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    Lexical grammar

    This section describes JavaScript's lexical grammar. The source text of ECMAScript scripts gets scanned from left to right and is converted into a sequence of input elements which are tokens, control characters, line terminators, comments or white space. ECMAScript also defines certain keywords and literals and has rules for automatic insertion of semicolons to end statements.

    Control characters

    Control characters have no visual representation but are used to control the interpretation of the text.

    Unicode format-control characters
    Code point Name Abbreviation Description
    U+200C Zero width non-joiner <ZWNJ> Placed between characters to prevent being connected into ligatures in certain languages (Wikipedia).
    U+200D Zero width joiner <ZWJ> Placed between characters that would not normally be connected in order to cause the characters to be rendered using their connected form in certain languages (Wikipedia).
    U+FEFF Byte order mark <BOM> Used at the start of the script to mark it as Unicode and the text's byte order (Wikipedia).

    White space

    White space characters improve the readability of source text and separate tokens from each other. These characters are usually unnecessary for the functionality of the code. Minification tools are often used to remove whitespace in order to reduce the amount of data that needs to be transferred.

    White space characters
    Code point Name Abbreviation Description Escape sequence
    U+0009 Character tabulation <HT> Horizontal tabulation \t
    U+000B Line tabulation <VT> Vertical tabulation \v
    U+000C Form feed <FF> Page breaking control character (Wikipedia). \f
    U+0020 Space <SP> Normal space  
    U+00A0 No-break space <NBSP> Normal space, but no point at which a line may break  
    Others Other Unicode space characters <USP> Spaces in Unicode on Wikipedia  

    Line terminators

    In addition to white space characters, line terminator characters are used to improve the readability of the source text. However, in some cases, line terminators can influence the the execution of JavaScript code as there are a few places where they are forbidden. Line terminators also affect the process of automatic semicolon insertion. Line terminators are matched by the \s class in regular expressions.

    Only the following Unicode code points are treated as line terminators in ECMAScript, other line breaking characters are treated as white space (for example, Next Line, NEL, U+0085 is considered as white space).

    Line terminator characters
    Code point Name Abbreviation Description Escape sequence
    U+000A Line Feed <LF> New line character in UNIX systems. \n
    U+000D Carriage Return <CR> New line character in Commodore and early Mac systems. \r
    U+2028 Line Separator <LS> Wikipedia  
    U+2029 Paragraph Separator <PS> Wikipedia  

    Comments

    Comments are used to add hints, notes, suggestions, or warnings to JavaScript code. This can make it easier to read and understand. They can also be used to disable code to prevent it from being executed; this can be a valuable debugging tool.

    JavaScript has two ways of assigning comments in its code.

    The first way is the // comment; this makes all text following it on the same line into a comment. For example:

    function comment() {
      // This is a one line JavaScript comment
      console.log("Hello world!");
    }
    comment();
    

    The second way is the /* */ style, which is much more flexible.

    For example, you can use it on a single line:

    function comment() {
      /* This is a one line JavaScript comment */
      console.log("Hello world!");
    }
    comment();

    You can also make multiple-line comments, like this:

    function comment() {
      /* This comment spans multiple lines. Notice
         that we don't need to end the comment until we're done. */
      console.log("Hello world!");
    }
    comment();

    You can also use it in the middle of a line, if you wish, although this can make your code harder to read so it should be used with caution:

    function comment(x) {
      console.log("Hello " + x /* insert the value of x */ + " !");
    }
    comment("world");

    In addition, you can use it to disable code to prevent it from running, by wrapping code in a comment, like this:

    function comment() {
      /* console.log("Hello world!"); */
    }
    comment();

    In this case, the console.log() call is never issued, since it's inside a comment. Any number of lines of code can be disabled this way.

    Keywords

    Reserved keywords as of ECMAScript 6

    Future reserved keywords

    The following are reserved as future keywords by the ECMAScript specification. They have no special functionality at present, but they might at some future time, so they cannot be used as identifiers. These keywords may not be used in either strict or non-strict mode.

    • enum
    • await

    The following are reserved as future keywords when they are found in strict mode code:

    • implements
    • package
    • protected
    • static
    • interface
    • private
    • public

    Future reserved keywords in older standards

    The following are reserved as future keywords by older ECMAScript specifications (ECMAScript 1 till 3).

    • abstract
    • boolean
    • byte
    • char
    • double
    • final
    • float
    • goto
    • int
    • long
    • native
    • short
    • synchronized
    • transient
    • volatile

    Additionally, the literals null, true, and false are reserved in ECMAScript for their normal uses.

    Reserved word usage

    Reserved words actually only apply to Identifiers (vs. IdentifierNames) . As described in es5.github.com/#A.1, these are all IdentifierNames which do not exclude ReservedWords.

    a.import
    a["import"]
    a = { import: "test" }.
    

    On the other hand the following is illegal because it's an Identifier, which is an IdentifierName without the reserved words. Identifiers are used for FunctionDeclaration and FunctionExpression.

    function import() {} // Illegal.

    Literals

    Null literal

    See also null for more information.

    null

    Boolean literal

    See also Boolean for more information.

    true
    false

    Numeric literals

    Decimal

    1234567890
    42
    
    // Caution when using leading zeros:
    
    0888 // 888 parsed as decimal
    0777 // parsed as octal, 511 in decimal
    

    Note that decimal literals can start with a zero (0) followed by another decimal digit, but If the next digit after the leading 0 is smaller than 8, the number gets parsed as an octal number. This won't throw in JavaScript, see bug 957513. See also the page about parseInt().

