https://repl.it/C1XH/183 created by sethopia

Change Key (1.5).js

/*
	TRANSPOSE MUSIC
	
	In music a keyboard has the following notes
		[A, A#, B, C, C#, D, D#, E, F, F#, G, G#]
		
		A # is "sharp" notation. A# or A sharp, is in between A and B
		
		
	Create a function transpose() that takes an array of notes and a string with the number of steps. The function should return an array with all the notes in the array transposed up that many steps
	
	ex)
		transpose(['E', 'F'], '1 step') -> ['F', 'F#']
		transpose(['G'], '5 steps') -> ['C']
*/

function transpose(arr, str) {
	var steps = parseInt(str.charAt(0));
	var returnArr = [];
	var keyboard = ["A", "A#", "B", "C", "C#", "D", "D#", "E", "F", "F#", "G", "G#"]
	for (var i = 0; i < arr.length; i++) {
		var newNote = 0;
		if(keyboard.indexOf(arr[i]) + steps > keyboard.length) {
			newNote = keyboard.indexOf(arr[i]) + steps - keyboard.length;
		} else {
			newNote = keyboard.indexOf(arr[i]) + steps;
		}
		returnArr.push(keyboard[newNote]);
	}
	return returnArr;
}

console.log(transpose(['E', 'F'], '1 step'));
console.log(transpose(['G'], '5 steps'));

Change Key (1.5)_console_dump.txt

Native Browser JavaScript
>>> [ 'F', 'F#' ]
[ 'C' ]