B Additional ECMAScript Features for Web Browsers

The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web browser. The content of this annex is normative but optional if the ECMAScript host is not a web browser.

Note

This annex describes various legacy features and other characteristics of web browser ECMAScript hosts. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex define the requirements for interoperable implementations of these legacy features.

These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.

B.1 Additional Syntax

B.1.1 HTML-like Comments

The syntax and semantics of 12.4 is extended as follows except that this extension is not allowed when parsing source text using the goal symbol Module:

Syntax

Comment :: MultiLineComment SingleLineComment SingleLineHTMLOpenComment SingleLineHTMLCloseComment SingleLineDelimitedComment MultiLineComment :: /* FirstCommentLineopt LineTerminator MultiLineCommentCharsopt */ HTMLCloseCommentopt FirstCommentLine :: SingleLineDelimitedCommentChars SingleLineHTMLOpenComment :: <!-- SingleLineCommentCharsopt SingleLineHTMLCloseComment :: LineTerminatorSequence HTMLCloseComment SingleLineDelimitedComment :: /* SingleLineDelimitedCommentCharsopt */ HTMLCloseComment :: WhiteSpaceSequenceopt SingleLineDelimitedCommentSequenceopt --> SingleLineCommentCharsopt SingleLineDelimitedCommentChars :: SingleLineNotAsteriskChar SingleLineDelimitedCommentCharsopt * SingleLinePostAsteriskCommentCharsopt SingleLineNotAsteriskChar :: SourceCharacter but not one of * or LineTerminator SingleLinePostAsteriskCommentChars :: SingleLineNotForwardSlashOrAsteriskChar SingleLineDelimitedCommentCharsopt * SingleLinePostAsteriskCommentCharsopt SingleLineNotForwardSlashOrAsteriskChar :: SourceCharacter but not one of / or * or LineTerminator WhiteSpaceSequence :: WhiteSpace WhiteSpaceSequenceopt SingleLineDelimitedCommentSequence :: SingleLineDelimitedComment WhiteSpaceSequenceopt SingleLineDelimitedCommentSequenceopt

Similar to a MultiLineComment that contains a line terminator code point, a SingleLineHTMLCloseComment is considered to be a LineTerminator for purposes of parsing by the syntactic grammar.

B.1.2 Regular Expressions Patterns

The syntax of 22.2.1 is modified and extended as follows. These changes introduce ambiguities that are broken by the ordering of grammar productions and by contextual information. When parsing using the following grammar, each alternative is considered only if previous production alternatives do not match.

This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The following grammar extensions include productions parameterized with the [UnicodeMode] parameter. However, none of these extensions change the syntax of Unicode patterns recognized when parsing with the [UnicodeMode] parameter present on the goal symbol.

Syntax

Term[UnicodeMode, N] :: [+UnicodeMode]Assertion[+UnicodeMode, ?N] [+UnicodeMode] Atom[+UnicodeMode, ?N] Quantifier [+UnicodeMode]Atom[+UnicodeMode, ?N] [~UnicodeMode] QuantifiableAssertion[?N] Quantifier [~UnicodeMode]Assertion[~UnicodeMode, ?N] [~UnicodeMode] ExtendedAtom[?N] Quantifier [~UnicodeMode]ExtendedAtom[?N] Assertion[UnicodeMode, N] :: ^ $ \ b \ B [+UnicodeMode] ( ? = Disjunction[+UnicodeMode, ?N] ) [+UnicodeMode] ( ? ! Disjunction[+UnicodeMode, ?N] ) [~UnicodeMode]QuantifiableAssertion[?N] ( ? <= Disjunction[?UnicodeMode, ?N] ) ( ? <! Disjunction[?UnicodeMode, ?N] ) QuantifiableAssertion[N] :: ( ? = Disjunction[~UnicodeMode, ?N] ) ( ? ! Disjunction[~UnicodeMode, ?N] ) ExtendedAtom[N] :: . \ AtomEscape[~UnicodeMode, ?N] \ [lookahead = c] CharacterClass[~UnicodeMode] ( Disjunction[~UnicodeMode, ?N] ) ( ? : Disjunction[~UnicodeMode, ?N] ) InvalidBracedQuantifier ExtendedPatternCharacter InvalidBracedQuantifier :: { DecimalDigits[~Sep] } { DecimalDigits[~Sep] , } { DecimalDigits[~Sep] , DecimalDigits[~Sep] } ExtendedPatternCharacter :: SourceCharacter but not one of ^ $ \ . * + ? ( ) [ | AtomEscape[UnicodeMode, N] :: [+UnicodeMode]DecimalEscape [~UnicodeMode] DecimalEscape but only if the CapturingGroupNumber of DecimalEscape is ≤ NcapturingParens CharacterClassEscape[?UnicodeMode] CharacterEscape[?UnicodeMode, ?N] [+N] k GroupName[?UnicodeMode] CharacterEscape[UnicodeMode, N] :: ControlEscape c ControlLetter 0 [lookahead ∉ DecimalDigit] HexEscapeSequence RegExpUnicodeEscapeSequence[?UnicodeMode] [~UnicodeMode]LegacyOctalEscapeSequence IdentityEscape[?UnicodeMode, ?N] IdentityEscape[UnicodeMode, N] :: [+UnicodeMode]SyntaxCharacter [+UnicodeMode]/ [~UnicodeMode]SourceCharacterIdentityEscape[?N] SourceCharacterIdentityEscape[N] :: [~N]SourceCharacter but not c [+N]SourceCharacter but not one of c or k ClassAtomNoDash[UnicodeMode, N] :: SourceCharacter but not one of \ or ] or - \ ClassEscape[?UnicodeMode, ?N] \ [lookahead = c] ClassEscape[UnicodeMode, N] :: b [+UnicodeMode]- [~UnicodeMode] c ClassControlLetter CharacterClassEscape[?UnicodeMode] CharacterEscape[?UnicodeMode, ?N] ClassControlLetter :: DecimalDigit _ Note

