There are basically two common ways to check if a string contains a substring:
const str = 'A quick brown fox jumps over the lazy dog';
Modern JavaScript way (ES6)
str.includes('fox'); // return true
str.includes('cat'); // returns false
This method was introduced in ES6 and hence is not available in some older browsers like Internet Explorer.
Alternative Way (E65)
This method is supported across all browsers.
str.indexOf('fox'); // returns 14 the starting index of the substring
str.indexOf('cat'); // return -1 since it is not present
indexOf returns 0 or greater if the substring is present otherwise it returns -1.
Case Insensitive
Both the methods are case-sensitive and if you want it to be case insensitive then make sure both the strings are in the same case by using the toLowerCase() or toUpperCase() method on them beforehand like this:
str.includes('a '); // false because str has "A " and not "a "
str.toLowerCase().includes('a '); // true
