Skip to content

11 ECMAScript Language: Lexical Grammar

The source text of an ECMAScript Script or Module is first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.

There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. The InputElementRegExpOrTemplateTail goal is used in syntactic grammar contexts where a RegularExpressionLiteral, a TemplateMiddle, or a TemplateTail is permitted. The InputElementRegExp goal symbol is used in all syntactic grammar contexts where a RegularExpressionLiteral is permitted but neither a TemplateMiddle, nor a TemplateTail is permitted. The InputElementTemplateTail goal is used in all syntactic grammar contexts where a TemplateMiddle or a TemplateTail is permitted but a RegularExpressionLiteral is not permitted. In all other contexts, InputElementDiv is used as the lexical goal symbol.

Note

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 RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 11.9); in examples such as the following:


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

where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in the same way as:


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

Syntax

InputElementDiv :: WhiteSpace LineTerminator Comment CommonToken DivPunctuator RightBracePunctuator InputElementRegExp :: WhiteSpace LineTerminator Comment CommonToken RightBracePunctuator RegularExpressionLiteral InputElementRegExpOrTemplateTail :: WhiteSpace LineTerminator Comment CommonToken RegularExpressionLiteral TemplateSubstitutionTail InputElementTemplateTail :: WhiteSpace LineTerminator Comment CommonToken DivPunctuator TemplateSubstitutionTail

11.1 Unicode Format-Control Characters

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 IdentifierName 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 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 11.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized in Table 31.

Code Point Name Abbreviation Usage
U+200C ZERO WIDTH NON-JOINER <ZWNJ> IdentifierPart
U+200D ZERO WIDTH JOINER <ZWJ> IdentifierPart
U+FEFF ZERO WIDTH NO-BREAK SPACE <ZWNBSP> WhiteSpace

11.2 White Space

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 StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail 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 32.

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

ECMAScript implementations must recognize as WhiteSpace code points listed in the “Space_Separator” (“Zs”) category.

Note

Other than for the code points listed in Table 32, ECMAScript WhiteSpace intentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in category “Space_Separator” (“Zs”).

Syntax

WhiteSpace :: <TAB> <VT> <FF> <SP> <NBSP> <ZWNBSP> <USP>

11.3 Line Terminators

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 (11.9). A line terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. Line terminators may only occur within a StringLiteral token as part of a LineContinuation.

A line terminator can occur within a MultiLineComment but cannot occur within a SingleLineComment.

Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.

The ECMAScript line terminator code points are listed in Table 33.

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 33 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 32. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of reporting line numbers.

Syntax

LineTerminator :: <LF> <CR> <LS> <PS> LineTerminatorSequence :: <LF> <CR> [lookahead ≠ <LF> ] <LS> <PS> <CR> <LF>

11.4 Comments

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

Because a single-line comment can contain any Unicode code point except a LineTerminator 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 LineTerminator 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 11.9).

Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code point, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

Syntax

Comment :: MultiLineComment SingleLineComment MultiLineComment :: /* MultiLineCommentCharsopt */ MultiLineCommentChars :: MultiLineNotAsteriskChar MultiLineCommentCharsopt * PostAsteriskCommentCharsopt PostAsteriskCommentChars :: MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt * PostAsteriskCommentCharsopt MultiLineNotAsteriskChar :: SourceCharacter but not * MultiLineNotForwardSlashOrAsteriskChar :: SourceCharacter but not one of / or * SingleLineComment :: // SingleLineCommentCharsopt SingleLineCommentChars :: SingleLineCommentChar SingleLineCommentCharsopt SingleLineCommentChar :: SourceCharacter but not LineTerminator

11.5 Tokens

Syntax

CommonToken :: IdentifierName Punctuator NumericLiteral StringLiteral Template Note

The DivPunctuator, RegularExpressionLiteral, RightBracePunctuator, and TemplateSubstitutionTail productions derive additional tokens that are not included in the CommonToken production.

11.6 Names and Keywords

IdentifierName and ReservedWord 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. ReservedWord is an enumerated subset of IdentifierName. The syntactic grammar defines Identifier as an IdentifierName that is not a ReservedWord. 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

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

Unicode escape sequences are permitted in an IdentifierName, where they contribute a single Unicode code point to the IdentifierName. The code point is expressed by the CodePoint of the UnicodeEscapeSequence (see 11.8.4). The \ preceding the UnicodeEscapeSequence and the u and { } code units, if they appear, do not contribute code points to the IdentifierName. A UnicodeEscapeSequence cannot be used to put a code point into an IdentifierName that would otherwise be illegal. In other words, if a \ UnicodeEscapeSequence sequence were replaced by the SourceCharacter it contributes, the result must still be a valid IdentifierName that has the exact same sequence of SourceCharacter elements as the original IdentifierName. All interpretations of IdentifierName 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 IdentifierNames that are canonically equivalent according to the Unicode standard are not equal unless, after replacement of each UnicodeEscapeSequence, they are represented by the exact same sequence of code points.