When the same left-hand sides occurs with both [+UnicodeMode] and [~UnicodeMode] guards it is to control the disambiguation priority.

B.1.2.1 Static Semantics: Early Errors

The semantics of 22.2.1.1 is extended as follows:

ExtendedAtom :: InvalidBracedQuantifier
  • It is a Syntax Error if any source text is matched by this production.

Additionally, the rules for the following productions are modified with the addition of the highlighted text:

NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges

B.1.2.2 Static Semantics: IsCharacterClass

The semantics of 22.2.1.3 is extended as follows:

ClassAtomNoDash :: \ [lookahead = c]
  1. Return false.

B.1.2.3 Static Semantics: CharacterValue

The semantics of 22.2.1.4 is extended as follows:

ClassAtomNoDash :: \ [lookahead = c]
  1. Return the code point value of U+005C (REVERSE SOLIDUS).
ClassEscape :: c ClassControlLetter
  1. Let ch be the code point matched by ClassControlLetter.
  2. Let i be ch's code point value.
  3. Return the remainder of dividing i by 32.
CharacterEscape :: LegacyOctalEscapeSequence
  1. Return the MV of LegacyOctalEscapeSequence (see 12.8.4.3).

B.1.2.4 Runtime Semantics: CompileSubpattern

The semantics of CompileSubpattern is extended as follows:

Within the rule for Term :: Atom Quantifier , references to “ Atom :: ( GroupSpecifier Disjunction ) ” are to be interpreted as meaning “ Atom :: ( GroupSpecifier Disjunction ) ” or “ ExtendedAtom :: ( Disjunction ) ”.

The rule for Term :: QuantifiableAssertion Quantifier is the same as for Term :: Atom Quantifier but with QuantifiableAssertion substituted for Atom.

The rule for Term :: ExtendedAtom Quantifier is the same as for Term :: Atom Quantifier but with ExtendedAtom substituted for Atom.

The rule for Term :: ExtendedAtom is the same as for Term :: Atom but with ExtendedAtom substituted for Atom.

B.1.2.5 Runtime Semantics: CompileAssertion

CompileAssertion rules for the Assertion :: ( ? = Disjunction ) and Assertion :: ( ? ! Disjunction ) productions are also used for the QuantifiableAssertion productions, but with QuantifiableAssertion substituted for Assertion.

B.1.2.6 Runtime Semantics: CompileAtom

CompileAtom rules for the Atom productions except for Atom :: PatternCharacter are also used for the ExtendedAtom productions, but with ExtendedAtom substituted for Atom. The following rules, with parameter direction, are also added:

ExtendedAtom :: \ [lookahead = c]
  1. Let A be the CharSet containing the single character \ U+005C (REVERSE SOLIDUS).
  2. Return ! CharacterSetMatcher(A, false, direction).
ExtendedAtom :: ExtendedPatternCharacter
  1. Let ch be the character represented by ExtendedPatternCharacter.
  2. Let A be a one-element CharSet containing the character ch.
  3. Return ! CharacterSetMatcher(A, false, direction).

