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A puzzle in JavaScript objects

This is a puzzle you can run into if you’re not careful, and sometimes, despite your best efforts, even if you are careful. It happened to me when I had a settings object that I passed into a view. The view then did things. When I made a new view from the settings object, it looked like the view I had just cleared. O.o

Here’s a contrived example:

See the Pen Set - ES6 by Swizec Teller (@swizec) on CodePen.

You have a Counter class that:

  • takes initial state as an argument
  • implements the inc() method for +1-ing state
  • returns current count on every inc() call

This allows you to instantiate as many counters as you want, starting from any number you want. They can only count up. The implementation is straightforward. They’re the kind I’ve seen many times in the wild.

You take initial state, save it in an instance property, then proceed to use it like it was your very own. The implementation is simple to reason about, easy to extend when further properties are needed, and quick to build.

There’s just one problem: Instantiating multiple counters from settings stored in memory makes them share state.

Wat? ?

Look, here’s the example again. Pay close attention to the output.

See the Pen Set - ES6 by Swizec Teller (@swizec) on CodePen.

1, 2, 3, not 1, 2, 1 as you would expect. ?

The rabbit hole goes deeper: Call counter.state == newCounter.state, and it prints true. Different objects, same state. Literally the same. Not same value, the exact same memory address.

Strange behavior, but it makes sense once you remember that in JavaScript, objects are always stored as references, not as values. And that sentence is hard to understand if you’ve never had a teacher who forced you to learn C and play with pointers.

You’re about to learn something awesome and core to software engineering!

We have memory with 7 addresses - A1 through A7. For simplicity, we’re giving our variables the same names.

When we set A1 to 5, a 5 appears at address A1 in memory. Set A2 to 6 and 6 appears at address A2. Simple values are stored at memory locations as the values.

When we set A3 to an array - [1, 2], we need two memory locations to store that value. So we set A3 to the address of our array – A4. At address A4 we store the value 1, and put 2 right next to it at A5.

Now JavaScript can do a memory optimization. When we set A6 to equal A3, it doesn’t have to create a new array at a new memory address. That would be wasteful. Instead, it gives A6 the same address reference - A4.

Both A3 and A6 point to the exact same memory location. Change one, and the other changes as well.

Savvy?

To avoid this problem, force JavaScript to make a copy before saving things into local object state. Use Lodash’s _.copyDeep if there’s a lot to copy, or explicitly set each property to build a new object from scratch.

Err on the side of explicit.

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Published on September 9th, 2016 in coding, Front End, JavaScript, Technical

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