The lastIndexOf()
method returns the last index at which a given element can be found in the array, or -1 if it is not present. The array is searched backwards, starting at fromIndex
.
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
arr.lastIndexOf(searchElement[, fromIndex])
Parameters
searchElement
- Element to locate in the array.
fromIndex
Optional- The index at which to start searching backwards. Defaults to the array's length minus one (
arr.length - 1
), i.e. the whole array will be searched. If the index is greater than or equal to the length of the array, the whole array will be searched. If negative, it is taken as the offset from the end of the array. Note that even when the index is negative, the array is still searched from back to front. If the calculated index is less than 0, -1 is returned, i.e. the array will not be searched.
Return value
The last index of the element in the array; -1 if not found.
Description
lastIndexOf
compares searchElement
to elements of the Array using strict equality (the same method used by the ===, or triple-equals, operator).
Polyfill
lastIndexOf
was added to the ECMA-262 standard in the 5th edition; as such it may not be present in other implementations of the standard. You can work around this by inserting the following code at the beginning of your scripts, allowing use of lastIndexOf
in implementations which do not natively support it. This algorithm is exactly the one specified in ECMA-262, 5th edition, assuming Object
, TypeError
, Number
, Math.floor
, Math.abs
, and Math.min
have their original values.
// Production steps of ECMA-262, Edition 5, 15.4.4.15 // Reference: http://es5.github.io/#x15.4.4.15 if (!Array.prototype.lastIndexOf) { Array.prototype.lastIndexOf = function(searchElement /*, fromIndex*/) { 'use strict'; if (this === void 0 || this === null) { throw new TypeError(); } var n, k, t = Object(this), len = t.length >>> 0; if (len === 0) { return -1; } n = len - 1; if (arguments.length > 1) { n = Number(arguments[1]); if (n != n) { n = 0; } else if (n != 0 && n != (1 / 0) && n != -(1 / 0)) { n = (n > 0 || -1) * Math.floor(Math.abs(n)); } } for (k = n >= 0 ? Math.min(n, len - 1) : len - Math.abs(n); k >= 0; k--) { if (k in t && t[k] === searchElement) { return k; } } return -1; }; }
Again, note that this implementation aims for absolute compatibility with lastIndexOf
in Firefox and the SpiderMonkey JavaScript engine, including in several cases which are arguably edge cases. If you intend to use this in real-world applications, you may be able to calculate from
with less complicated code if you ignore those cases.
Examples
Using lastIndexOf
The following example uses lastIndexOf
to locate values in an array.
var numbers = [2, 5, 9, 2]; numbers.lastIndexOf(2); // 3 numbers.lastIndexOf(7); // -1 numbers.lastIndexOf(2, 3); // 3 numbers.lastIndexOf(2, 2); // 0 numbers.lastIndexOf(2, -2); // 0 numbers.lastIndexOf(2, -1); // 3
Finding all the occurrences of an element
The following example uses lastIndexOf
to find all the indices of an element in a given array, using push
to add them to another array as they are found.
var indices = []; var array = ['a', 'b', 'a', 'c', 'a', 'd']; var element = 'a'; var idx = array.lastIndexOf(element); while (idx != -1) { indices.push(idx); idx = (idx > 0 ? array.lastIndexOf(element, idx - 1) : -1); } console.log(indices); // [4, 2, 0]
Note that we have to handle the case idx == 0
separately here because the element will always be found regardless of the fromIndex
parameter if it is the first element of the array. This is different from the indexOf
method.
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Array.prototype.lastIndexOf' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
lastIndexOf | Chrome Full support 1 | Edge Full support 12 | Firefox Full support 1.5 | IE Full support 9 | Opera Full support 9.5 | Safari Full support 3 | WebView Android Full support ≤37 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support 10.1 | Safari iOS Full support 1 | Samsung Internet Android Full support 1.0 | nodejs Full support 0.1.100 |
Legend
- Full support
- Full support