B.1.2.7 Runtime Semantics: CompileToCharSet

The semantics of 22.2.2.9 is extended as follows:

The following two rules replace the corresponding rules of CompileToCharSet.

NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges
  1. Let A be CompileToCharSet of the first ClassAtom.
  2. Let B be CompileToCharSet of the second ClassAtom.
  3. Let C be CompileToCharSet of ClassRanges.
  4. Let D be ! CharacterRangeOrUnion(A, B).
  5. Return the union of D and C.
NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges
  1. Let A be CompileToCharSet of ClassAtomNoDash.
  2. Let B be CompileToCharSet of ClassAtom.
  3. Let C be CompileToCharSet of ClassRanges.
  4. Let D be ! CharacterRangeOrUnion(A, B).
  5. Return the union of D and C.

In addition, the following rules are added to CompileToCharSet.

ClassEscape :: c ClassControlLetter
  1. Let cv be the CharacterValue of this ClassEscape.
  2. Let c be the character whose character value is cv.
  3. Return the CharSet containing the single character c.
ClassAtomNoDash :: \ [lookahead = c]
  1. Return the CharSet containing the single character \ U+005C (REVERSE SOLIDUS).
Note
This production can only be reached from the sequence \c within a character class where it is not followed by an acceptable control character.

B.1.2.7.1 CharacterRangeOrUnion ( A, B )

The abstract operation CharacterRangeOrUnion takes arguments A (a CharSet) and B (a CharSet). It performs the following steps when called:

  1. If Unicode is false, then
    1. If A does not contain exactly one character or B does not contain exactly one character, then
      1. Let C be the CharSet containing the single character - U+002D (HYPHEN-MINUS).
      2. Return the union of CharSets A, B and C.
  2. Return ! CharacterRange(A, B).

B.2 Additional Built-in Properties

When the ECMAScript host is a web browser the following additional properties of the standard built-in objects are defined.

B.2.1 Additional Properties of the Global Object

The entries in Table 94 are added to Table 8.

Table 94: Additional Well-known Intrinsic Objects
Intrinsic Name Global Name ECMAScript Language Association
%escape% escape The escape function (B.2.1.1)
%unescape% unescape The unescape function (B.2.1.2)

B.2.1.1 escape ( string )

The escape function is a property of the global object. It computes a new version of a String value in which certain code units have been replaced by a hexadecimal escape sequence.

For those code units being replaced whose value is 0x00FF or less, a two-digit escape sequence of the form %xx is used. For those characters being replaced whose code unit value is greater than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.

The escape function is the %escape% intrinsic object. When the escape function is called with one argument string, the following steps are taken:

  1. Set string to ? ToString(string).
  2. Let length be the number of code units in string.
  3. Let R be the empty String.
  4. Let k be 0.
  5. Repeat, while k < length,
    1. Let char be the code unit (represented as a 16-bit unsigned integer) at index k within string.
    2. If char is one of the code units in "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./", then
      1. Let S be the String value containing the single code unit char.
    3. Else if char ≥ 256, then
      1. Let n be the numeric value of char.
      2. Let S be the string-concatenation of:
        • "%u"
        • the String representation of n, formatted as a four-digit uppercase hexadecimal number, padded to the left with zeroes if necessary
    4. Else,
      1. Assert: char < 256.
      2. Let n be the numeric value of char.
      3. Let S be the string-concatenation of:
        • "%"
        • the String representation of n, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary
    5. Set R to the string-concatenation of R and S.
    6. Set k to k + 1.
  6. Return R.
Note

The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC 3986.

B.2.1.2 unescape ( string )

The unescape function is a property of the global object. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by the escape function is replaced with the code unit that it represents.

The unescape function is the %unescape% intrinsic object. When the unescape function is called with one argument string, the following steps are taken:

  1. Set string to ? ToString(string).
  2. Let length be the number of code units in string.
  3. Let R be the empty String.
  4. Let k be 0.
  5. Repeat, while klength,
    1. Let c be the code unit at index k within string.
    2. If c is the code unit 0x0025 (PERCENT SIGN), then
      1. Let hexEscape be the empty String.
      2. Let skip be 0.
      3. If klength - 6 and the code unit at index k + 1 within string is the code unit 0x0075 (LATIN SMALL LETTER U), then
        1. Set hexEscape to the substring of string from k + 2 to k + 6.
        2. Set skip to 5.
      4. Else if klength - 3, then
        1. Set hexEscape to the substring of string from k + 1 to k + 3.
        2. Set skip to 2.
      5. If hexEscape can be interpreted as an expansion of HexDigits[~Sep], then
        1. Let hexIntegerLiteral be the string-concatenation of "0x" and hexEscape.
        2. Let n be ! ToNumber(hexIntegerLiteral).
        3. Set c to the code unit whose value is (n).
        4. Set k to k + skip.
    3. Set R to the string-concatenation of R and c.
    4. Set k to k + 1.
  6. Return R.

