Array.prototype.every() - JavaScript 编辑

The every() method tests whether all elements in the array pass the test implemented by the provided function. It returns a Boolean value.

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.

Syntax

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

Parameters

callback

A function to test for each element, taking 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 every was called upon.

thisArg Optional

A value to use as this when executing callback.

Return value

true if the callback function returns a truthy value for every array element. Otherwise, false.

Description

The every method executes the provided callback function once for each element present in the array until it finds the one where callback returns a falsy value. If such an element is found, the every method immediately returns false. Otherwise, if callback returns a truthy value for all elements, every returns true.

Caution: Calling this method on an empty array will return true for any condition!

callback is invoked only for array indexes 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 every, it will be used as callback's this value. Otherwise, the value undefined will be used as its this value. The this value ultimately observable by callback is determined according to the usual rules for determining the this seen by a function.

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

The range of elements processed by every is set before the first invocation of callback. Therefore, callback will not run on elements that are appended to the array after the call to every begins. If existing elements of the array are changed, their value as passed to callback will be the value at the time every visits them. Elements that are deleted are not visited.

every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true. (It is vacuously true that all elements of the empty set satisfy any given condition.)

Polyfill

every was added to the ECMA-262 standard in the 5^th edition, and it may not be present in other implementations of the standard. You can work around this by inserting the following code at the beginning of your scripts, allowing use of every in implementations which do not natively support it.

This algorithm is exactly the one specified in ECMA-262, 5^th edition, assuming Object and TypeError have their original values, and that callbackfn.call evaluates to the original value of Function.prototype.call.

if (!Array.prototype.every) {
  Array.prototype.every = function(callbackfn, thisArg) {
    'use strict';
    var T, k;

    if (this == null) {
      throw new TypeError('this is null or not defined');
    }

    // 1. Let O be the result of calling ToObject passing the this
    //    value as the argument.
    var O = Object(this);

    // 2. Let lenValue be the result of calling the Get internal method
    //    of O with the argument "length".
    // 3. Let len be ToUint32(lenValue).
    var len = O.length >>> 0;

    // 4. If IsCallable(callbackfn) is false, throw a TypeError exception.
    if (typeof callbackfn !== 'function' && Object.prototype.toString.call(callbackfn) !== '[object Function]') {
      throw new TypeError();
    }

    // 5. If thisArg was supplied, let T be thisArg; else let T be undefined.
    if (arguments.length > 1) {
      T = thisArg;
    }

    // 6. Let k be 0.
    k = 0;

    // 7. Repeat, while k < len
    while (k < len) {

      var kValue;

      // a. Let Pk be ToString(k).
      //   This is implicit for LHS operands of the in operator
      // b. Let kPresent be the result of calling the HasProperty internal
      //    method of O with argument Pk.
      //   This step can be combined with c
      // c. If kPresent is true, then
      if (k in O) {
        var testResult;
        // i. Let kValue be the result of calling the Get internal method
        //    of O with argument Pk.
        kValue = O[k];

        // ii. Let testResult be the result of calling the Call internal method
        // of callbackfn with T as the this value if T is not undefined
        // else is the result of calling callbackfn
        // and argument list containing kValue, k, and O.
        if(T) testResult = callbackfn.call(T, kValue, k, O);
        else testResult = callbackfn(kValue,k,O)

        // iii. If ToBoolean(testResult) is false, return false.
        if (!testResult) {
          return false;
        }
      }
      k++;
    }
    return true;
  };
}

Examples

Testing size of all array elements

The following example tests whether all elements in the array are bigger than 10.

function isBigEnough(element, index, array) {
  return element >= 10;
}
[12, 5, 8, 130, 44].every(isBigEnough);   // false
[12, 54, 18, 130, 44].every(isBigEnough); // true

Using arrow functions

Arrow functions provide a shorter syntax for the same test.

[12, 5, 8, 130, 44].every(x => x >= 10);   // false
[12, 54, 18, 130, 44].every(x => x >= 10); // true

Affecting Initial Array (modifying, appending, and deleting)

The following examples tests the behaviour of the every method when the array is modified.

// ---------------
// Modifying items
// ---------------
let arr = [1, 2, 3, 4];
arr.every( (elem, index, arr) => {
  arr[index+1] -= 1
  console.log(`[${arr}][${index}] -> ${elem}`)
  return elem < 2
})

// Loop runs for 3 iterations, but would
// have run 2 iterations without any modification
//
// 1st iteration: [1,1,3,4][0] -> 1
// 2nd iteration: [1,1,2,4][1] -> 1
// 3rd iteration: [1,1,2,3][2] -> 2

// ---------------
// Appending items
// ---------------
arr = [1, 2, 3];
arr.every( (elem, index, arr) => {
  arr.push('new')
  console.log(`[${arr}][${index}] -> ${elem}`)
  return elem < 4
})

// Loop runs for 3 iterations, even after appending new items
//
// 1st iteration: [1, 2, 3, new][0] -> 1
// 2nd iteration: [1, 2, 3, new, new][1] -> 2
// 3rd iteration: [1, 2, 3, new, new, new][2] -> 3

// ---------------
// Deleting items
// ---------------
arr = [1, 2, 3, 4];
arr.every( (elem, index, arr) => {
  arr.pop()
  console.log(`[${arr}][${index}] -> ${elem}`)
  return elem < 4
})

// Loop runs for 2 iterations only, as the remaining
// items are `pop()`ed off
//
// 1st iteration: [1,2,3][0] -> 1
// 2nd iteration: [1,2][1] -> 2

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Array.prototype.forEach()
• Array.prototype.some()
• Array.prototype.find()
• TypedArray.prototype.every()