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    This is an experimental technology, part of the ECMAScript 6 (Harmony) proposal.
    Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future version of browsers as the spec changes.


    The findIndex() method returns an index in the array, if an element in the array satisfies the provided testing function. Otherwise -1 is returned.

    See also the find() method, which returns the value of a found element in the array instead of its index.


    arr.findIndex(callback[, thisArg])


    Function to execute on each value in the array, taking three arguments:
    The current element being processed in the array.
    The index of the current element being processed in the array.
    The array findIndex was called upon.
    Optional. Object to use as this when executing callback.


    The findIndex method executes the callback function once for each element present in the array until it finds one where callback returns a true value. If such an element is found, findIndex immediately returns the index of that element. Otherwise, findIndex returns -1. callback is invoked only for indexes of the array which have assigned values; it is not invoked for indexes which have been deleted or which have never been assigned values.

    callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.

    If a thisArg parameter is provided to findIndex, it will be used as the this for each invocation of the callback. If it is not provided, then undefined is used.

    findIndex does not mutate the array on which it is called.

    The range of elements processed by findIndex is set before the first invocation of callback. Elements that are appended to the array after the call to findIndex begins will not be visited by callback. If an existing, unvisited element of the array is changed by callback, its value passed to the visiting callback will be the value at the time that findIndex visits that element's index; elements that are deleted are not visited.


    Example: Find the index of a prime number in an array

    The following example finds the index of an element in the array that is a prime number (or returns -1 if there is no prime number).

    function isPrime(element, index, array) {
      var start = 2;
      while (start <= Math.sqrt(element)) {
        if (element % start++ < 1) {
          return false;
      return element > 1;
    console.log([4, 6, 8, 12].findIndex(isPrime)); // -1, not found
    console.log([4, 6, 7, 12].findIndex(isPrime)); // 2


    This method has been added to the ECMAScript 6 specification and may not be available in all JavaScript implementations yet. However, you can polyfill Array.prototype.findIndex with the following snippet:

    if (!Array.prototype.findIndex) {
      Array.prototype.findIndex = function(predicate) {
        if (this == null) {
          throw new TypeError('Array.prototype.findIndex called on null or undefined');
        if (typeof predicate !== 'function') {
          throw new TypeError('predicate must be a function');
        var list = Object(this);
        var length = list.length >>> 0;
        var thisArg = arguments[1];
        var value;
        for (var i = 0; i < length; i++) {
          value = list[i];
          if (, value, i, list)) {
            return i;
        return -1;


    Specification Status Comment
    ECMAScript 6 (ECMA-262)
    The definition of 'Array.prototype.findIndex' in that specification.
    Release Candidate Initial definition.

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support Not supported 25.0 (25.0) Not supported Not supported 7.1
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support Not supported Not supported 25.0 (25.0) Not supported Not supported iOS 8

    See also

    Document Tags and Contributors

    Last updated by: javierbeaumont,
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