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:
You have a
Counter class that:
- takes initial state as an argument
- implements the
- returns current count on every
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.
Look, here’s the example again. Pay close attention to the output.
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.
You’re about to learn something awesome and core to software engineering!
We have memory with 7 addresses -
A7. For simplicity, we’re giving our variables the same names.
When we set
5 appears at address
A1 in memory. Set
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
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 -
A6 point to the exact same memory location. Change one, and the other changes as well.
_.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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