How to count string occurrence in string?

The g in the regular expression (short for global) says to search the whole string rather than just find the first occurrence. This matches is twice:

var temp = "This is a string.";
var count = (temp.match(/is/g) || []).length;
console.log(count);

Expand snippet

And, if there are no matches, it returns 0:

var temp = "Hello World!";
var count = (temp.match(/is/g) || []).length;
console.log(count);