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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 7. 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. The Unicode identifier grammar is based on both normative and informative character categories specified by the Unicode Standard.

The characters in the specified categories in version 3. Unicode escape sequences are also permitted in an IdentifierName , where they contribute a single character to the IdentifierName , as computed by the CV of the UnicodeEscapeSequence see 7.

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A UnicodeEscapeSequence cannot be used to put a character into an IdentifierName that would otherwise be illegal. All interpretations of identifiers within this specification are based upon their actual characters regardless of whether or not an escape sequence was used to contribute any particular characters. Two IdentifierName that are canonically equivalent according to the Unicode standard are not equal unless they are represented by the exact same sequence of code units in other words, conforming ECMAScript implementations are only required to do bitwise comparison on IdentifierName values.

The intent is that the incoming source text has been converted to normalised form C before it reaches the compiler. If portability is a concern, programmers should only employ identifier characters defined in Unicode 3. 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. The following tokens are also considered to be FutureReservedWords when they occur within strict mode code see The occurrence of any of these tokens within strict mode code in any context where the occurrence of a FutureReservedWord would produce an error must also produce an equivalent error:.

The value of the null literal null is the sole value of the Null type, namely null.

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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. DecimalDigits opt ExponentPart opt. A numeric literal stands for a value of the Number type.

JavaScript language resources

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. The MV of DecimalLiteral Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type. A digit is significant if it is not part of an ExponentPart and.

A conforming implementation, when processing strict mode code see A string literal is zero or more characters enclosed in single or double quotes. Each character may be represented by an escape sequence. All characters may appear literally in a string literal except for the closing quote character, backslash, carriage return, line separator, paragraph separator, and line feed.

Any character may appear in the form of an escape sequence. SingleEscapeCharacter DecimalDigit x u.

What is the Java Language Specification?

The definitions of the nonterminal HexDigit is given in 7. SourceCharacter is defined in clause 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 7. The SV of StringLiteral :: "" is the empty character sequence.

The SV of StringLiteral :: '' is the empty character sequence. NOTE A line terminator character cannot appear in a string literal, except as part of a LineContinuation to produce the empty character sequence. A regular expression literal is an input element that is converted to a RegExp object see A RegExp object may also be created at runtime by new RegExp see 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 must not extend the RegularExpressionBody and RegularExpressionFlags productions or the productions used by these productions. A regular expression literal evaluates to a value of the Object type that is an instance of the standard built-in constructor RegExp.

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 each time the literal is evaluated, a new object is created as if by the expression new RegExp Pattern, Flags where RegExp is the standard built-in constructor with that name. The newly constructed object becomes the value of the RegularExpressionLiteral.

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If the call to new RegExp would generate an error as specified in 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. When, as the program 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. When, as the program 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 complete ECMAScript Program , then a semicolon is automatically inserted at the end of the input stream.

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 When a continue , break , return , or throw token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue , break , return , or throw token.

How to Read the ECMAScript Specification

An Expression in a return or throw statement should start on the same line as the return or throw token. A Identifier in a break or continue statement should be on the same line as the break or continue token. In contrast, the source. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement. 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.

Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation.

Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables. The Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value has the value undefined. The Boolean type represents a logical entity having two values, called true and false. The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a code unit value see Clause 6.

Each element is regarded as occupying a position within the sequence. These positions are indexed with nonnegative integers.

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The first element if any is at position 0, the next element if any at position 1, and so on. The length of a String is the number of elements i. The empty String has length zero and therefore contains no elements. When a String contains actual textual data, each element is considered to be a single UTF code unit.

Whether or not this is the actual storage format of a String, the characters within a String are numbered by their initial code unit element position as though they were represented using UTF All operations on Strings except as otherwise stated treat them as sequences of undifferentiated bit unsigned integers; they do not ensure the resulting String is in normalised form, nor do they ensure language-sensitive results.

NOTE The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. The intent is that textual data coming into the execution environment from outside e.

What's New in JavaScript for 12222

Usually this would occur at the same time incoming text is converted from its original character encoding to Unicode and would impose no additional overhead. Since it is recommended that ECMAScript source code be in Normalised Form C, string literals are guaranteed to be normalised if source text is guaranteed to be normalised , as long as they do not contain any Unicode escape sequences. Note that the NaN value is produced by the program expression NaN. In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.

There are two other special values, called positive Infinity and negative Infinity. Half of these are positive numbers and half are negative numbers; for every finite positive Number value there is a corresponding negative value having the same magnitude. Note that there is both a positive zero and a negative zero. A finite number has an odd significand if it is nonzero and the integer m used to express it in one of the two forms shown above is odd.

Otherwise, it has an even significand. Choose the member of this set that is closest in value to x.