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

The JavaScript language spec does not mandate the time complexity of these functions, as far as I know.

It is certainly possible to implement an array-like data structure (O(1) random access) with O(1) push and unshift operations. The C++ std::deque is an example. A Javascript implementation that used C++ deques to represent Javascript arrays internally would therefore have O(1) push and unshift operations.

But if you need to guarantee such time bounds, you will have to roll your own, like this:

http://code.stephenmorley.org/javascript/queues/

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());

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}`);