String.lastIndexOf()

I doubt it will be, and you can define your own function or just use:

var str = 'Hello planet earth, you are a great planet.';
var search = 'planet';

var lastIndex = (str.split('').reverse().join('').indexO­f(search.split('').reverse().join('')) - str.length + search.length) * (-1)

Please test this, I haven't :P