The source text of an ECMAScript program is first converted into a sequence of input elements, which are either tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of characters as the next input element.
There are two goal symbols for the lexical grammar. The InputElementDiv symbol is used in those syntactic grammar contexts where a division (/) or division-assignment (/=) operator is permitted. The InputElementRegExp symbol is used in other syntactic grammar contexts.
Note that contexts exist in the syntactic grammar where both a division and a RegularExpressionLiteral are permitted by the syntactic grammar; however, since the lexical grammar uses the InputElementDiv goal symbol in such cases, the opening slash is not recognised as starting a regular expression literal in such a context. As a workaround, one may enclose the regular expression literal in parentheses.
Syntax
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 these in source text to facilitate editing and display.
The format control characters can occur anywhere in the source text of an ECMAScript program. These characters are removed from the source text before applying the lexical grammar. Since these characters are removed before processing string and regular expression literals, one must use a Unicode escape sequence (see 7.6) to include a Unicode format-control character inside a string or regular expression literal.
White space characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space may occur between any two tokens, and may occur within strings (where they are considered significant characters forming part of the literal string value), but cannot appear within any other kind of token.
The following characters are considered to be white space:
| Code Point Value | Name | Formal Name |
|---|---|---|
| \u0009 | Tab | <TAB> |
| \u000B | Vertical Tab | <VT> |
| \u000C | Form Feed | <FF> |
| \u0020 | Space | <SP> |
| \u00A0 | No-break space | <NBSP> |
| Other category "Zs" | Any other Unicode "space separator" | <USP> |
Syntax
Like white space characters, line terminator characters are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space characters, 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. A line terminator cannot occur within any token, not even a string. Line terminators also affect the process of automatic semicolon insertion (7.9).
The following characters are considered to be line terminators:
| Code Point Value | Name | Formal Name |
|---|---|---|
| \u000A | Line Feed | <LF> |
| \u000D | Carriage Return | <CR> |
| \u2028 | Line separator | <LS> |
| \u2029 | Paragraph separator | <PS> |
Syntax
Description
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any character except a LineTerminator character, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all characters 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 recognised 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 (7.9).
Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator character, then the entire comment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.
Syntax
Syntax
Description
Reserved words cannot be used as identifiers.
Syntax
The following tokens are ECMAScript keywords and may not be used as identifiers in ECMAScript programs.
Syntax
The following words are used as keywords in proposed extensions and are therefore reserved to allow for the possibility of future adoption of those extensions.
Syntax
Description
Identifiers are interpreted according to the grammar given in Section 5.16 of the upcoming version 3.0 of the Unicode standard, with some small modifications. This grammar is based on both normative and informative character categories specified by the Unicode standard. The characters in the specified categories in version 2.1 of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations; however, conforming ECMAScript implementations may allow additional legal identifier characters based on the category assignment from later versions of Unicode.
This standard specifies one departure from the grammar given in the Unicode standard: The dollar sign ($) and the underscore (_) are permitted anywhere in an identifier. The dollar sign is intended for use only in mechanically generated code.
Unicode escape sequences are also permitted in identifiers, where they contribute a single character to the identifier, as computed by the CV of the UnicodeEscapeSequence (see 7.8.4). The \ preceding the UnicodeEscapeSequence does not contribute a character to the identifier. A UnicodeEscapeSequence cannot be used to put a character into an identifier that would otherwise be illegal. In other words, if a \ UnicodeEscapeSequence sequence were replaced by its UnicodeEscapeSequence's CV, the result must still be a valid Identifier that has the exact same sequence of characters as the original Identifier.
Two identifiers that are canonically equivalent according to the Unicode standard are not equal unless they are represented by the exact same sequence of code points (in other words, conforming ECMAScript implementations are only required to do bitwise comparison on identifiers). The intent is that the incoming source text has been converted to normalised form C before it reaches the compiler.
Syntax
Syntax
Syntax
Syntax
Semantics
The value of the null literal null is the sole value of the Null type, namely null.
Syntax
Semantics
The value of the Boolean literal true is a value of the Boolean type, namely true.
The value of the Boolean literal false is a value of the Boolean type, namely false.
Syntax
The source character 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.
Semantics
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 (in the sense defined in 8.5), 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
A string literal is zero or more characters enclosed in single or double quotes. Each character may be represented by an escape sequence.
Syntax
The definitions of the nonterminal HexDigit is given in section 7.8.3. SourceCharacter is described in sections 2 and 6.
A string literal stands for a value of the String type. The string value (SV) of the literal is described in terms of character values (CV) contributed by the various parts of the string literal. As part of this process, some characters within the string literal are interpreted as having a mathematical value (MV), as described below or in section 7.8.3.
| Escape Sequence | Code Point Value | Name | Symbol |
|---|---|---|---|
| \b | \u0008 | backspace | <BS> |
| \t | \u0009 | horizontal tab | <HT> |
| \n | \u000A | line feed (new line) | <LF> |
| \v | \u000B | vertical tab | <VT> |
| \f | \u000C | form feed | <FF> |
| \r | \u000D | carriage return | <CR> |
| \" | \u0022 | double quote | " |
| \' | \u0027 | single quote | ' |
| \\ | \u005C | backslash | \ |
NOTE
A 'LineTerminator' character cannot appear in a string literal, even if
preceded by a backslash \. The correct
way to cause a line terminator character to be part of the string value
of a string literal is to use an escape sequence such as
\n or \u000A.
A regular expression literal is an input element that is converted to a RegExp object (section 15.10) when it is scanned. The object is created before evaluation of the containing program or function begins. Evaluation of the literal produces a reference to that object; it does not create a new object. 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 (section 15.10.4) or calling the RegExp constructor as a function (section 15.10.3).
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 strings of characters comprising the RegularExpressionBody and the RegularExpressionFlags are passed uninterpreted to the regular expression constructor, which interprets them according to its own, more stringent grammar. An implementation may extend the regular expression constructor's grammar, but it should not extend the RegularExpressionBody and RegularExpressionFlags productions or the productions used by these productions.
Syntax
NOTE
Regular expression literals may not be empty; instead of representing
an empty regular expression literal, the characters
// start a single-line comment. To specify an
empty regular expression, use /(?:)/.
Semantics
A regular expression literal stands for a value of the Object type. This value is determined in two steps: first, the characters comprising the regular expression's RegularExpressionBody and RegularExpressionFlags production expansions are collected uninterpreted into two strings Pattern and Flags, respectively. Then the new RegExp constructor is called with two arguments Pattern and Flags and the result becomes the value of the RegularExpressionLiteral. If the call to new RegExp generates an error, an implementation may, at its discretion, either report the error immediately while scanning the program, or it may defer the error until the regular expression literal is evaluated in the course of program execution.
Certain ECMAScript statements (empty statement, variable statement, expression statement, do-while statement, continue statement, break statement, return statement, and throw statement) must be terminated with semicolons. 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.
1. The offending token is separated from the previous token by at least one LineTerminator.
2. The offending token is }.
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 (section 12.6.3).
NOTE
These are the only restricted productions in the grammar:
The practical effect of these restricted productions is as follows:
The resulting practical advice to ECMAScript programmers is:
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
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
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 parenthesised 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.