Time complexity of unshift() vs. push() in Javascript

Question

I know what is the difference between unshift() and push() methods in JavaScript, but I'm wondering what is the difference in time complexity?

I suppose for push() method is O(1) because you're just adding an item to the end of array, but I'm not sure for unshift() method, because, I suppose you must "move" all the other existing elements forward and I suppose that is O(log n) or O(n)?

Answer 1

push() is faster.

js>function foo() {a=[]; start = new Date; for (var i=0;i<100000;i++) a.unshift(1); return((new Date)-start)} js>foo() 2190 js>function bar() {a=[]; start = new Date; for (var i=0;i<100000;i++) a.push(1); return((new Date)-start)} js>bar() 10 

function foo() {a=[]; start = new Date; for (var i=0;i<100000;i++) a.unshift(1); return((new Date)-start)} console.log(foo())  function bar() {a=[]; start = new Date; for (var i=0;i<100000;i++) a.push(1); return((new Date)-start)} console.log(bar());

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Update

The above does not take into consideration the order of the arrays. If you want to compare them properly, you must reverse the pushed array. However, push then reverse is still faster by ~10ms for me on chrome with this snippet:

var a=[];  var start = new Date;  for (var i=0;i<100000;i++) {   a.unshift(1); } var end = (new Date)-start; console.log(`Unshift time: ${end}`);  var a=[]; var start = new Date; for (var i=0;i<100000;i++) {   a.push(1); }  a.reverse(); var end = (new Date)-start; console.log(`Push and reverse time: ${end}`);