JavaScript (ES2026) reference · String Methods

JavaScript String toWellFormed() Method

Return a copy where lone surrogates are replaced with U+FFFD, so the string can be encoded safely.

JavaScript (ES2026) Since ES2024

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Syntax

string.toWellFormed()

A new, well-formed string.

Fix a broken stringTry it yourself

const bad = "ab\uD800";
console.log(bad.isWellFormed(), bad.toWellFormed().isWellFormed());

Output

false true

Avoid URIError when encodingTry it yourself

try {
  encodeURIComponent("\uD800");
} catch (e) {
  console.log(e.name);
}
console.log(encodeURIComponent("\uD800".toWellFormed()));

Output

URIError
%EF%BF%BD

Version	Change
JavaScript ES2024	Added

string.isWellFormed() · Return true if the string has no lone surrogates, i.e. it can be safely encoded as UTF-8.
string.normalize() · Return the Unicode normalization form of a string so visually identical text compares equal (NFC, NFD, NFKC, NFKD).
JavaScript Strings · tutorial
JavaScript String Methods · tutorial