B.2.2 Additional Properties of the String.prototype Object

B.2.2.1 String.prototype.substr ( start, length )

The substr method takes two arguments, start and length, and returns a substring of the result of converting the this value to a String, starting from index start and running for length code units (or through the end of the String if length is undefined). If start is negative, it is treated as sourceLength + start where sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:

  1. Let O be ? RequireObjectCoercible(this value).
  2. Let S be ? ToString(O).
  3. Let size be the length of S.
  4. Let intStart be ? ToIntegerOrInfinity(start).
  5. If intStart is -∞, set intStart to 0.
  6. Else if intStart < 0, set intStart to max(size + intStart, 0).
  7. If length is undefined, let intLength be size; otherwise let intLength be ? ToIntegerOrInfinity(length).
  8. If intStart is +∞, intLength ≤ 0, or intLength is +∞, return the empty String.
  9. Let intEnd be min(intStart + intLength, size).
  10. If intStartintEnd, return the empty String.
  11. Return the substring of S from intStart to intEnd.
Note

The substr function is intentionally generic; it does not require that its this value be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

B.2.2.2 String.prototype.anchor ( name )

When the anchor method is called with argument name, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "a", "name", name).

B.2.2.2.1 CreateHTML ( string, tag, attribute, value )

The abstract operation CreateHTML takes arguments string, tag (a String), attribute (a String), and value. It performs the following steps when called:

  1. Let str be ? RequireObjectCoercible(string).
  2. Let S be ? ToString(str).
  3. Let p1 be the string-concatenation of "<" and tag.
  4. If attribute is not the empty String, then
    1. Let V be ? ToString(value).
    2. Let escapedV be the String value that is the same as V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the six code unit sequence "&quot;".
    3. Set p1 to the string-concatenation of:
      • p1
      • the code unit 0x0020 (SPACE)
      • attribute
      • the code unit 0x003D (EQUALS SIGN)
      • the code unit 0x0022 (QUOTATION MARK)
      • escapedV
      • the code unit 0x0022 (QUOTATION MARK)
  5. Let p2 be the string-concatenation of p1 and ">".
  6. Let p3 be the string-concatenation of p2 and S.
  7. Let p4 be the string-concatenation of p3, "</", tag, and ">".
  8. Return p4.

B.2.2.3 String.prototype.big ( )

When the big method is called with no arguments, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "big", "", "").

B.2.2.4 String.prototype.blink ( )

When the blink method is called with no arguments, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "blink", "", "").

B.2.2.5 String.prototype.bold ( )

When the bold method is called with no arguments, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "b", "", "").

B.2.2.6 String.prototype.fixed ( )

When the fixed method is called with no arguments, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "tt", "", "").

B.2.2.7 String.prototype.fontcolor ( color )

When the fontcolor method is called with argument color, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "font", "color", color).

B.2.2.8 String.prototype.fontsize ( size )

When the fontsize method is called with argument size, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "font", "size", size).

B.2.2.9 String.prototype.italics ( )

When the italics method is called with no arguments, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "i", "", "").

B.2.2.10 String.prototype.link ( url )

When the link method is called with argument url, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "a", "href", url).

B.2.2.11 String.prototype.small ( )

When the small method is called with no arguments, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "small", "", "").

B.2.2.12 String.prototype.strike ( )

When the strike method is called with no arguments, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "strike", "", "").

B.2.2.13 String.prototype.sub ( )

When the sub method is called with no arguments, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "sub", "", "").

B.2.2.14 String.prototype.sup ( )

When the sup method is called with no arguments, the following steps are taken:

  1. Let S be the this value.
  2. Return ? CreateHTML(S, "sup", "", "").

B.2.2.15 String.prototype.trimLeft ( )

Note

The property "trimStart" is preferred. The "trimLeft" property is provided principally for compatibility with old code. It is recommended that the "trimStart" property be used in new ECMAScript code.