Syntax

IdentifierName :: IdentifierStart IdentifierName IdentifierPart IdentifierStart :: UnicodeIDStart $ _ \ UnicodeEscapeSequence IdentifierPart :: UnicodeIDContinue $ \ UnicodeEscapeSequence <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 UnicodeEscapeSequence is given in 11.8.4.

Note 2

The nonterminal IdentifierPart derives _ via UnicodeIDContinue.

 Note 3

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”.

11.6.1 Identifier Names

11.6.1.1 Static Semantics: Early Errors

IdentifierStart :: \ UnicodeEscapeSequence IdentifierPart :: \ UnicodeEscapeSequence

11.6.1.2 Static Semantics: StringValue

IdentifierName :: IdentifierStart IdentifierName IdentifierPart
  1. Return the String value consisting of the sequence of code units corresponding to IdentifierName. In determining the sequence any occurrences of \ UnicodeEscapeSequence are first replaced with the code point represented by the UnicodeEscapeSequence and then the code points of the entire IdentifierName are converted to code units by UTF16Encoding each code point.

11.6.2 Reserved Words

A reserved word is an IdentifierName that cannot be used as an Identifier.

Syntax

ReservedWord :: Keyword FutureReservedWord NullLiteral BooleanLiteral Note

The ReservedWord definitions are specified as literal sequences of specific SourceCharacter elements. A code point in a ReservedWord cannot be expressed by a \ UnicodeEscapeSequence.

11.6.2.1 Keywords

The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript programs.

Syntax

Keyword :: one of await break case catch class const continue debugger default delete do else export extends finally for function if import in instanceof new return super switch this throw try typeof var void while with yield Note

In some contexts yield and await are given the semantics of an Identifier. See 12.1.1. In strict mode code, let and static are treated as reserved words through static semantic restrictions (see 12.1.1, 13.3.1.1, 13.7.5.1, and 14.6.1) rather than the lexical grammar.

11.6.2.2 Future Reserved Words

The following tokens are reserved for use as keywords in future language extensions.

Syntax

FutureReservedWord :: enum Note

Use of the following tokens within strict mode code is also reserved. That usage is restricted using static semantic restrictions (see 12.1.1) rather than the lexical grammar:

implements package protected
interface private public

11.7 Punctuators

Syntax

Punctuator :: one of { ( ) [ ] . ... ; , < > <= >= == != === !== + - * % ** ++ -- << >> >>> & | ^ ! ~ && || ? : = += -= *= %= **= <<= >>= >>>= &= |= ^= => DivPunctuator :: / /= RightBracePunctuator :: }

11.8 Literals

11.8.1 Null Literals

Syntax

NullLiteral :: null

11.8.2 Boolean Literals

Syntax

BooleanLiteral :: true false

11.8.3 Numeric Literals

Syntax

NumericLiteral :: DecimalLiteral BinaryIntegerLiteral OctalIntegerLiteral HexIntegerLiteral DecimalLiteral :: DecimalIntegerLiteral . DecimalDigitsopt ExponentPartopt . DecimalDigits ExponentPartopt DecimalIntegerLiteral ExponentPartopt DecimalIntegerLiteral :: 0 NonZeroDigit DecimalDigitsopt DecimalDigits :: DecimalDigit DecimalDigits 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 :: ExponentIndicator SignedInteger ExponentIndicator :: one of e E SignedInteger :: DecimalDigits + DecimalDigits - DecimalDigits BinaryIntegerLiteral :: 0b BinaryDigits 0B BinaryDigits BinaryDigits :: BinaryDigit BinaryDigits BinaryDigit BinaryDigit :: one of 0 1 OctalIntegerLiteral :: 0o OctalDigits 0O OctalDigits OctalDigits :: OctalDigit OctalDigits OctalDigit OctalDigit :: one of 0 1 2 3 4 5 6 7 HexIntegerLiteral :: 0x HexDigits 0X HexDigits HexDigits :: HexDigit HexDigits 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 prod-annexB-LegacyOctalIntegerLiteral, nor extend the syntax of DecimalIntegerLiteral to include prod-annexB-NonOctalDecimalIntegerLiteral.

