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How to Check if an Array Includes a Value in JavaScript?

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Checking if an array includes a value is something that we’ve to do often in a JavaScript app.

In this article, we’ll look at how to check if an array includes a given value in JavaScript.

Array.prototype.includes

The includes method is a method that’s included with the array instance.

It takes a value and compares it with === to check if an element is included.

For instance, we can write:

console.log(['apple', 'orange', 'grape'].includes('orange'));

If it’s included, then it returns true .

Otherwise, it returns false .

Array.prototype.indexOf

We can also use the indexOf method of an array instance to check if it includes a given element.

It also uses === for comparison.

And it returns the index of the first instance of an item if it exists.

Otherwise, it returns -1.

To use it, we write:

console.log(['apple', 'orange', 'grape'].indexOf('orange') >= 0);

Write Our Own

We can write our own function to search for a value.

For instance, we can write:

function contains(a, obj) {
  let i = a.length;
  while (i--) {
    if (a[i] === obj) {
      return true;
    }
  }
  return false;
}

console.log(contains(['apple', 'orange', 'grape'] , 'orange'));

We create the contains function that uses a while loop to search for an item.

If a[i] has the same value as obj then we return true .

If we loop through the whole a array and didn’t find anything that matches, then we return false .

Array.prototype.some

The some method is another array instance method that comes with JavaScript arrays.

It lets us pass in a callback to check if any items match the given condition.

For instance, we can write:

const items = [{
  a: '1'
}, {
  a: '2'
}, {
  a: '3'
}]

console.log(items.some(item => item.a === '3'))

We have an items array that has a bunch of objects.

And we call items.some with a callback to check if any items entry with a property equal to 3 exists.

some returns true if an item that matches the given condition exists and false otherwise.

Conclusion

There’re many ways to find if an array item exists in JavaScript.

By John Au-Yeung

Web developer specializing in React, Vue, and front end development.

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