Answer to cassidoo's Interview question "Implement the substring() function."

mySubstring.js

String.prototype.mySubstring = function(start, end) {
	let substr = '';
	for (i = (start || 0); i <= ((end || this.length) - 1); i++) {
		substr += this[i];
	}
	return substr;
}


console.log("hello world!".mySubstring(1, 5)); // => ello
console.log("hello world!".mySubstring(6)); // => world!