See John Code

Of course when you assign an existing object to a variable in JavaScript, you actually receive a reference to the object.  So modifications on the reference, cause changes on the original.  This shouldn’t be a surprise at all, and just to illustrate:

var a = [1, 2, 3];
var b = a;
b.push(4);
a; // [1, 2, 3, 4]

It’s common to see code for making a copy of an array by iterating over the elements:

var a = [1, 2, 3];
var b = new Array(a.length);
for (var i = 0; i < a.length; i++) {
  b[i] = a[i];
}

After which we’re safe since we have an element-by-element copy of the original (safe if the elements are primitives!):

b.push(4);
a; // [1, 2, 3]

Pretty ugly code for such a simple, common operation.  Luckily JavaScript has a nifty method on Array called “slice”, which is for selecting a segment of an existing Array.  We can abuse slice to make the copy for us:

var a = [1, 2, 3];
var b = a.slice(0, a.length);

b.push(4);
a; // [1, 2, 3]

And wouldn’t you know it – the second argument is optional:

var a = [1, 2, 3];
var b = a.slice(0);

AND, don’t tell the old-school JavaScript developers but if you’re programming in a decently new version of JavaScript (hey node.js guys) – that first argument is also optional:

var a = [1, 2, 3];
var b = a.slice();