How to check a substring present in a string in Javascript?

1. ES6 and above

includes is what you are looking for.

let big_string = 'javascript';
let small_string = 'java';

if( big_string.includes (small_string) ){
  // you will get here 
}

2. Older Javascript

You can achieve what you are looking for using indexOf

let big_string = 'javascript';
let small_string = 'java';

if( big_string.indexOf (small_string) !== -1  ){
  // you will get here 
}