Intent to prototype: Well-Formed Unicode Strings

[Yoshi Cheng-Hao Huang]

Summary:

Add String.prototype.isWellFormed and String.prototype.toWellFormed methods.

isWellFormed() returns a boolean to indicate whether the string is a well-formed UTF-16.

toWellFormed() will return a string with the all unpaired surrogate code points converted to U+FFFD (REPLACEMENT CHARACTER)

This is currently at stage 3 of TC39 proposals.

Bug:

https://bugzilla.mozilla.org/show_bug.cgi?id=1803523

Standards body:

https://tc39.es/proposal-is-usv-string/

Platform coverage:

All

Preference:

N/A yet

Other browsers:

Chrome has shipped this since 111

https://chromestatus.com/feature/5200195346759680

flag:--harmony-string-is-well-formed

which is enabled by default

Safari: https://bugs.webkit.org/show_bug.cgi?id=248588

This feature has shipped this in Safari Technology Preview 160

https://webkit.org/blog/13639/release-notes-for-safari-technology-preview-160/

flag: useStringWellFormed

which is enabled by default

web-platform-tests:

Tests are located in in test262/

https://github.com/tc39/test262/tree/main/test/built-ins/String/prototype/toWellFormed

https://github.com/tc39/test262/tree/main/test/built-ins/String/prototype/isWellFormed