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The below excerpts refer to ECMAScript 2017.

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. .... Any code points may appear in the form of an escape sequence.

11.8.4 String Literals, Syntax

Nonterminal symbol EscapeSequence has the following lexical grammar production:

EscapeSequence :: CharacterEscapeSequence 0 [lookahead ∉ DecimalDigit] HexEscapeSequence UnicodeEscapeSequence

Nonterminal symbol CharacterEscapeSequence has the following lexical grammar production:

CharacterEscapeSequence :: SingleEscapeCharacter NonEscapeCharacter

11.8.4.3 Static Semantics: SV

Contains descriptions such as:

The SV of DoubleStringCharacter :: \ EscapeSequence is the SV of the EscapeSequence

Questions

  1. What is meant by escape sequence in Note 1? Trying to understand what an escape sequence actually does, rather than just the lexical grammar for it
  2. Why does CharacterEscapeSequence include NonEscapeCharacter?
  3. The descriptions in 11.8.4.3 Static Semantics: SV do not seem to follow the normal ECMAScript convention for lexical grammar productions. What is meant by those descriptions?
  4. Added question: Does Note 1 state that code points can be within quotes or alternatively after an escape sequence (such as backslash)? Is that what is meant by Any code points may appear in the form of an escape sequence?
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  1. What is meant by escape sequence in Note 1?

    The EscapeSequence from your next question.

  2. Why does CharacterEscapeSequence include NonEscapeCharacter?

    Because invalid escapes just have their backslash ignored – for example, '\c' === 'c'. Backwards compatibility can't be broken.

  3. 11.8.4.3 Static Semantics: SV contains descriptions such as “The SV of DoubleStringCharacter :: \ EscapeSequence is the SV of the EscapeSequence”. Those lines do not follow the normal ECMAScript convention for lexical grammar productions. What is meant by those descriptions?

    It means that you should refer to the rule in the same section corresponding to the EscapeSequence. For example, if you had "\x20", the \x20 would be a DoubleStringCharacter consisting of \ and the EscapeSequence x20, which in turn is a HexEscapeSequence x HexDigit HexDigit, whose SV is given by

    The SV of HexEscapeSequence :: x HexDigit HexDigit is the code unit value that is (16 times the MV of the first HexDigit) plus the MV of the second HexDigit.

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10 Comments

Thank you, Ryan. Under Q3, is the spec saying that a lexical grammar production DoubleStringCharacter :: \ EscapeSequence has a string value? I am having a bit of trouble grasping what it is saying. Also, 11.8.4.2 seems to define StringValue as a StringLiteral. It says something I don't get either: Return the String value whose elements are the SV of this StringLiteral.. Any thoughts there? Thanks again.
Just as a heads up: I added some clarifications and an extra question to the OP.
@Magnus: Yes, it’s saying DoubleStringCharacter has a string value determined by what it matched. The string value of a literal is the string (the primitive type), “String value”, with elements determined by the concatenation of the SVs of the parts of the literal.
@Magnus: And as for question 4: yes, it means that "a" (containing the code point a) can also be represented by an escape sequence as "\x61".
Thanks again, Ryan. What happens if I have the string literal "abc\u000Axyz"? According to 10.1 escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator. However, 11.8.4. (and your x61 example) seem to indicate that the escape sequence will be interpreted.
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