String.prototype.toWellFormed()

Baseline 2023

Newly available

Since October 2023, this feature works across the latest devices and browser versions. This feature might not work in older devices or browsers.

The toWellFormed() method of String values returns a string where all lone surrogates of this string are replaced with the Unicode replacement character U+FFFD.

Syntax

js
toWellFormed()

Parameters

None.

Return value

A new string that is a copy of this string, with all lone surrogates replaced with the Unicode replacement character U+FFFD. If str is well formed, a new string is still returned (essentially a copy of str).

Description

Strings in JavaScript are UTF-16 encoded. UTF-16 encoding has the concept of surrogate pairs, which is introduced in detail in the UTF-16 characters, Unicode code points, and grapheme clusters section.

toWellFormed() iterates through the code units of this string, and replaces any lone surrogates with the Unicode replacement character U+FFFD �. This ensures that the returned string is well-formed and can be used in functions that expect well-formed strings, such as encodeURI. Compared to a custom implementation, toWellFormed() is more efficient, as engines can directly access the internal representation of strings.

When ill-formed strings are used in certain contexts, such as TextEncoder, they are automatically converted to well-formed strings using the same replacement character. When lone surrogates are rendered, they are also rendered as the replacement character (a diamond with a question mark inside).

Examples

Using toWellFormed()

js
const strings = [
  // Lone leading surrogate
  "ab\uD800",
  "ab\uD800c",
  // Lone trailing surrogate
  "\uDFFFab",
  "c\uDFFFab",
  // Well-formed
  "abc",
  "ab\uD83D\uDE04c",
];

for (const str of strings) {
  console.log(str.toWellFormed());
}
// Logs:
// "ab�"
// "ab�c"
// "�ab"
// "c�ab"
// "abc"
// "ab😄c"

Avoiding errors in encodeURI()

encodeURI throws an error if the string passed is not well-formed. This can be avoided by using toWellFormed() to convert the string to a well-formed string first.

js
const illFormed = "https://example.com/search?q=\uD800";

try {
  encodeURI(illFormed);
} catch (e) {
  console.log(e); // URIError: URI malformed
}

console.log(encodeURI(illFormed.toWellFormed())); // "https://example.com/search?q=%EF%BF%BD"

Specifications

Specification
ECMAScript Language Specification
# sec-string.prototype.towellformed

Browser compatibility

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See also