Why would you use Array.some() or Array.every() over Array.filter()

Yesterday, one of my students asked me why you would ever need Array.some() or Array.every() when Array.filter() exists.

If you’re not familiar with these methods yet:

1. Array.some() returns true if at least one item in your array matches some criteria you specify as part of a callback function.
2. Array.every() returns true if all items in your array matches some criteria you specify as part of a callback function.
3. Array.filter() returns a new array of items, removing any from the original array that don’t match some criteria you specify as part of a callback function.

In most cases, if I want I’m looking to see if some items meet some criteria, I then want to do something with them.

I wouldn’t use Array.some(). I’d use Array.filter(), check that it has at least one item using the .length property, and then do something with those items.

var fruits = ['apples', 'oranges', 'pears', 'apples', 'bananas'];

// Get a new array with only apples
var apples = fruits.filter(function (fruit) {
	return fruit === 'apples';
});

// If there were any apples, count them
if (apples.length > 0) {
	alert('You have ' + apples.length + ' apples!');
}

So, when would you ever need Array.some() or Array.every()?

Let’s say you have an array of items in a cart, and if one item in particular is there, you want to apply a discount. You don’t need to filter down the whole array… just apply a discount if that one item is there. You’re still going to use all items in the original array.

This is the perfect use-case for Array.some()!

var cart = [
	{
		item: 'apples',
		price: 1
	},
	{
		item: 'pears',
		price: 0.5
	},
	{
		item: 'bread',
		price: 3
	}
];

// Check if the cart has pears in it
var hasPears = cart.some(function (item) {
	return item.item === 'pears';
});

// If it does, set a discount of 20%
var discount = hasPears ? 0.2 : 0;