value must be the

Number value

for the MV (as

specified in

6.1.6.1

and the literal has more than 20 significant digits, in which

for the MV of a literal produced by replacing each significant

digit after the 20th with a

digit or the

Number value

for the MV of a literal produced by replacing each significant

digit after the 20th with a

digit and then incrementing the literal at the 20th significant digit position. A digit is

significant

if it is not part of an

and

it is not

; or

there is a non-zero digit to its left and there is a non-zero digit, not in the

, to its right.

1. Return the BigInt value that represents the MV of

1. Return

ℤ

1. Return the BigInt value that represents the MV of

be the number of code points in

1. Let

, excluding all occurrences of

2. Let

be (the MV of

10) plus the MV of

3. Return

NOTE 1

A string literal is 0 or more Unicode code points enclosed in single or double quotes. Unicode
code points may also be represented by an escape sequence. All code points may appear literally
in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS),
U+000D (CARRIAGE RETURN), and U+000A (LINE FEED). Any code points may appear in the
form of an escape sequence. String literals evaluate to ECMAScript String values. When
generating these String values Unicode code points are UTF-16 encoded as defined in

11.1.1

Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element
of the string. All other code points are encoded as two code unit elements of the string.

12.8.3.2 Static Semantics: NumericValue

12.8.4 String Literals

289

DoubleStringCharacters

opt

SingleStringCharacters