The initial value of the "trimLeft" property is %String.prototype.trimStart%, defined in 22.1.3.32.

B.2.2.16 String.prototype.trimRight ( )

Note

The property "trimEnd" is preferred. The "trimRight" property is provided principally for compatibility with old code. It is recommended that the "trimEnd" property be used in new ECMAScript code.

The initial value of the "trimRight" property is %String.prototype.trimEnd%, defined in 22.1.3.31.

B.2.3 Additional Properties of the Date.prototype Object

B.2.3.1 Date.prototype.getYear ( )

Note

The getFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

When the getYear method is called with no arguments, the following steps are taken:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, return NaN.
  3. Return YearFromTime(LocalTime(t)) - 1900𝔽.

B.2.3.2 Date.prototype.setYear ( year )

Note

The setFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

When the setYear method is called with one argument year, the following steps are taken:

  1. Let t be ? thisTimeValue(this value).
  2. If t is NaN, set t to +0𝔽; otherwise, set t to LocalTime(t).
  3. Let y be ? ToNumber(year).
  4. If y is NaN, then
    1. Set the [[DateValue]] internal slot of this Date object to NaN.
    2. Return NaN.
  5. Let yi be ! ToIntegerOrInfinity(y).
  6. If 0 ≤ yi ≤ 99, let yyyy be 1900𝔽 + 𝔽(yi).
  7. Else, let yyyy be y.
  8. Let d be MakeDay(yyyy, MonthFromTime(t), DateFromTime(t)).
  9. Let date be UTC(MakeDate(d, TimeWithinDay(t))).
  10. Set the [[DateValue]] internal slot of this Date object to TimeClip(date).
  11. Return the value of the [[DateValue]] internal slot of this Date object.

B.2.3.3 Date.prototype.toGMTString ( )

Note

The toUTCString method is preferred. The toGMTString method is provided principally for compatibility with old code.

The initial value of the "toGMTString" property is %Date.prototype.toUTCString%, defined in 21.4.4.43.

B.2.4 Additional Properties of the RegExp.prototype Object

B.2.4.1 RegExp.prototype.compile ( pattern, flags )

When the compile method is called with arguments pattern and flags, the following steps are taken:

  1. Let O be the this value.
  2. Perform ? RequireInternalSlot(O, [[RegExpMatcher]]).
  3. If Type(pattern) is Object and pattern has a [[RegExpMatcher]] internal slot, then
    1. If flags is not undefined, throw a TypeError exception.
    2. Let P be pattern.[[OriginalSource]].
    3. Let F be pattern.[[OriginalFlags]].
  4. Else,
    1. Let P be pattern.
    2. Let F be flags.
  5. Return ? RegExpInitialize(O, P, F).
Note

The compile method completely reinitializes the this value RegExp with a new pattern and flags. An implementation may interpret use of this method as an assertion that the resulting RegExp object will be used multiple times and hence is a candidate for extra optimization.

B.3 Other Additional Features

B.3.1 Labelled Function Declarations

Prior to ECMAScript 2015, the specification of LabelledStatement did not allow for the association of a statement label with a FunctionDeclaration. However, a labelled FunctionDeclaration was an allowable extension for non-strict code and most browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015 and later, the grammar production for LabelledStatement permits use of FunctionDeclaration as a LabelledItem but 14.13.1 includes an Early Error rule that produces a Syntax Error if that occurs. That rule is modified with the addition of the highlighted text:

LabelledItem : FunctionDeclaration
  • It is a Syntax Error if any source text that is strict mode code is matched by this production.
Note

The early error rules for WithStatement, IfStatement, and IterationStatement prevent these statements from containing a labelled FunctionDeclaration in non-strict code.

B.3.2 Block-Level Function Declarations Web Legacy Compatibility Semantics

Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a FunctionDeclaration as an element of a Block statement's StatementList. However, support for that form of FunctionDeclaration was an allowable extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the semantics of such declarations differ among those implementations. Because of these semantic differences, existing web ECMAScript code that uses Block level function declarations is only portable among browser implementations if the usage only depends upon the semantic intersection of all of the browser implementations for such declarations. The following are the use cases that fall within that intersection semantics:

  1. A function is declared and only referenced within a single block.

    • One or more FunctionDeclarations whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
    • No other declaration of f that is not a var declaration occurs within the function code of g.
    • All occurrences of f as an IdentifierReference are within the StatementList of the Block containing the declaration of f.
  2. A function is declared and possibly used within a single Block but also referenced by an inner function definition that is not contained within that same Block.

