Ian McNally

Equality of shallow javascript arrays

April 02, 2015

Today, I was working on a UI bit where you click on a modal to add or remove some items from a list. I wanted the modal to have a shallow copy of the items list, so that a cancel wouldn’t update the original items list. Only save would. Since code is worth a thousand explanations:


var items = ['Clean room', 'Do laundry'];

openModal = function() {
  modal.open(items.slice()); // passing in items to modal


remove = function(item) {
  _.pull(items, item);

cancel = function() {

So when modal.cancel is called, controller.items should be unmodified.

In my unit test (jasmine, below), I wanted to test that modal.items was a shallow copy.

// Fail

But that doesn’t work. slice returns an new array, but maintains the references inside the array. This subtlety is lost on toEqual, and lodash/underscore’s _.isEqual. Plain javascript to the rescue:

expect(modal.items !== controller.items).toBe(true);
// Pass

Oh yeah.

To cover my bases, I still used toEqual to test that controller.items was passed to modal.items so my test looked like:

expect(modal.items !== controller.items).toBe(true);
// Pass

Ian McNally

Hey, I'm Ian. I build websites and write about what I learn as I go. Follow me on Twitter.