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