As I known.

The shift command is come from the binary bit shift [1]. for Example.

    001100
0 < 011000 // when you shift left
|
Yay!

I think it is quite simple it is just like you push it from behind. So this makes sense for me.

The unshift is the opposite way of shift.

    001100
1 > 001100 // they called it unshift
    1001100
    |
    Yay!

So that's it, Hope this helps!

[1] http://en.wikipedia.org/wiki/Bitwise_operation#Bit_shifts

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}`);