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Check If an Array Contains and Item in JavaScript

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There’re a few ways to check if an array contains an item in JavaScript.

We can use the includes, some, filter, find, findIndex, or indexOf to do the check. All methods are part of the array instance.

includes

The includes method takes in a value and returns true if it’s in the array or false otherwise. The check is done with === operator.

For instance, we can call includes as follows:

const arr = [1, 2, 3];
const result = arr.includes(1);

In the code above, we called includes on arr and then check if 1 is in it.

Therefore, it should return true so result is true.

some

The some method takes a callback with the condition of the item to check for. We can use it as follows:

const arr = [1, 2, 3];
const result = arr.some(a => a === 1);

In the code above, we passed in a callback to the some method to check if any array entry, which is the value of a is 1.

It returns true if it’s in the array and false otherwise, so we should get the same result.

filter

The filter method returns and array with the items that meet the condition returned by the callback included.

For instance, we can call it as follows:

const arr = [1, 2, 3];
const result = arr.filter(a => a === 1).length > 0;

In the code above, we called the filter method with the same callback, which returns an array with 1 in it. Then we get the length property of it and the make sure it’s bigger than 0.

Therefore, we get the same result as before by calling filter and adding the length check.

find

The find method returns the first entry in the array that meets the given condition returned in the callback.

If the item isn’t found, then undefined is returned.

For instance, we can write the following:

const arr = [1, 2, 3];
const result = typeof arr.find(a => a === 1) !== 'undefined'

In the code above, we use the typeof operator to check if the array item returned by find is undefined or not. If it’s not undefined, then it’s found. Otherwise, it’s not.

Then we should get the same result as before.

findIndex

The findIndex method returns the index of the first element in the array that meets the given condition returned in the callback.

If the item isn’t found, then -1 is returned.

For instance, we can write the following code to use findIndex:

const arr = [1, 2, 3];
const result = arr.findIndex(a => a === 1) !== -1;

In the code above, we called findIndex with the same callback used in the previous examples to search if any item in the array is 1. Then we check if the returned index isn’t -1.

Therefore, it should return the same boolean result as before, which is true.

indexOf

indexOf returns the index of the item that matches the value passed in as the argument.

It returns -1 if the value isn’t found.

For instance, we can use it as follows:

const arr = [1, 2, 3];
const result = arr.indexOf(1) !== -1;

We checked the returned array against -1 so that we can check if it’s in the array.

So we should get the same results as before.

some, find and findIndex are suitable for searching for any object, while the rest are suitable for primitive values since they let search with any condition instead of just letting us pass in a value or variable.

Objects are compared with the === operator so it doesn’t compare the structure of an object.

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