12.8 Literals

12.8.1 Null Literals

Syntax

NullLiteral :: null

12.8.2 Boolean Literals

Syntax

BooleanLiteral :: true false

12.8.3 Numeric Literals

Syntax

NumericLiteralSeparator :: _ NumericLiteral :: DecimalLiteral DecimalBigIntegerLiteral NonDecimalIntegerLiteral[+Sep] NonDecimalIntegerLiteral[+Sep] BigIntLiteralSuffix DecimalBigIntegerLiteral :: 0 BigIntLiteralSuffix NonZeroDigit DecimalDigits[+Sep]opt BigIntLiteralSuffix NonZeroDigit NumericLiteralSeparator DecimalDigits[+Sep] BigIntLiteralSuffix NonDecimalIntegerLiteral[Sep] :: BinaryIntegerLiteral[?Sep] OctalIntegerLiteral[?Sep] HexIntegerLiteral[?Sep] BigIntLiteralSuffix :: n DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits[+Sep]opt ExponentPart[+Sep]opt . DecimalDigits[+Sep] ExponentPart[+Sep]opt DecimalIntegerLiteral ExponentPart[+Sep]opt DecimalIntegerLiteral :: 0 NonZeroDigit NonZeroDigit NumericLiteralSeparatoropt DecimalDigits[+Sep] DecimalDigits[Sep] :: DecimalDigit DecimalDigits[?Sep] DecimalDigit [+Sep] DecimalDigits[+Sep] NumericLiteralSeparator DecimalDigit DecimalDigit :: one of 0 1 2 3 4 5 6 7 8 9 NonZeroDigit :: one of 1 2 3 4 5 6 7 8 9 ExponentPart[Sep] :: ExponentIndicator SignedInteger[?Sep] ExponentIndicator :: one of e E SignedInteger[Sep] :: DecimalDigits[?Sep] + DecimalDigits[?Sep] - DecimalDigits[?Sep] BinaryIntegerLiteral[Sep] :: 0b BinaryDigits[?Sep] 0B BinaryDigits[?Sep] BinaryDigits[Sep] :: BinaryDigit BinaryDigits[?Sep] BinaryDigit [+Sep] BinaryDigits[+Sep] NumericLiteralSeparator BinaryDigit BinaryDigit :: one of 0 1 OctalIntegerLiteral[Sep] :: 0o OctalDigits[?Sep] 0O OctalDigits[?Sep] OctalDigits[Sep] :: OctalDigit OctalDigits[?Sep] OctalDigit [+Sep] OctalDigits[+Sep] NumericLiteralSeparator OctalDigit OctalDigit :: one of 0 1 2 3 4 5 6 7 HexIntegerLiteral[Sep] :: 0x HexDigits[?Sep] 0X HexDigits[?Sep] HexDigits[Sep] :: HexDigit HexDigits[?Sep] HexDigit [+Sep] HexDigits[+Sep] NumericLiteralSeparator HexDigit HexDigit :: one of 0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

The SourceCharacter immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.

Note

For example: 3in is an error and not the two input elements 3 and in.

A conforming implementation, when processing strict mode code, must not extend, as described in B.1.1, the syntax of NumericLiteral to include LegacyOctalIntegerLiteral, nor extend the syntax of DecimalIntegerLiteral to include NonOctalDecimalIntegerLiteral.

12.8.3.1 Static Semantics: MV

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

12.8.3.2 Static Semantics: NumericValue

NumericLiteral :: DecimalLiteral
  1. Return the Number value that results from rounding the MV of DecimalLiteral as described below.
NumericLiteral :: NonDecimalIntegerLiteral
  1. Return the Number value that results from rounding the MV of NonDecimalIntegerLiteral 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 +0𝔽; otherwise, the rounded value must be the Number value for the MV (as specified in 6.1.6.1), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits, in which case the Number value may be either the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th significant digit position. A digit is significant if it is not part of an ExponentPart and