    Binary

    Binary number syntax uses a leading zero followed by a lowercase or uppercase Latin letter "B" (0b or 0B). Because this syntax is new in ECMAScript 6, see the browser compatibility table, below. If the digits after the 0b are not 0 or 1, the following SyntaxError is thrown: "Missing binary digits after 0b".

    var FLT_SIGNBIT  = 0b10000000000000000000000000000000; // 2147483648
    var FLT_EXPONENT = 0b01111111100000000000000000000000; // 2139095040
    var FLT_MANTISSA = 0B00000000011111111111111111111111; // 8388607

    Octal

    Octal number syntax uses a leading zero followed by a lowercase or uppercase Latin letter "O" (0o or 0O). Because this syntax is new in ECMAScript 6, see the browser compatibility table, below. If the digits after the 0o are outside the range (01234567), the following SyntaxError is thrown:  "Missing octal digits after 0o".

    var n = 0O755; // 493
    var m = 0o644; // 420
    
    // Also possible with leading zeros (see note about decimals above)
    0755
    0644
    

    Hexadecimal

    Hexadecimal number syntax uses a leading zero followed by a lowercase or uppercase Latin letter "X" (0x or 0X). If the digits after 0x are outside the range (0123456789ABCDEF),  the following SyntaxError is thrown: "Identifier starts immediately after numeric literal".

    0xFFFFFFFFFFFFFFFFF // 295147905179352830000
    0x123456789ABCDEF   // 81985529216486900
    0XA                 // 10
    

    Object literals

    See also Object and Object initializer for more information.

    var o = { a: "foo", b: "bar", c: 42 };
    
    // shorthand notation. New in ES6
    var a = "foo", b = "bar", c = 42;
    var o = {a, b, c};
    // instead of
    var o = { a: a, b: b, c: c };
    

    Array literals

    See also Array for more information.

    [1954, 1974, 1990, 2014]

    String literals

    'foo'
    "bar"

    Hexadecimal escape sequences

    '\xA9' // "©"
    

    Unicode escape sequences

    The Unicode escape sequences require at least four characters following \u.

    '\u00A9' // "©"

    Unicode code point escapes

    New in ECMAScript 6. With Unicode code point escapes, any character can be escaped using hexadecimal numbers so that it is possible to use Unicode code points up to 0x10FFFF. With simple Unicode escapes it is often necessary to write the surrogate halves separately to achieve the same.

    See also String.fromCodePoint() or String.prototype.codePointAt().

    '\u{2F804}'
    
    // the same with simple Unicode escapes
    '\uD87E\uDC04'

    Regular expression literals

    See also RegExp for more information.

    /ab+c/g
    
    // An "empty" regular expression literal
    // The empty non-capturing group is necessary 
    // to avoid ambiguity with single-line comments.
    /(?:)/

    Template literals

    See also template strings for more information.

    `string text`
    
    `string text line 1
     string text line 2`
    
    `string text ${expression} string text`
    
    tag `string text ${expression} string text`

    Automatic semicolon insertion

    Some JavaScript statements must be terminated with semicolons and are therefore affected by automatic semicolon insertion (ASI):

    • Empty statement
    • let, const, variable statement
    • import, export, module declaration
    • Expression statement
    • debugger
    • continue, break, throw
    • return

    The ECMAScript specification mentions three rules of semicolon insertion.

    1.  A semicolon is inserted before, when a Line terminator or "}" is encountered that is not allowed by the grammar.

    { 1 2 } 3 
    
    // is transformed by ASI into 
    
    { 1 2 ;} 3;

    2.  A semicolon is inserted at the end, when the end of the input stream of tokens is detected and the the parser is unable to parse the single input stream as a complete program.

    Here ++ is not treated as a postfix operator applying to variable b, because a line terminator occurs between b and ++.

    a = b
    ++c
    
    // is transformend by ASI into
    
    a = b;
    ++c;
    

    3. A semicolon is inserted at the end, when a statement with restricted productions in the grammar is followed by a line terminator. These statements with "no LineTerminator here" rules are:

    • PostfixExpressions (++ and --)
    • continue
    • break
    • return
    • yield, yield*
    • module
    return
    a + b
    
    // is transformed by ASI into
    
    return;
    a + b;
    

    Specifications

    Specification Status Comment
    ECMAScript 1st Edition. Standard Initial definition.
    ECMAScript 5.1 (ECMA-262)
    The definition of 'Lexical Conventions' in that specification.
    Standard  
    ECMAScript 6 (ECMA-262)
    The definition of 'Lexical Grammar' in that specification.
    Release Candidate Added: Binary and Octal Numeric literals, Unicode code point escapes, Templates

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
    Binary and octal numeric literals
    (0b and 0o)
    41 25 (25) ? 28 ?
    Unicode code point escapes
    (\u{})
    ? Not supported
    bug 952985
    ? ? ?
    Shorthand notation for object literals Not supported 33 (33) Not supported Not supported Not supported
    Template literals Not supported 34 (34) Not supported Not supported Not supported
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
    Binary and octal numeric literals ? 41 33.0 (33) ? ? ?
    Unicode code point escapes ? ? Not supported
    bug 952985
    ? ? ?
    Shorthand notation for object literals Not supported Not supported 33.0 (33) Not supported Not supported Not supported
    Template literals Not supported Not supported 34.0 (34) Not supported Not supported Not supported

    Firefox-specific notes

    • Prior to Firefox 5 (JavaScript 1.8.6), future reserved keywords could be used when not in strict mode. This ECMAScript violation was fixed in Firefox 5.

    See also

    Document Tags and Contributors

    Last updated by: KiraAndMaxim,
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