Array.prototype.find() - JavaScript 编辑

The find() method returns the value of the first element in the provided array that satisfies the provided testing function. If no values satisfies the testing function, undefined is returned.

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.

• If you need the index of the found element in the array, use findIndex().
• If you need to find the index of a value, use Array.prototype.indexOf(). (It’s similar to findIndex(), but checks each element for equality with the value instead of using a testing function.)
• If you need to find if a value exists in an array, use Array.prototype.includes(). Again, it checks each element for equality with the value instead of using a testing function.
• If you need to find if any element satisfies the provided testing function, use Array.prototype.some().

Syntax

arr.find(callback(element[, index[, array]])[, thisArg])

Parameters

callback

Function to execute on each value in the array, taking 3 arguments:

element

The current element in the array.

index Optional

The index (position) of the current element in the array.

array Optional

The array that find was called on.

thisArg Optional

Object to use as this inside callback.

Return value

The value of the first element in the array that satisfies the provided testing function. Otherwise, undefined is returned.

Description

The find method executes the callback function once for each index of the array until the callback returns a truthy value. If so, find immediately returns the value of that element. Otherwise, find returns undefined.

callback is invoked for every index of the array, not just those with assigned values. This means it may be less efficient for sparse arrays, compared to methods that only visit assigned values.

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

The find method does not mutate the array on which it is called, but the function provided to callback can. If so, the elements processed by find are set before the first invocation of callback. Therefore:

• callback will not visit any elements added to the array after the call to find begins.
• If an existing, yet-unvisited element of the array is changed by callback, its value passed to the callback will be the value at the time find visits that element's index.
• Elements that are deleted are still visited.

Polyfill

This method has been added to the ECMAScript 2015 specification and may not be available in all JavaScript implementations yet. However, you can polyfill Array.prototype.find with the following snippet:

// https://tc39.github.io/ecma262/#sec-array.prototype.find
if (!Array.prototype.find) {
  Object.defineProperty(Array.prototype, 'find', {
    value: function(predicate) {
      // 1. Let O be ? ToObject(this value).
      if (this == null) {
        throw 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 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, « kValue, k, O »)).
        // d. If testResult is true, return kValue.
        var kValue = o[k];
        if (predicate.call(thisArg, kValue, k, o)) {
          return kValue;
        }
        // e. Increase k by 1.
        k++;
      }

      // 7. Return undefined.
      return undefined;
    },
    configurable: true,
    writable: true
  });
}

If you need to support truly obsolete JavaScript engines that don't support Object.defineProperty, it is best not to polyfill Array.prototype at all, as you cannot make it non-enumerable.

Examples

Find an object in an array by one of its properties

const inventory = [
  {name: 'apples', quantity: 2},
  {name: 'bananas', quantity: 0},
  {name: 'cherries', quantity: 5}
];

function isCherries(fruit) {
  return fruit.name === 'cherries';
}

console.log(inventory.find(isCherries));
// { name: 'cherries', quantity: 5 }

Using arrow function and destructuring

const inventory = [
  {name: 'apples', quantity: 2},
  {name: 'bananas', quantity: 0},
  {name: 'cherries', quantity: 5}
];

const result = inventory.find( ({ name }) => name === 'cherries' );

console.log(result) // { name: 'cherries', quantity: 5 }

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) {
  let 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

The following examples show that nonexistent and deleted elements are visited, and that the value passed to the callback is their value when visited:

// Declare array with no elements at indexes 2, 3, and 4
const array = [0,1,,,,5,6];

// Shows all indexes, not just those with assigned values
array.find(function(value, index) {
  console.log('Visited index ', index, ' with value ', value);
});

// Shows all indexes, including deleted
array.find(function(value, index) {
  // Delete element 5 on first iteration
  if (index === 0) {
    console.log('Deleting array[5] with value ', array[5]);
    delete array[5];
  }
  // Element 5 is still visited even though deleted
  console.log('Visited index ', index, ' with value ', value);
});
// expected output:
// Deleting array[5] with value 5
// Visited index 0 with value 0
// Visited index 1 with value 1
// Visited index 2 with value undefined
// Visited index 3 with value undefined
// Visited index 4 with value undefined
// Visited index 5 with value undefined
// Visited index 6 with value 6

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Array.prototype.findIndex() – find and return an index
• Array.prototype.includes() – test whether a value exists in the array
• Array.prototype.filter() – remove all non-matching elements
• Array.prototype.every() – test all elements
• Array.prototype.some() – test until one element matches