  • it is not 0; or
  • there is a non-zero digit to its left and there is a non-zero digit, not in the ExponentPart, to its right.
NumericLiteral :: NonDecimalIntegerLiteral BigIntLiteralSuffix
  1. Return the BigInt value that represents the MV of NonDecimalIntegerLiteral.
DecimalBigIntegerLiteral :: 0 BigIntLiteralSuffix
  1. Return 0.
DecimalBigIntegerLiteral :: NonZeroDigit BigIntLiteralSuffix
  1. Return the BigInt value that represents the MV of NonZeroDigit.
DecimalBigIntegerLiteral :: NonZeroDigit DecimalDigits BigIntLiteralSuffix NonZeroDigit NumericLiteralSeparator DecimalDigits BigIntLiteralSuffix
  1. Let n be the number of code points in DecimalDigits, excluding all occurrences of NumericLiteralSeparator.
  2. Let mv be (the MV of NonZeroDigit × 10) plus the MV of DecimalDigits.
  3. Return (mv).

12.8.4 String Literals

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.

Syntax

StringLiteral :: " DoubleStringCharactersopt " ' SingleStringCharactersopt ' DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharactersopt SingleStringCharacters :: SingleStringCharacter SingleStringCharactersopt DoubleStringCharacter :: SourceCharacter but not one of " or \ or LineTerminator <LS> <PS> \ EscapeSequence LineContinuation SingleStringCharacter :: SourceCharacter but not one of ' or \ or LineTerminator <LS> <PS> \ EscapeSequence LineContinuation LineContinuation :: \ LineTerminatorSequence EscapeSequence :: CharacterEscapeSequence 0 [lookahead ∉ DecimalDigit] HexEscapeSequence UnicodeEscapeSequence

A conforming implementation, when processing strict mode code, must not extend the syntax of EscapeSequence to include LegacyOctalEscapeSequence or NonOctalDecimalEscapeSequence as described in B.1.2.

CharacterEscapeSequence :: SingleEscapeCharacter NonEscapeCharacter SingleEscapeCharacter :: one of ' " \ b f n r t v NonEscapeCharacter :: SourceCharacter but not one of EscapeCharacter or LineTerminator EscapeCharacter :: SingleEscapeCharacter DecimalDigit x u HexEscapeSequence :: x HexDigit HexDigit UnicodeEscapeSequence :: u Hex4Digits u{ CodePoint } Hex4Digits :: HexDigit HexDigit HexDigit HexDigit

The definition of the nonterminal HexDigit is given in 12.8.3. SourceCharacter is defined in 11.1.

Note 2

<LF> and <CR> cannot appear in a string literal, except as part of a LineContinuation to produce the empty code points sequence. The proper way to include either in the String value of a string literal is to use an escape sequence such as \n or \u000A.

12.8.4.1 Static Semantics: SV

A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms of String values contributed by the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having a mathematical value (MV), as described below or in 12.8.3.

Table 36: String Single Character Escape Sequences
Escape Sequence Code Unit Value Unicode Character Name Symbol
\b 0x0008 BACKSPACE <BS>
\t 0x0009 CHARACTER TABULATION <HT>
\n 0x000A LINE FEED (LF) <LF>
\v 0x000B LINE TABULATION <VT>
\f 0x000C FORM FEED (FF) <FF>
\r 0x000D CARRIAGE RETURN (CR) <CR>
\" 0x0022 QUOTATION MARK "
\' 0x0027 APOSTROPHE '
\\ 0x005C REVERSE SOLIDUS \

12.8.4.2 Static Semantics: MV

12.8.5 Regular Expression Literals

Note 1

A regular expression literal is an input element that is converted to a RegExp object (see 22.2) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp or calling the RegExp constructor as a function (see 22.2.3).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and the RegularExpressionFlags are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (22.2.1).

An implementation may extend the ECMAScript Regular Expression grammar defined in 22.2.1, but it must not extend the RegularExpressionBody and RegularExpressionFlags productions defined below or the productions used by these productions.

