The substr() method of String values returns a portion of this string, starting at the specified index and extending for a given number of characters afterwards.

Try it

const str = "Mozilla";

console.log(str.substr(1, 2));
// Expected output: "oz"

console.log(str.substr(2));
// Expected output: "zilla"

Syntax

substr(start)
substr(start, length)

Parameters

start

The index of the first character to include in the returned substring.

length Optional

The number of characters to extract.

Return value

A new string containing the specified part of the given string.

Description

A string's substr() method extracts length characters from the string, counting from the start index.

• If start >= str.length, an empty string is returned.
• If start < 0, the index starts counting from the end of the string. More formally, in this case the substring starts at max(start + str.length, 0).
• If start is omitted or undefined, it's treated as 0.
• If length is omitted or undefined, or if start + length >= str.length, substr() extracts characters to the end of the string.
• If length < 0, an empty string is returned.
• For both start and length, NaN is treated as 0.

Although you are encouraged to avoid using substr(), there is no trivial way to migrate substr() to either slice() or substring() in legacy code without essentially writing a polyfill for substr(). For example, str.substr(a, l), str.slice(a, a + l), and str.substring(a, a + l) all have different results when str = "01234", a = 1, l = -2 — substr() returns an empty string, slice() returns "123", while substring() returns "0". The actual refactoring path depends on the knowledge of the range of a and l.

Examples

Using substr()

const string = "Mozilla";

console.log(string.substr(0, 1)); // 'M'
console.log(string.substr(1, 0)); // ''
console.log(string.substr(-1, 1)); // 'a'
console.log(string.substr(1, -1)); // ''
console.log(string.substr(-3)); // 'lla'
console.log(string.substr(1)); // 'ozilla'
console.log(string.substr(-20, 2)); // 'Mo'
console.log(string.substr(20, 2)); // ''

Specifications

Browser compatibility

See also

• Polyfill of String.prototype.substr in core-js
• es-shims polyfill of String.prototype.substr
• String.prototype.slice()
• String.prototype.substring()