What is the best way to shift first three elements of array to the end?

Question

I have the array ["a", "b", "c", "d", "e"] I need a function to shift abc to the end of the array - ["d", "e", "a", "b", "c"]

Answer 1

function rotate(a, n) {
  // The modulo here is a performance optimization... rotating by the length of the array has no effect. E.g. in this example, rotating 8 is the same as rotating 3.
  for (var i = 0; i < n % a.length; i++) {
    a.push(a.shift());
  }
}

var a = ["a", "b", "c", "d", "e"];

rotate(a, 3);

console.log(a);

// Output:
// [ 'd', 'e', 'a', 'b', 'c' ]

EDIT

Non-destructive version using slice:

function rotate(a, n) {
  n %= a.length;
  return a.slice(n).concat(a.slice(0, n));
}

var a = ["a", "b", "c", "d", "e"];

console.log(rotate(a, 3));

// Output:
// [ 'd', 'e', 'a', 'b', 'c' ]

EDIT2

In response to the follow-up question in a comment, here's how you would copy the elements instead of moving them. (This is the non-destructive version.)

function copy(a, n) {
  n %= a.length;
  return a.concat(a.slice(0, n));
}

var a = ["a", "b", "c", "d", "e"];

console.log(copy(a, 3));

// Output:
// [ 'a', 'b', 'c', 'd', 'e', 'a', 'b', 'c' ]

Or here's an in-place version of copy:

function copy(a, n) {
  for (var i = 0; i < n % a.length; i++) {
    a.push(a[i]);
  }
}

var a = ["a", "b", "c", "d", "e"];

copy(a, 3);

console.log(a);

// Output:
// [ 'a', 'b', 'c', 'd', 'e', 'a', 'b', 'c' ]