SOURCE

String.prototype.padstart = function(targetLength, str) {
	if (this.length > targetLength) {
		return String(this);
	} else {
		if (targetLength - this.length > str.length) {
			str += str.repeat((targetLength - this.length) / str.length);
		}
	}	
    return str.slice(0, (targetLength - this.length)) + String(this);
}

console.log("123".padstart(7, "abc"));