    • One or more FunctionDeclarations whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
    • No other declaration of f that is not a var declaration occurs within the function code of g.
    • There may be occurrences of f as an IdentifierReference within the StatementList of the Block containing the declaration of f.
    • There is at least one occurrence of f as an IdentifierReference within another function h that is nested within g and no other declaration of f shadows the references to f from within h.
    • All invocations of h occur after the declaration of f has been evaluated.
  3. A function is declared and possibly used within a single block but also referenced within subsequent blocks.

    • One or more FunctionDeclaration whose BindingIdentifier is the name f occur within the function code of an enclosing function g and that declaration is nested within a Block.
    • No other declaration of f that is not a var declaration occurs within the function code of g.
    • There may be occurrences of f as an IdentifierReference within the StatementList of the Block containing the declaration of f.
    • There is at least one occurrence of f as an IdentifierReference within the function code of g that lexically follows the Block containing the declaration of f.

The first use case is interoperable with the semantics of Block level function declarations provided by ECMAScript 2015. Any pre-existing ECMAScript code that employs that use case will operate using the Block level function declarations semantics defined by clauses 10, 14, and 15.

ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause 10, clause 15, clause 19.2.1 and clause 16.1.7 semantics.

If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should be produced when code contains a FunctionDeclaration for which these compatibility semantics are applied and introduce observable differences from non-compatibility semantics. For example, if a var binding is not introduced because its introduction would create an early error, a warning message should not be produced.

B.3.2.1 Changes to FunctionDeclarationInstantiation

During FunctionDeclarationInstantiation the following steps are performed in place of step 29:

  1. If strict is false, then
    1. For each FunctionDeclaration f that is directly contained in the StatementList of a Block, CaseClause, or DefaultClause, do
      1. Let F be StringValue of the BindingIdentifier of f.
      2. If replacing the FunctionDeclaration f with a VariableStatement that has F as a BindingIdentifier would not produce any Early Errors for func and F is not an element of parameterNames, then
        1. NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName, the name of a formal parameter, or another FunctionDeclaration.
        2. If initializedBindings does not contain F and F is not "arguments", then
          1. Perform ! varEnv.CreateMutableBinding(F, false).
          2. Perform varEnv.InitializeBinding(F, undefined).
          3. Append F to instantiatedVarNames.
        3. When the FunctionDeclaration f is evaluated, perform the following steps in place of the FunctionDeclaration Evaluation algorithm provided in 15.2.6:
          1. Let fenv be the running execution context's VariableEnvironment.
          2. Let benv be the running execution context's LexicalEnvironment.
          3. Let fobj be ! benv.GetBindingValue(F, false).
          4. Perform ! fenv.SetMutableBinding(F, fobj, false).
          5. Return NormalCompletion(empty).

B.3.2.2 Changes to GlobalDeclarationInstantiation

During GlobalDeclarationInstantiation the following steps are performed in place of step 12:

  1. Let strict be IsStrict of script.
  2. If strict is false, then
    1. Let declaredFunctionOrVarNames be the list-concatenation of declaredFunctionNames and declaredVarNames.
    2. For each FunctionDeclaration f that is directly contained in the StatementList of a Block, CaseClause, or DefaultClause Contained within script, do
      1. Let F be StringValue of the BindingIdentifier of f.
      2. If replacing the FunctionDeclaration f with a VariableStatement that has F as a BindingIdentifier would not produce any Early Errors for script, then
        1. If env.HasLexicalDeclaration(F) is false, then
          1. Let fnDefinable be ? env.CanDeclareGlobalVar(F).
          2. If fnDefinable is true, then
            1. NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName nor the name of another FunctionDeclaration.
            2. If declaredFunctionOrVarNames does not contain F, then
              1. Perform ? env.CreateGlobalVarBinding(F, false).
              2. Append F to declaredFunctionOrVarNames.
            3. When the FunctionDeclaration f is evaluated, perform the following steps in place of the FunctionDeclaration Evaluation algorithm provided in 15.2.6:
              1. Let genv be the running execution context's VariableEnvironment.
              2. Let benv be the running execution context's LexicalEnvironment.
              3. Let fobj be ! benv.GetBindingValue(F, false).
              4. Perform ? genv.SetMutableBinding(F, fobj, false).
              5. Return NormalCompletion(empty).

