Template literals should allow the embedding of languages (DSLs etc.). But restrictions on escape sequences make this problematic.
For example, consider making a latex processor with templates:
function latex(strings) {
// ...
}
let document = latex`
\newcommand{\fun}{\textbf{Fun!}} // works just fine
\newcommand{\unicode}{\textbf{Unicode!}} // Illegal token!
\newcommand{\xerxes}{\textbf{King!}} // Illegal token!
Breve over the h goes \u{h}ere // Illegal token!
`The problem here is that \u is the start of a unicode escape, but ES grammar forces it to be of the form \u00FF or \u{42}
and considers the token \unicode illegal.
Similarly \x is the start of a hex escape like \xFF but \xerxes is illegal. Octal literal escapes have the same problem; \0100 is illegal.
Remove the restriction on escape sequences.
Lifting the restriction raises the question of how to handle cooked template values that contain illegal escape sequences. Currently, cooked template values are supposed to replace escape sequences with the "Unicode code point represented by the escape sequence" but this can't happen if the escape sequence is not valid.
The proposed solution is to set the cooked value to undefined for template values that contain illegal escape sequences. The raw value is still accessible via .raw so embedded DSLs that might contain undefined cooked values can just use the raw string:
function tag(strs) {
strs[0] === undefined
strs.raw[0] === "\\unicode and \\u{55}";
}
tag`\unicode and \u{55}`This loosening of the escape sequence restriction only applies to tagged template literals; untagged templates still throw an
let bad = `bad escape sequence: \unicode`; // throws early errorA conforming implementation must not use the extended definition of
A template literal component is interpreted as a sequence of Unicode code points. The Template Value (TV) of a literal component is described in terms of code unit values (SV,
TV excludes the code units of
© 2016 Tim Disney
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