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19. Well-formed JSON.stringify

This chapter covers the ES2019 feature “Well-formed JSON.stringify” by Richard Gibson.

According to the RFC for JSON, if you exchange JSON “in public”, you must encode it as UTF-8. That can be a problem if you use JSON.stringify(), because it may return sequences of UTF-16 code units that can’t be encoded as UTF-8.

How can that happen? If a JavaScript string contains a lone surrogate (a JavaScript character in the range 0xD800–0xDFFF) then JSON.stringify() produces a string with a lone surrogate:

  1. assert.equal(JSON.stringify('\u{D800}'), '"\u{D800}"');

Lone UTF-16 surrogates cannot be encoded as UTF-8, which is why this proposal changes JSON.stringify() so that it represents them via code unit escape sequences:

  1. assert.equal(JSON.stringify('\u{D800}'), '"\\ud800"');

Note: JSON supports code unit escape sequences (e.g. \uD800), but not code point escape sequences (e.g. \u{D800}).