11.8.3.1 Static Semantics: MV

A numeric literal stands for a value of the Number type. This value is determined in two steps: first, a mathematical value (MV) is derived from the literal; second, this mathematical value is rounded 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 +0; otherwise, the rounded value must be the Number value for the MV (as specified in 6.1.6), 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 nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

11.8.4 String Literals

Note 1

A string literal is zero 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), U+2028 (LINE SEPARATOR), U+2029 (PARAGRAPH SEPARATOR), 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 10.1.1. 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 \ EscapeSequence LineContinuation SingleStringCharacter :: SourceCharacter but not one of ' or \ or LineTerminator \ 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 prod-annexB-LegacyOctalEscapeSequence 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 11.8.3. SourceCharacter is defined in 10.1.

Note 2

A line terminator code point cannot appear in a string literal, except as part of a LineContinuation to produce the empty code points sequence. The proper way to cause a line terminator code point to be part of the String value of a string literal is to use an escape sequence such as \n or \u000A.

11.8.4.1 Static Semantics: StringValue

StringLiteral :: " DoubleStringCharactersopt " ' SingleStringCharactersopt '
  1. Return the String value whose elements are the SV of this StringLiteral.

11.8.4.2 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 code unit 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 11.8.3.

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 \

11.8.5 Regular Expression Literals

Note 1

A regular expression literal is an input element that is converted to a RegExp object (see 21.2) 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 21.2.3).

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 (21.2.1).

An implementation may extend the ECMAScript Regular Expression grammar defined in 21.2.1, 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: /(?:)/.

11.8.5.1 Static Semantics: Early Errors

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

11.8.5.2 Static Semantics: BodyText

11.8.5.3 Static Semantics: FlagText

11.8.6 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 but only if MV of HexDigits > 0x10FFFF CodePoint :: HexDigits 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.

11.8.6.1 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 code unit values (SV, 11.8.4) 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, 11.8.3). 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.

11.9 Automatic Semicolon Insertion

Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.

11.9.1 Rules of Automatic Semicolon Insertion

In the following rules, “token” means the actual recognized lexical token determined using the current lexical goal symbol as described in clause 11.

There are three basic rules of semicolon insertion:

  1. When, as the source text is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:

    • The offending token is separated from the previous token by at least one LineTerminator.
    • The offending token is }.
    • The previous token is ) and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (13.7.2).
  2. When, as the source text is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single instance of the goal nonterminal, then a semicolon is automatically inserted at the end of the input stream.
  3. When, as the source text is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[no LineTerminator here]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least one LineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see 13.7.4).

Note

The following are the only restricted productions in the grammar:

UpdateExpression[Yield, Await] : LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] ++ LeftHandSideExpression[?Yield, ?Await] [no LineTerminator here] -- ContinueStatement[Yield, Await] : continue ; continue [no LineTerminator here] LabelIdentifier[?Yield, ?Await] ; BreakStatement[Yield, Await] : break ; break [no LineTerminator here] LabelIdentifier[?Yield, ?Await] ; ReturnStatement[Yield, Await] : return ; return [no LineTerminator here] Expression[+In, ?Yield, ?Await] ; ThrowStatement[Yield, Await] : throw [no LineTerminator here] Expression[+In, ?Yield, ?Await] ; ArrowFunction[In, Yield, Await] : ArrowParameters[?Yield, ?Await] [no LineTerminator here] => ConciseBody[?In] YieldExpression[In, Await] : yield [no LineTerminator here] * AssignmentExpression[?In, +Yield, ?Await] yield [no LineTerminator here] AssignmentExpression[?In, +Yield, ?Await]

The practical effect of these restricted productions is as follows:

  • When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.
  • When a continue, break, return, throw, or yield token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.

The resulting practical advice to ECMAScript programmers is:

  • A postfix ++ or -- operator should appear on the same line as its operand.
  • An Expression in a return or throw statement or an AssignmentExpression in a yield expression should start on the same line as the return, throw, or yield token.
  • A LabelIdentifier in a break or continue statement should be on the same line as the break or continue token.

11.9.2 Examples of Automatic Semicolon Insertion

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source


        { 1
        2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:


        { 1
        ;2 ;} 3;

which is a valid ECMAScript sentence.

The source


        for (a; b
        )

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.

The source


        return
        a + b

is transformed by automatic semicolon insertion into the following:


        return;
        a + b;
Note 1

The expression a + b is not treated as a value to be returned by the return statement, because a LineTerminator separates it from the token return.

The source


        a = b
        ++c

is transformed by automatic semicolon insertion into the following:


        a = b;
        ++c;
Note 2

The token ++ is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs between b and ++.

The source


        if (a > b)
        else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source


        a = b + c
        (d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:

a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.