String.prototype.isWellFormed()

The isWellFormed() method of String values returns a boolean indicating whether this string contains any lone surrogates.

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.

Syntax example: isWellFormed()