Array.prototype.every()

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

callbackFn

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 callbackFn.

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

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

callbackFn 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.

callbackFn 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 callbackFn. Therefore, callbackFn 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 callbackFn 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.)

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

The following example tests if all the elements of an array are present in another array.

function isSubset(array1, array2) {
  // returns true if array2 is a subset of array1

  return array2.every(function (element) {
    return array1.includes(element);
  });
}

console.log(isSubset([1, 2, 3, 4, 5, 6, 7], [5, 7, 6])); // true
console.log(isSubset([1, 2, 3, 4, 5, 6, 7], [5, 8, 7])); // false

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

The following examples tests the behavior 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

• A polyfill of Array.prototype.every is available in core-js
• Array.prototype.forEach()
• Array.prototype.some()
• Array.prototype.find()
• TypedArray.prototype.every()