How to Check if a JavaScript String Contains a Substring in a Case Insensitive Manner?

Sometimes, we may want to check if a substring is in a JavaScript string in a case-insensitive manner.

In this article, we’ll check if a JavaScript string has a substring in a case-insensitive manner.

String.prototype.toLowerCase and String.prototype.indexOf

We can convert both the string and the substring we’re checking for the lower case to check if a substring is in a JavaScript string is in a case-insensitive manner.

For instance, we can write:

const includes = 'ABCDE'.toLowerCase().indexOf("abc".toLowerCase()) !== -1  
console.log(includes)

We call toLowerCase on 'ABCDE' and 'abc' to convert them both to lower case.

And then we call indexOf to check if “abc”.toLowerCase() to if included in 'ABCDE' in a case-insensitive manner.

Since 'abc' is in 'ABCDE' when they’re compared in a case-insensitive manner, indexOf should return an index other than -1.

And so includes is true .

Case-Insensitive Regex Search

We can also do a case-insensitive regex search in our code.

For instance, we can write:

const includes = /abc/i.test('ABCDE')  
console.log(includes)

The i flag lets us search for a pattern in the string we pass into test in a case-insensitive manner.

And so includes is also true in this example.

Case-Insensitive Regex Search with RegExp Constructor

Alternatively, we can do a case-insensitive regex search with the RegExp constructor.

For instance, we can write:

const includes = new RegExp("abc", "i").test('ABCDE')  
console.log(includes)

to do the same as we did before.

String.prototype.toLowerCase and String.prototype.includes

We can also substitute the indexOf method in the first example with includes .

Using the includes method, we don’t have to compare against -1.

Instead, we get true if the substring is included in a string and false otherwise.

So we can write:

const included = 'ABCDE'.toLowerCase().includes("abc".toLowerCase())  
console.log(included)

We convert them both the string and substring to lower case as usual, but we use includes to check if the substring we pass into includes is included in 'ABCDE' .

And so we should get the same result as before.

String.prototype.toLocaleLowerCase and String.prototype.includes

If we’re checking a JavaScript substring with a string that has text other than English, we may want to use the toLocaleLowerCase method since it works with non-English locales.

For instance, we can write:

const included = 'ABCDE'.toLocaleLowerCase().includes("abc".toLocaleLowerCase())  
console.log(included)

And we get the same result as before.

Conclusion

We can use various string or regex methods to check whether a substring is included in a JavaScript string in a case insensitive manner.