Kelvin Wangonya

Posted on Aug 26, 2018 • Updated on Sep 3, 2019 • Originally published at wangonya.com

Javascript array iteration with 'some()' and 'every()'

#javascript #arrays #beginners #webdev

If you're using an array in your code, chances are, you'll need to iterate over the values in the array. There's a couple of ways you can do that, some better and more efficient than others depending on what you want to accomplish.

For this post, I'll focus on two ways: some() and every().

`some()`

The some() method tests whether at least one element in the array passes the test implemented by the provided function. It checks the elements one by one, and if it finds an array element where the function returns a truthy value, some() returns true and does not check the remaining values. Otherwise it returns false.

Lets say you want to check if a contact exists in your contact list:

const contacts = ['Stewie', 'Meg', 'Quagmire', 'Cleveland'];

function checkContacts(arr, val) {
  return arr.some(arrVal => val === arrVal);
}

checkContacts(contacts, 'Lois');   // false
checkContacts(contacts, 'Meg'); // true

When checking for Lois, some() checks the array elements beginning at Stewie to the end, and having not found a match, returns false. When checking for Meg, it stops at Meg and returns true, ignoring the rest of the elements.

`every()`

This method tests whether all elements in the array pass the test implemented by the provided function. It checks the elements one by one, and if it finds an array element where the function returns a falsy value, every() returns false and does not check the remaining values. Otherwise it returns true.

Lets check if all the names in our contacts list have more than 3 characters:

['Stewie', 'Meg', 'Quagmire', 'Cleveland'].every(contact => contact.length >= 4); // false
['Stewie', 'Megan', 'Quagmire', 'Cleveland'].every(contact => contact.length >= 4); // true

The first test returns false since Meg has only 3 characters. Remember: for every(), all the elements in the array have to be truthy for it to return true. Changing Meg to Megan in the second test therefore returns true.

These two methods can come in handy if you need to perform somewhat similar tasks as described above. But of course, they're not the only way. Hope someone finds this useful! :)

Top comments (8)

JavaScript Joel • Aug 28 '18

Great article!

With curry and partial application, you can also use some like this:

const contacts = ['Stewie', 'Meg', 'Quagmire', 'Cleveland'];

const equals = a => b => a === b

contacts.some(equals('Lois')) // false
contacts.some(equals('Meg')) // true

You could also write every like this:

const propGte = prop => val => obj => obj[prop] >= val;
const lengthGte = propGte('length');

['Stewie', 'Meg', 'Quagmire', 'Cleveland'].every(lengthGte(4)); // false
['Stewie', 'Megan', 'Quagmire', 'Cleveland'].every(lengthGte(4)); // true