B.3.2.3 Changes to EvalDeclarationInstantiation

During EvalDeclarationInstantiation the following steps are performed in place of step 11:

  1. If strict is false, then
    1. Let declaredFunctionOrVarNames be the list-concatenation of declaredFunctionNames and declaredVarNames.
    2. For each FunctionDeclaration f that is directly contained in the StatementList of a Block, CaseClause, or DefaultClause Contained within body, do
      1. Let F be StringValue of the BindingIdentifier of f.
      2. If replacing the FunctionDeclaration f with a VariableStatement that has F as a BindingIdentifier would not produce any Early Errors for body, then
        1. Let bindingExists be false.
        2. Let thisEnv be lexEnv.
        3. Assert: The following loop will terminate.
        4. Repeat, while thisEnv is not the same as varEnv,
          1. If thisEnv is not an object Environment Record, then
            1. If thisEnv.HasBinding(F) is true, then
              1. Let bindingExists be true.
          2. Set thisEnv to thisEnv.[[OuterEnv]].
        5. If bindingExists is false and varEnv is a global Environment Record, then
          1. If varEnv.HasLexicalDeclaration(F) is false, then
            1. Let fnDefinable be ? varEnv.CanDeclareGlobalVar(F).
          2. Else,
            1. Let fnDefinable be false.
        6. Else,
          1. Let fnDefinable be true.
        7. If bindingExists is false and fnDefinable is true, then
          1. If declaredFunctionOrVarNames does not contain F, then
            1. If varEnv is a global Environment Record, then
              1. Perform ? varEnv.CreateGlobalVarBinding(F, true).
            2. Else,
              1. Let bindingExists be varEnv.HasBinding(F).
              2. If bindingExists is false, then
                1. Perform ! varEnv.CreateMutableBinding(F, true).
                2. Perform ! varEnv.InitializeBinding(F, undefined).
            3. Append F to declaredFunctionOrVarNames.
          2. When the FunctionDeclaration f is evaluated, perform the following steps in place of the FunctionDeclaration Evaluation algorithm provided in 15.2.6:
            1. Let genv be the running execution context's VariableEnvironment.
            2. Let benv be the running execution context's LexicalEnvironment.
            3. Let fobj be ! benv.GetBindingValue(F, false).
            4. Perform ? genv.SetMutableBinding(F, fobj, false).
            5. Return NormalCompletion(empty).

B.3.2.4 Changes to Block Static Semantics: Early Errors

The rules for the following production in 14.2.1 are modified with the addition of the highlighted text:

Block : { StatementList }

B.3.2.5 Changes to switch Statement Static Semantics: Early Errors

The rules for the following production in 14.12.1 are modified with the addition of the highlighted text:

SwitchStatement : switch ( Expression ) CaseBlock

B.3.2.6 Changes to BlockDeclarationInstantiation

During BlockDeclarationInstantiation the following steps are performed in place of step 3.a.ii.1:

  1. If env.HasBinding(dn) is false, then
    1. Perform ! env.CreateMutableBinding(dn, false).

During BlockDeclarationInstantiation the following steps are performed in place of step 3.b.iii:

  1. If the binding for fn in env is an uninitialized binding, then
    1. Perform env.InitializeBinding(fn, fo).
  2. Else,
    1. Assert: d is a FunctionDeclaration.
    2. Perform env.SetMutableBinding(fn, fo, false).

B.3.3 FunctionDeclarations in IfStatement Statement Clauses

The following augments the IfStatement production in 14.6:

IfStatement[Yield, Await, Return] : if ( Expression[+In, ?Yield, ?Await] ) FunctionDeclaration[?Yield, ?Await, ~Default] else Statement[?Yield, ?Await, ?Return] if ( Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] else FunctionDeclaration[?Yield, ?Await, ~Default] if ( Expression[+In, ?Yield, ?Await] ) FunctionDeclaration[?Yield, ?Await, ~Default] else FunctionDeclaration[?Yield, ?Await, ~Default] if ( Expression[+In, ?Yield, ?Await] ) FunctionDeclaration[?Yield, ?Await, ~Default] [lookahead ≠ else]

This production only applies when parsing non-strict code. Source text matched by this production is processed as if each matching occurrence of FunctionDeclaration[?Yield, ?Await, ~Default] was the sole StatementListItem of a BlockStatement occupying that position in the source text. The semantics of such a synthetic BlockStatement includes the web legacy compatibility semantics specified in B.3.2.

