Can I check if a JavaScript string contains a substring, regardless of case?

As with most languages, JavaScript's substring matching is case-sensitive by default. If you need to check if a string contains a substring, but you don't care about the case, you'll need to roll up your own solution.
Converting both strings to lowercase before comparing them is the naive approach, but it's generally discouraged for performance reasons. Instead, you can use a regular expression with the 'i' flag to perform a case-insensitive search.
In order to do so, you'll have to use the RegExp() constructor to create a regular expression that matches the given searchString, ignoring the case. Then, you can use the RegExp.prototype.test() method to check if the string contains the substring.

const includesCaseInsensitive = (str, searchString) =>
  new RegExp(searchString, 'i').test(str);

includesCaseInsensitive('Blue Whale', 'blue'); // true

In order to do so, you'll have to use the RegExp() constructor to create a regular expression that matches the given searchString, ignoring the case. Then, you can use the RegExp.prototype.test() method to check if the string contains the substring.

From 30-seconds-of-code: includes-case-insensitive