encrypt.js

/* ==========================================================================
 *
 *    Use:
 *    'Hello World!'.encrypt('mysecretkey')
 *    => 'Zijvw Gnhvh!'
 *
 * ========================================================================== */

String.prototype.encrypt = function(key) {

	function between(x, min, max) {return x >= min && x <= max;}

	let msg = [];
	let output = '';
	for (let char of this) {
		let code = char.charCodeAt(0)
		if (between(code, 65, 90)) {msg.push([code - 65, 0])}
		else if (between(code, 97, 122)) {msg.push([code - 97, 1])}
		else {msg.push(char)}
	}

	key = key.toLowerCase().split('').map(function(c) {
		return c.charCodeAt(0) - 65
	})

	for (var i = 0; i < msg.length; i++) {
		if (typeof msg[i] === 'string') {output += msg[i]}
		else {
			let value   = (msg[i][0] + key[i % key.length]) % 26
			output 	   += String.fromCharCode(value + 65 + msg[i][1] * 32)
		}
	}

	return output

}
module.exports = (text, key) => text.encrypt(key)