B.3.4 VariableStatements in Catch Blocks

The content of subclause 14.15.1 is replaced with the following:

Catch : catch ( CatchParameter ) Block Note

The Block of a Catch clause may contain var declarations that bind a name that is also bound by the CatchParameter. At runtime, such bindings are instantiated in the VariableDeclarationEnvironment. They do not shadow the same-named bindings introduced by the CatchParameter and hence the Initializer for such var declarations will assign to the corresponding catch parameter rather than the var binding.

This modified behaviour also applies to var and function declarations introduced by direct eval calls contained within the Block of a Catch clause. This change is accomplished by modifying the algorithm of 19.2.1.3 as follows:

Step 3.d.i.2.a.i is replaced by:

  1. If thisEnv is not the Environment Record for a Catch clause, throw a SyntaxError exception.

Step 11.b.ii.4.a.i.i is replaced by:

  1. If thisEnv is not the Environment Record for a Catch clause, let bindingExists be true.

B.3.5 Initializers in ForIn Statement Heads

The following augments the ForInOfStatement production in 14.7.5:

ForInOfStatement[Yield, Await, Return] : for ( var BindingIdentifier[?Yield, ?Await] Initializer[~In, ?Yield, ?Await] in Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]

This production only applies when parsing non-strict code.

The static semantics of ContainsDuplicateLabels in 8.2.1 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Return ContainsDuplicateLabels of Statement with argument labelSet.

The static semantics of ContainsUndefinedBreakTarget in 8.2.2 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Return ContainsUndefinedBreakTarget of Statement with argument labelSet.

The static semantics of ContainsUndefinedContinueTarget in 8.2.3 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Return ContainsUndefinedContinueTarget of Statement with arguments iterationSet and « ».

The static semantics of IsDestructuring in 14.7.5.2 are augmented with the following:

BindingIdentifier : Identifier yield await
  1. Return false.

The static semantics of VarDeclaredNames in 8.1.6 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Let names1 be the BoundNames of BindingIdentifier.
  2. Let names2 be the VarDeclaredNames of Statement.
  3. Return the list-concatenation of names1 and names2.

The static semantics of VarScopedDeclarations in 8.1.7 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Let declarations1 be a List whose sole element is BindingIdentifier.
  2. Let declarations2 be the VarScopedDeclarations of Statement.
  3. Return the list-concatenation of declarations1 and declarations2.

The runtime semantics of ForInOfLoopEvaluation in 14.7.5.5 are augmented with the following:

ForInOfStatement : for ( var BindingIdentifier Initializer in Expression ) Statement
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId).
  3. If IsAnonymousFunctionDefinition(Initializer) is true, then
    1. Let value be NamedEvaluation of Initializer with argument bindingId.
  4. Else,
    1. Let rhs be the result of evaluating Initializer.
    2. Let value be ? GetValue(rhs).
  5. Perform ? PutValue(lhs, value).
  6. Let keyResult be ? ForIn/OfHeadEvaluation(« », Expression, enumerate).
  7. Return ? ForIn/OfBodyEvaluation(BindingIdentifier, Statement, keyResult, enumerate, varBinding, labelSet).

B.3.6 The [[IsHTMLDDA]] Internal Slot

An [[IsHTMLDDA]] internal slot may exist on host-defined objects. Objects with an [[IsHTMLDDA]] internal slot behave like undefined in the ToBoolean and IsLooselyEqual abstract operations and when used as an operand for the typeof operator.

Note

Objects with an [[IsHTMLDDA]] internal slot are never created by this specification. However, the document.all object in web browsers is a host-defined exotic object with this slot that exists for web compatibility purposes. There are no other known examples of this type of object and implementations should not create any with the exception of document.all.

B.3.6.1 Changes to ToBoolean

The result column in Table 14 for an argument type of Object is replaced with the following algorithm:

  1. If argument has an [[IsHTMLDDA]] internal slot, return false.
  2. Return true.

B.3.6.2 Changes to IsLooselyEqual

The following steps replace step 4 of the IsLooselyEqual algorithm:

  1. If Type(x) is Object and x has an [[IsHTMLDDA]] internal slot and y is either null or undefined, return true.
  2. If x is either null or undefined and Type(y) is Object and y has an [[IsHTMLDDA]] internal slot, return true.

B.3.6.3 Changes to the typeof Operator

The following table entry is inserted into Table 42 immediately preceding the entry for "Object (implements [[Call]])":

Table 95: Additional typeof Operator Results
Type of val Result
Object (has an [[IsHTMLDDA]] internal slot) "undefined"