String.prototype..substring()

string.substring(start[, end])

Returns: string · Added in vES1 · Updated March 13, 2026 · String Methods

javascript string substring methods core

The substring() method returns a portion of a string between the start index and the character before the end index. It does not modify the original string—instead, it creates and returns a new string.

Syntax

str.substring(startIndex)
str.substring(startIndex, endIndex)

Parameters

Parameter	Type	Description
`startIndex`	`number`	The index where extraction begins. Characters at this index are included.
`endIndex`	`number`	Optional. The index where extraction ends. Characters at this index are excluded.

Return Value

A new string containing the extracted portion. If endIndex is omitted, extraction continues to the end of the string.

Description

The substring() method swaps its arguments if startIndex is greater than endIndex. It treats any value that is NaN or negative as 0. If either index exceeds the string length, they are treated as the string length.

Unlike slice(), substring() does not accept negative indices. For negative index support, use slice() instead.

Examples

Basic Usage

const str = "Hello, World!";

console.log(str.substring(0, 5));   // "Hello"
console.log(str.substring(7));      // "World!"
console.log(str.substring(7, 12));  // "World"

Swapping Arguments

If startIndex is greater than endIndex, the arguments are automatically swapped:

const str = "Hello";

console.log(str.substring(5, 0));  // "Hello" — same as substring(0, 5)
console.log(str.substring(2, 5)); // "llo"

Out of Bounds Indices

Indices beyond the string length are clamped:

const str = "Hello";

console.log(str.substring(0, 100)); // "Hello" — endIndex clamped to string length
console.log(str.substring(5));      // "" — empty string, startIndex equals length
console.log(str.substring(10));     // "" — empty string, startIndex exceeds length

Negative and NaN Indices

Negative indices and NaN are treated as 0:

const str = "Hello";

console.log(str.substring(-3, 2)); // "He" — -3 treated as 0
console.log(str.substring(2, NaN)); // "He" — NaN treated as 0
console.log(str.substring("abc"));  // "Hello" — NaN from string conversion

Extracting a Single Character

To extract a single character, use the same index for both parameters:

const str = "Hello";

console.log(str.substring(1, 2)); // "e"

Common Use Cases

Removing a Prefix or Suffix

const url = "https://example.com";
const domain = url.substring(8); // "example.com"

const text = "Hello, World!";
const trimmed = text.substring(0, 5); // "Hello"

Extracting Between Delimiters

const email = "user@example.com";
const username = email.substring(0, email.indexOf("@")); // "user"

Performance Notes

substring() is fast and widely supported across all JavaScript environments. For most substring extraction tasks, it gets the job done efficiently.