String.includes()
Pushes support back to at least IE6.
/**
* String.prototype.includes() polyfill
* https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/includes#Polyfill
*/
if (!String.prototype.includes) {
String.prototype.includes = function(search, start) {
'use strict';
if (search instanceof RegExp) {
throw TypeError('first argument must not be a RegExp');
}
if (start === undefined) { start = 0; }
return this.indexOf(search, start) !== -1;
};
}