Learn the JavaScript Array.every() and Array.some() Methods

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Array.every() and Array.some() are handy JavaScript array methods that can help you test an array against specified criteria. In this post, we'll quickly learn how to use them.

Video Tutorial

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Array.every()

Array.every takes a callback function as an argument. If the function returns true for each item in the array, Array.every returns true. Let's check it out.

function test(el) {
  return el < 10;
}

[1, 2, 3, 4, 5, 6].every(test);
// true

Since every item in the array is less than 10, the Array.every method returns true.

It's common to pass an anonymous arrow function directly to the every method, so let's do that to see a more familiar syntax:

[1, 2, 3, 4, 5, 6].every(el => el < 10);
// true

Let's see another example. This time it'll fail because not every element will pass the test.

[1, 2, 3, 4, 5, 6].every(el => el < 5);
// false

One nice thing that happens here is that, not only does it fail, but it exits early as soon as an element fails the test. That means it won't even run the test on the last element of the array.

Array.some()

The Array.some method tests to see if at least one element of an array passes the test. This time, we'll start with a failing example:

[1, 2, 3, 4, 5, 6].some(el => el > 10);
// false

Since none of the elements are greater than 10, Array.some returns false after testing each element.

Now a scenario that returns true:

[1, 2, 3, 4, 5, 6].some(el => el > 3);
// true

Not only does it return true, it returns true as soon as the first element passes the test. In this cases, since the number 4 passes the test, 5 and 6 aren't even tested.

Other Functionality

Now that we generally know how the every and some methods work, here are some additional functionality you can get out of them:

The Array.every and Array.some can take a second argument that will be used as this in the execution of the callback function.
The callback function can take two additional arguments: the current item's index and a reference to the array upon which the method was called.

Let's cram all these additional features into an example:

const obj = { name: "Daffodil" };

[1, 2, 3, 4, 5, 6].every(function(el, i, arr) {
  return el > i && arr[i] === el && this === obj;
}, obj);
// true

So why is this true?

Each element is greater than its index
arr[i] (where i is the current index` is identical to the current element)
Since we provided a reference to obj as the this argument, this is equal to obj