String.prototype.contains method, introduced with ECMAScript 6 and Firefox 18, had caused considerable site compatibility issues, notably with MooTools. The library has already fixed the issue, but in order to be consistent with
Array.prototype.includes, which has also renamed from
contains due to compatibility issues, the specification has renamed the method to
String.prototype.includes. The Firefox implementation has been updated to match the spec. While the deprecated
contains method still remains as an alias of
includes, it will be removed in the near future.