Syntax

RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags RegularExpressionBody :: RegularExpressionFirstChar RegularExpressionChars RegularExpressionChars :: [empty] RegularExpressionChars RegularExpressionChar RegularExpressionFirstChar :: RegularExpressionNonTerminator but not one of * or \ or / or [ RegularExpressionBackslashSequence RegularExpressionClass RegularExpressionChar :: RegularExpressionNonTerminator but not one of \ or / or [ RegularExpressionBackslashSequence RegularExpressionClass RegularExpressionBackslashSequence :: \ RegularExpressionNonTerminator RegularExpressionNonTerminator :: SourceCharacter but not LineTerminator RegularExpressionClass :: [ RegularExpressionClassChars ] RegularExpressionClassChars :: [empty] RegularExpressionClassChars RegularExpressionClassChar RegularExpressionClassChar :: RegularExpressionNonTerminator but not one of ] or \ RegularExpressionBackslashSequence RegularExpressionFlags :: [empty] RegularExpressionFlags IdentifierPart Note 2

Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.

12.8.5.1 Static Semantics: Early Errors

RegularExpressionFlags :: RegularExpressionFlags IdentifierPart
  • It is a Syntax Error if IdentifierPart contains a Unicode escape sequence.

12.8.5.2 Static Semantics: BodyText

RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags
  1. Return the source text that was recognized as RegularExpressionBody.

12.8.5.3 Static Semantics: FlagText

RegularExpressionLiteral :: / RegularExpressionBody / RegularExpressionFlags
  1. Return the source text that was recognized as RegularExpressionFlags.

12.8.6 Template Literal Lexical Components

Syntax

Template :: NoSubstitutionTemplate TemplateHead NoSubstitutionTemplate :: ` TemplateCharactersopt ` TemplateHead :: ` TemplateCharactersopt ${ TemplateSubstitutionTail :: TemplateMiddle TemplateTail TemplateMiddle :: } TemplateCharactersopt ${ TemplateTail :: } TemplateCharactersopt ` TemplateCharacters :: TemplateCharacter TemplateCharactersopt TemplateCharacter :: $ [lookahead ≠ {] \ EscapeSequence \ NotEscapeSequence LineContinuation LineTerminatorSequence SourceCharacter but not one of ` or \ or $ or LineTerminator NotEscapeSequence :: 0 DecimalDigit DecimalDigit but not 0 x [lookahead ∉ HexDigit] x HexDigit [lookahead ∉ HexDigit] u [lookahead ∉ HexDigit] [lookahead ≠ {] u HexDigit [lookahead ∉ HexDigit] u HexDigit HexDigit [lookahead ∉ HexDigit] u HexDigit HexDigit HexDigit [lookahead ∉ HexDigit] u { [lookahead ∉ HexDigit] u { NotCodePoint [lookahead ∉ HexDigit] u { CodePoint [lookahead ∉ HexDigit] [lookahead ≠ }] NotCodePoint :: HexDigits[~Sep] but only if MV of HexDigits > 0x10FFFF CodePoint :: HexDigits[~Sep] but only if MV of HexDigits ≤ 0x10FFFF

A conforming implementation must not use the extended definition of EscapeSequence described in B.1.2 when parsing a TemplateCharacter.

Note

TemplateSubstitutionTail is used by the InputElementTemplateTail alternative lexical goal.

12.8.6.1 Static Semantics: TV and TRV

A template literal component is interpreted as a sequence of Unicode code points. The Template Value (TV) of a literal component is described in terms of String values (SV, 12.8.4) contributed by the various parts of the template literal component. As part of this process, some Unicode code points within the template component are interpreted as having a mathematical value (MV, 12.8.3). In determining a TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence. The Template Raw Value (TRV) is similar to a Template Value with the difference that in TRVs escape sequences are interpreted literally.

Note

TV excludes the code units of LineContinuation while TRV includes them. <CR><LF> and <CR> LineTerminatorSequences are normalized to <LF> for both TV and TRV. An explicit EscapeSequence is needed to include a <CR> or <CR><LF> sequence.