How to check if a string “StartsWith” another string?

Question

How would I write the equivalent of C#\'s String.StartsWith in JavaScript?

var haystack = \'hello world\';
var needle = \'he\';

haystack.startsWith(needle)          


        

Answer 1

Based on the answers here, this is the version I am now using, as it seems to give the best performance based on JSPerf testing (and is functionally complete as far as I can tell).

if(typeof String.prototype.startsWith != 'function'){
    String.prototype.startsWith = function(str){
        if(str == null) return false;
        var i = str.length;
        if(this.length < i) return false;
        for(--i; (i >= 0) && (this[i] === str[i]); --i) continue;
        return i < 0;
    }
}

This was based on startsWith2 from here: http://jsperf.com/startswith2/6. I added a small tweak for a tiny performance improvement, and have since also added a check for the comparison string being null or undefined, and converted it to add to the String prototype using the technique in CMS's answer.

Note that this implementation doesn't support the "position" parameter which is mentioned in this Mozilla Developer Network page, but that doesn't seem to be part of the ECMAScript proposal anyway.