The findIndex()
method returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1
, indicating that no element passed the test.
See also the find()
method, which returns the value of an array element, instead of its index.
Syntax
arr.findIndex(callback( element[, index[, array]] )[, thisArg])
Parameters
callback
-
A function to execute on each value in the array until the function returns
true
, indicating that the satisfying element was found.It takes three arguments:
element
- The current element being processed in the array.
index
Optional- The index of the current element being processed in the array.
array
Optional- The array
findIndex()
was called upon.
thisArg
Optional- Optional object to use as
this
when executingcallback
.
Return value
The index of the first element in the array that passes the test. Otherwise, -1
.
Description
The findIndex()
method executes the callback
function once for every index in the array until it finds the one where callback
returns a truthy value.
If such an element is found, findIndex()
immediately returns the element's index. If callback
never returns a truthy value (or the array's length
is 0
), findIndex()
returns -1
.
Edge case alert: Unlike other array methods such as Array.some()
, callback
is run even for indexes with unassigned values.
callback
is invoked with three arguments:
- The value of the element
- The index of the element
- The Array object being traversed
If a thisArg
parameter is passed to findIndex()
, it will be used as the this
inside each invocation of the callback
. If it is not provided, then undefined
is used.
The range of elements processed by findIndex()
is set before the first invocation of callback
. callback
will not process the elements appended to the array after the call to findIndex()
begins. If an existing, unvisited element of the array is changed by callback
, its value passed to the callback
will be the value at the time findIndex()
visits the element's index.
Elements that are deleted are still visited.
Polyfill
// https://tc39.github.io/ecma262/#sec-array.prototype.findindex if (!Array.prototype.findIndex) { Object.defineProperty(Array.prototype, 'findIndex', { value: function(predicate) { // 1. Let O be ? ToObject(this value). if (this == null) { throw new TypeError('"this" is null or not defined'); } var o = Object(this); // 2. Let len be ? ToLength(? Get(O, "length")). var len = o.length >>> 0; // 3. If IsCallable(predicate) is false, throw a TypeError exception. if (typeof predicate !== 'function') { throw new TypeError('predicate must be a function'); } // 4. If thisArg was supplied, let T be thisArg; else let T be undefined. var thisArg = arguments[1]; // 5. Let k be 0. var k = 0; // 6. Repeat, while k < len while (k < len) { // a. Let Pk be ! ToString(k). // b. Let kValue be ? Get(O, Pk). // c. Let testResult be ToBoolean(? Call(predicate, T, ยซ kValue, k, O ยป)). // d. If testResult is true, return k. var kValue = o[k]; if (predicate.call(thisArg, kValue, k, o)) { return k; } // e. Increase k by 1. k++; } // 7. Return -1. return -1; }, configurable: true, writable: true }); }
If you need to support truly obsolete JavaScript engines that do not support Object.defineProperty
, it is best not to polyfill Array.prototype
methods at all, as you cannot make them non-enumerable.
Examples
Find the index of a prime number in an array
The following example returns the index of the first element in the array that is a prime number, or -1
if there is no prime number.
function isPrime(num) { for (let i = 2; num > i; i++) { if (num % i == 0) { return false; } } return num > 1; } console.log([4, 6, 8, 9, 12].findIndex(isPrime)); // -1, not found console.log([4, 6, 7, 9, 12].findIndex(isPrime)); // 2 (array[2] is 7)
Find index using arrow function
The following example finds the index of a fruit using an arrow function:
const fruits = ["apple", "banana", "cantaloupe", "blueberries", "grapefruit"]; const index = fruits.findIndex(fruit => fruit === "blueberries"); console.log(index); // 3 console.log(fruits[index]); // blueberries
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Array.prototype.findIndex' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
findIndex | Chrome Full support 45 | Edge Full support 12 | Firefox Full support 25 | IE No support No | Opera Full support 32 | Safari Full support 8 | WebView Android Full support 45 | Chrome Android Full support 45 | Firefox Android Full support 4 | Opera Android Full support 32 | Safari iOS Full support 8 | Samsung Internet Android Full support 5.0 | nodejs
Full support
4.0.0
|
Legend
- Full support
- Full support
- No support
- No support
- User must explicitly enable this feature.
- User must explicitly enable this feature.