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object-copying-failures-and-partial-solutions.md

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Object copying failures and partial solutions

JavaScript can do a lot! It's the language of the internet and also pays my bills. Still, there are some surprising frustrations about JavaScript. Some of them seem unbelievable. One of these is deep-copying Objects. 🤦‍♂️

tl;dr

JavaScript does not support deep-copying Objects. ES6 introduced some useful ways to do shallow-copying, like Object.assign and the spread operator, but it's important to remember these DO NOT deep-copy. Nested objects remain a reference to the original object.

DON'T do this

const obj = {a: 0} // obj.a = 0} 🙂
const clone = obj  // obj.a = 0} 🙂     clone.a = 0} 🙂
obj.a++            // obj.a = 1} 🤬     clone.a = 1} 🤬
clone.a = 5        // obj.a = 5} 🤬     clone.a = 5} 🤬

The complete technical explanation can get complex, but the tl;dr is that clone is linked to obj. It's not a proper clone. It's a reference to the original. Changing properties for one of them will change the same property for the other. 🤦‍♂️

🤞 ES6 to the rescue?

Kind of. The Object.assign method and spread operator only do shallow-copying. I'll just show the spread operator because as far as I know, they work the same in this case.

const obj = {a: 0}      // obj.a = 0 🙂
const clone = {...obj}  // obj.a = 0 🙂     clone.a = 0 🙂
obj.a++                 // obj.a = 1 🙂     clone.a = 0 🙂
clone.a = 5             // obj.a = 1 🙂     clone.a = 5 🙂

Great, right?

Unfortunately they don't work on nested objects. 🤦‍♂️

const obj = {a: 0, b:{c: 0}}   // obj.b.c = 0 🙂
const clone = {...obj}         // obj.b.c = 0 🙂     clone.b.c = 0 🙂
obj.b.c++                      // obj.b.c = 1 🤬     clone.b.c = 1 🤬
clone.b.c = 5                  // obj.b.c = 5 🤬     clone.b.c = 5 🤬

The nested objects remain linked to the original object just like the first example. It's important to be aware of this when using the ES6 features mentioned.

JSON.stringify??

Apparently const clone = JSON.parse(JSON.stringify(obj)) works except for:

  • Dates
  • Functions
  • undefined
  • regExp
  • Infinity

Conclusion

It's a shame that JavaScript needs to rely on libraries, custom algorithms, or sketchy JSON workarounds. 😞

At least the ES6 goodies did improve the status quo a bit.

Resources

  • Long StackOverflow page with an alarming number of answers suggesting the JSON.stringify method.