Syntax

Parameters

None.

Return value

Returns true if this string does not contain any lone surrogates, false otherwise.

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.

isWellFormed() allows you to test whether a string is well-formed (i.e., does not contain any lone surrogates). Compared to a custom implementation, isWellFormed() is more efficient, as engines can directly access the internal representation of strings. If you need to convert a string to a well-formed string, use the toWellFormed() method. isWellFormed() allows you to handle ill-formed strings differently from well-formed strings, such as throwing an error or marking it as invalid.

Examples

Using isWellFormed()

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.isWellFormed());
}
// Logs:
// false
// false
// false
// false
// true
// true

Avoiding errors in encodeURI()

encodeURI throws an error if the string passed is not well-formed. This can be avoided by using isWellFormed() to test the string before passing it to encodeURI().

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

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

if (illFormed.isWellFormed()) {
  console.log(encodeURI(illFormed));
} else {
  console.warn("Ill-formed strings encountered."); // Ill-formed strings encountered.
}

Specifications

Browser compatibility

See also

• Polyfill of String.prototype.isWellFormed in core-js
• es-shims polyfill of String.prototype.isWellFormed
• String.prototype.toWellFormed()
• String.prototype.normalize()