Password decryption for Hack This Site's basic level 6.

password-decrypt.js

function decrypt(password) {
	var letters = 'abcdefghijklmnopqrstuvwxyz',
		decrypt = '';
    for (var x = 0; x < password.length; x++) {
        var value = password[x];
        if (isNaN(value)) decrypt += letters[letters.indexOf(value) - x];
        else decrypt += value - x;
    }
	return decrypt;
}

decrypt(['a', 5, 'h', 7, 7, 9, 'i', 'h']);