How to count number of occurrences of a substring in JavaScript?

daldhei.js

String.prototype.count=function(s1) { 
    return (this.length - this.replace(new RegExp(s1,"g"), '').length) / s1.length;
}

console.log("Marek Sautter".count("e")); // --> 2

console.log("I got a letter from Marek Sautter".count("tt")); // --> 2

// https://stackoverflow.com/questions/881085/count-the-number-of-occurrences-of-a-character-in-a-string-in-javascript