Array.prototype.find()

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该特性处于 ECMAScript 6 规范草案中,目前的实现在未来可能会发生微调,请谨慎使用。

Summary

find()方法返回数组中满足测试条件的一个元素,如果没有满足条件的元素,则返回undefined

区别:findIndex()方法返回的是满足条件的元素的索引,而非它的值。

Syntax

arr.find(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 find was called upon.
thisArg
Object to use as this when executing callback.

Description

The find 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, find immediately returns the value of that element. Otherwise, find returns undefined. 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 find, it will be used as the this for each invocation of the callback. If it is not provided, then undefined is used.

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

The range of elements processed by find is set before the first invocation of callback. Elements that are appended to the array after the call to find 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 find visits that element's index; elements that are deleted are not visited.

Examples

Example: Find a prime number in an array

The following example finds an element in the array that is a prime number (or returns undefined 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].find(isPrime) ); // undefined, not found
console.log( [4, 5, 8, 12].find(isPrime) ); // 5

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.find with the following snippet:

if (!Array.prototype.find) {
  Object.defineProperty(Array.prototype, 'find', {
    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 value;
          }
        }
      }
      return undefined;
    }
  });
}
 

Specifications

Specification Status Comment
ECMAScript 6 (ECMA-262)
Array.prototype.find
Release Candidate Initial definition.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 未实现 25.0 (25.0) 未实现 未实现 未实现
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support 未实现 未实现 25.0 (25.0) 未实现 未实现 未实现

See also

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