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Showing content from http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/toWellFormed below:

String.prototype.toWellFormed() - JavaScript | MDN

Baseline 2023

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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.

None.

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).

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).

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"

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.

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"

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