JavaScript implementation of the PHP function `strspn`

strspn.js

String.prototype.strspn = function (chars, start, length) {
	var end;

	if (start == undefined)
		start = 0;
	else if (start < 0)
		start += this.length;

	if (length == undefined)
		length = this.length;

	start = Math.max(0, Math.min(start, this.length));
	end   = Math.max(start, Math.min(start + length, this.length));

	for (var i = start; i < end; i ++) {
		if (chars.indexOf(this[i]) == -1)
			return i - start;
	}

	return end - start;
};