How do you check if an item is present in an array in javascript?

You can use the includes() method in JavaScript to check if an item exists in an array. You can also use it to check if a substring exists within a string.

It returns true if the item is found in the array/string and false if the item doesn't exist.

In this article, you'll see how to use the includes() method in JavaScript to check if an item is in an Array, and if a substring exists within a string.

Here's the syntax for using the includes() method to check if an item is in an array:

array.includes(item, fromIndex)

Let's break down the syntax above:

array denotes the name of the array which will be searched through to check if an item exists.

The includes() method takes in two parameters – item and fromIndex.

• item is the particular item you are searching for.
• fromIndex, which is an optional parameter, specifies the index from which to start the search. If you don't include this parameter, the default index will be set to 0 (the first index).

Here are some examples to show how to use the includes() method to check if an item exists in an array:

const nums = [ 1, 3, 5, 7];
console.log(nums.includes(3));
// true

In the example above, we created an array called nums with four numbers – 1, 3, 5, 7.

Using dot notation, we attached the includes() method to the nums array.

In the includes() method's parameter, we passed in 3. This is the item we want to search for.

We got true returned because 3 exists in the nums array.

Let's try searching for a number that doesn't exist in the array.

const nums = [ 1, 3, 5, 7];
console.log(nums.includes(8));
// false

As expected, we got false returned in the example above because 8 is not an item in the nums array.

How to Check if an Item is in an Array in JavaScript Using Array.includes() Starting From a Specified Index

In the last section, we saw how to check if an item existed in an array without using the second parameter in the includes() method.

As a reminder, the second parameter is used to specify the index to start from when searching for an item in an array.

The index of an array starts from 0. So the first item is 0, the second item is 1, the third item is 2, and so on.

Here's an example to show how we can use the includes() method's second parameter:

const nums = [ 1, 3, 5, 7];
console.log(nums.includes(3,2));
// false

The example above returned false even though we had 3 as an item in the array. Here's why:

Using the second parameter, we told the includes() method to search for the number 3 but starting from index 2: nums.includes(3,2).

This is the array:  [ 1, 3, 5, 7]

Index 0 = 1.

Index 1 = 3.

Index 2 =  5.

Index 3 = 7.

So starting from the second index which is 5, we have only 5 and 7 ([5,7]) to be searched through. This is why searching for 3 from index 2 returned false.

If you change the index to start the search from to 1 then you'd get true returned because 3 can be found within that range. That is:

const nums = [ 1, 3, 5, 7];
console.log(nums.includes(3,1));
// true

How to Check if a Substring is in a String in JavaScript Using the includes() Method

Similar to the previous examples, you have to attach the includes() method to the name of the string to be searched through using dot notation.

Here's what the syntax looks like:

string.includes(substring, fromIndex)

Here's an example:

const bio = "I am a web developer";
console.log(bio.includes("web"));
// true

In the example above, the bio variable had a value of "I am a web developer".

Using the includes() method, we searched for the substring "web".

We got true returned because "web" is in the bio string.

You can also use the second parameter to specify where the search will begin, but note that each character in a string represents an index and the spaces between each substring also represents an index.

Here is an example to demonstrate that:

let bio = "I am a web developer";
console.log(bio.includes("web",9));
// false

We are getting false because index 9 is the e in "web".

Starting from index 9, the string would look like this: "eb developer". The substring "web" doesn't exist in the string so false gets returned.

Summary

In this article, we talked about the includes() method in JavaScript. You use it to check if an item exists in an array. You can also use it to check if a substring can be found within a string.

We saw some examples that explained its use to check for an item in an array starting from the first index, then another example from a specified index.

Lastly, we saw how to use the includes() method to check if a substring exists within a string from the first index and from a specified index.

Happy coding!