How to verify that a string has a substring in JavaScript?

In ECMAScript 6 has String.prototype.includes:

const string = "foo";
const substring = "oo";

console.log(string.includes(substring));

Old browsers do not support includes. So for old browsers use String.prototype.indexOf that returns -1 if a substring not found:

var string = "foo";
var substring = "oo";

console.log(string.indexOf(substring) !== -1);