String.prototype.lastIndexOf() - JavaScript 编辑

The lastIndexOf() method returns the index within the calling String object of the last occurrence of the specified value, searching backwards from fromIndex. Returns -1 if the value is not found.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

str.lastIndexOf(searchValue[, fromIndex])

Parameters

searchValue

A string representing the value to search for. If searchValue is an empty string, then fromIndex is returned.

fromIndex Optional

The index of the last character in the string to be considered as the beginning of a match. The default value is +Infinity. If fromIndex >= str.length, the whole string is searched. If fromIndex < 0, the behavior will be the same as if it would be 0.

Return value

The index of the last occurrence of searchValue; -1 if not found.

Description

Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character is str.length - 1.

'canal'.lastIndexOf('a');     // returns 3
'canal'.lastIndexOf('a', 2);  // returns 1
'canal'.lastIndexOf('a', 0);  // returns -1
'canal'.lastIndexOf('x');     // returns -1
'canal'.lastIndexOf('c', -5); // returns 0
'canal'.lastIndexOf('c', 0);  // returns 0
'canal'.lastIndexOf('');      // returns 5
'canal'.lastIndexOf('', 2);   // returns 2

Note: 'abab'.lastIndexOf('ab', 2) will return 2 and not 0, as fromIndex limits only the beginning of the match.

Case-sensitivity

The lastIndexOf() method is case sensitive. For example, the following expression returns -1:

'Blue Whale, Killer Whale'.lastIndexOf('blue'); // returns -1

Examples

Using indexOf() and lastIndexOf()

The following example uses indexOf() and lastIndexOf() to locate values in the string "Brave new world".

let anyString = 'Brave new world';

console.log('The index of the first w from the beginning is ' + anyString.indexOf('w'));
// logs 8
console.log('The index of the first w from the end is ' + anyString.lastIndexOf('w'));
// logs 10
console.log('The index of "new" from the beginning is ' + anyString.indexOf('new'));
// logs 6
console.log('The index of "new" from the end is ' + anyString.lastIndexOf('new'));
// logs 6

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'String.prototype.lastIndexOf' in that specification.

Browser compatibility

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The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

See also

• String.prototype.charAt()
• String.prototype.indexOf()
• String.prototype.split()
• Array.prototype.indexOf()
• Array.prototype.lastIndexOf()