JavaScript String isWellFormed() Method (original) (raw)

Last Updated : 17 Jul, 2023

**JavaScript String.isWellFormed() method is used to check if the string contains a lone surrogate or not. A lone surrogate is a string that contains Unicode characters in the range 0xD800–0xDBFF and 0xDC00–0xDFFF. It returns trues if the lone surrogate is not present. If the string is not well-formed it will give an error if we use the encodeURI() method on it.

**Syntax:

isWellFormed()

**Parameters: It does not take any parameter

**Return Value: A boolean, true if no lone surrogate is present, and false if the lone surrogate is present.

**Example 1: This example uses the isWellFormed() method on strings.

JavaScript `

const str1 = "ab\uD800"; const str2 = "new\uD800"; const str3 = "hello";

console.log(str1.isWellFormed()); console.log(str2.isWellFormed()); console.log(str3.isWellFormed());

`

**Output:

false
false
true

**Example 2: This example will perform the encodeURI function only on well-formed strings.

JavaScript `

const badFormed = "ab\uD800"; const wellFormed = "hello";

function checkFormation(str) { if(str.isWellFormed()){ console.log(encodeURI(str)) } } checkFormation(wellFormed); checkFormation(badFormed);

`

**Output:

hello

**Supported Browsers:

We have a complete list of Javascript string methods, to check those please go through this Javascript String reference article.