[RFC]: add `@stdlib/assert/is-well-formed-string`

[kgryte]

Description

This RFC proposes adding support for asserting whether an input value is a well-formed string (i.e. does not contain lone surrogates).

Package: @stdlib/assert/is-well-formed-string
Alias: isWellFormedString

Related Issues

None.

Questions

No.

Other

Resources:

• Lone surrogates: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String#utf-16_characters_unicode_code_points_and_grapheme_clusters
• isWellFormed: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/isWellFormed

Prior art in stdlib:

• @stdlib/assert/is-string
• @stdlib/assert/is-empty-string

Checklist

• I have read and understood the Code of Conduct.
• Searched for existing issues and pull requests.
• The issue name begins with RFC:.

[gunjjoshi]

I would like to work on this issue. Can you please assign this to me ?

[kgryte]

@gunjjoshi When working on this issue, I suggest first studying some of the implementations in @stdlib/string/base and also consulting the surrogate pair regexp in @stdlib/regexp.

[gunjjoshi]

@kgryte I would surely do that.
Also, I was thinking of checking 2 things -

1. Input value is a string, using is-string
2. Input value is well formed, using isWellFormed function

Is this approach correct ?

[kgryte]

The isWellFormed function is not universally supported. We typically roll our own implementations. If the built-in exists, you can defer to it, but if it is missing, we need to fallback to a functional polyfill.

[gunjjoshi]

Got it. I will try to implement it from scratch, after having a look at the packages you mentioned earlier.

[gunjjoshi]

@kgryte I found @stdlib/regexp/utf16-surrogate-pair to be somewhat useful for this.

Firstly we can check if an input string str contains any surrogate pairs by using reUtf16SurrogatePair(). Also, we will be checking each character individually in the string, based on the conditions for lone surrogates.

var reUtf16SurrogatePair = require('@stdlib/regexp/utf16-surrogate-pair');

var RE_UTF16_SURROGATE_PAIR = reUtf16SurrogatePair();

function containsLoneSurrogates(str) {
    var hasSurrogatePairs = RE_UTF16_SURROGATE_PAIR.test(str);
    if (hasSurrogatePairs) {
        return false;
    }
    for (var i = 0; i < str.length; i++) {
        var charCode = str.charCodeAt(i);
        if ((charCode >= 0xD800 && charCode <= 0xDBFF) || (charCode >= 0xDC00 && charCode <= 0xDFFF)) {
            return false;
        }
    }
    return true;
}

Is this approach correct ? Any suggestions ?