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    Array.prototype.findIndex()

    This is an experimental technology, part of the Harmony (ECMAScript 6) 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.

    Summary

    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.

    Syntax

    arr.findIndex(callback[, thisArg])

    Parameters

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

    Description

    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 callbackIf 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.

    Examples

    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
    

    Polyfill

    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) {
      Object.defineProperty(Array.prototype, 'findIndex', {
        enumerable: false,
        configurable: true,
        writable: true,
        value: function(predicate) {
          if (this == null) {
            throw new TypeError('Array.prototype.find 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++) {
            if (i in list) {
              value = list[i];
              if (predicate.call(thisArg, value, i, list)) {
                return i;
              }
            }
          }
          return -1;
        }
      });
    }

    Specifications

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

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support Not supported 25.0 (25.0) Not supported Not supported Not supported
    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 Not supported

    See also

    Document Tags and Contributors

    Contributors to this page: bbecquet, rwaldron, miql, fscholz
    Last updated by: bbecquet,
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