Introduction

If you are building apps with react or redux for a long time you probably have seen some block of code like this

case 'some_case':
              return {
                ...state,
                  keyOne:{
                    ...state.keyOne,
                    someProp: payload
                  }
              }

And there's nothing uglier than a bunch of redundant lines of code so that your state can understand you're doing something with it. And that was one of the most fundamental Architectural flaws/questions React team faced after the introduction of frameworks like Vue, svelte, where every state you create is immutable by nature and listens to the state change by default even if you mutate the nested data.
and even libraries like Redux toolkit uses immer under the hood to make the developer experience more seamless.

Why does the scenario happen?

Well in React all states are immutable by nature, which means whenever you need to change state the data needs to be copied to a new memory address, or in other terms, some cost needs to be there, and this is not a problem for any primitive data types like strings or numbers. but because of the very nature of JS objects and how they are maintained in memory, changing one property inside the object doesn't change the top-level reference of that object, it only mutates it. and that's a problem for the setState method and useState hook. because unless they get a new reference they won't be able to know if the state has been mutated (because by default React uses shallow comparison )

-- one question you might have is: "why go for shallow compare at all?" well let's think about the object as a tree data structure and each property is a node in the tree now what's the time complexity of a deep compare in this case? O(n) [n being the number of nodes in the tree ], but the actual updated data would most likely be in O(1) so if you think about it, a deep comparison would be really expensive most of the time and that's why React's core team probably decided to go with shallow compare as a default choice, but you could override that behavior by using shouldComponentUpdate() method in Class-based components.

and that means we probably can somehow do better than this

and that's how Immer
was introduced, and not only Immer but most of the modern UI frameworks also go for a smiler solution to handle immutable states in a manner that seems mutable.

This approach follows a very ancient but powerful software design pattern called Proxy

On a very high-level proxy is an object that behaves like an interaction layer between the usage and the actual object from which it was created
Source : Patterns.dev

And this alone gives us the superpower to control how the updates happen to our data and subscribe to get/set events as well. for this very reason, a lot of the modern UI frameworks use Proxies under the hood.

and today let's try to re-create a very popular hook useImmer
that uses Immer.js to use the proxy pattern to handle state updates

Scope of this custom hook

[we are gonna be using typescript for this solution but it would be similar in plain javascript as well ]

import useCustomImmer from "./hooks/useCustomImmer";
import { useEffect } from "react";
export default function App() {
  const [value, setValue] = useCustomImmer({
    count: 0,
    name: {
      firstName: "Akashdeep",
      lastName: "Patra",
      count: 2,
      arr: [1, 2, 3]
    }
  });

  const handleClick = () => {
    setValue((prev) => {
      prev.count += 1;
      prev.name.count += 1;
      prev.name.arr.push(99);
    });
  };
  useEffect(() => {
    console.log("render");
  });
  return (
    <div className="App">
      <button onClick={handleClick}>Click me </button>
      <br />
      <span>{JSON.stringify(value)}</span>
    </div>
  );
}

For the sake of this example, we have a state that has some nested data and in the handle click method, we are mutating the properties provided by our setter function in the useCustomImmer hook. [This is supposed to trigger a re-render with the updated state ]

now let's look at the solution

export default function useCustomImmer<StateType>(
  initialValue: StateType
): [value: StateType, setter: (func: (value: StateType) => void) => void] {
  const [value, setValue] = useState<StateType>(initialValue);

const listener =useCallback(debounce((target:Target) => {
    setValue((prevValue)=>({...updateNestedValue(prevValue,target.path,target.value)}))
  },100),[])

  // @ts-ignore
  const valueProxy = useMemo<StateType>(
    () => new Proxy(value, createHandler([], listener)),
    [value, listener]
  );

  const setterFunction = (func: (value: StateType) => void) => {
    func?.(valueProxy);
  };

  return [value, setterFunction];
}

The hook signature would be similar to useState, where it returns an array where the first value is the state itself and the 2nd value is the setter function, this function would take a callback that has an argument of the most updated state (similar to how useState's functional approach goes but here the argument state would not just be a normal object, instead it'll be a proxy of the original state )

we will create a Proxy with the state and keep it updated with the state changes.

Now here comes the problem, the listener we are attaching with the Proxy to monitor get/set events for properties on the state object only listens for top-level properties by default and they do not apply for nested properties [Check this link for a more detailed explanation of the API ].

To be able to solve this we need to recursively attach a handler to our object, and for that, I have written a custom createHandler method that does the same

const createHandler = <T>(path: string[] = [],listener:(args:Target)=>void) => ({
  get: (target: T, key: keyof T): any => {
    if (typeof target[key] === 'object' && target[key] != null )
      return new Proxy(
        target[key],
        createHandler<any>([...path, key as string],listener)
      );
    return target[key];
  },
  set: (target: T, key: keyof T, value: any,newValue:any) => {
    console.log(`Setting ${[...path, key]} to: `, value);
    if(Array.isArray(target)){
      listener?.({
        path:[...path],
        value: newValue
      })
    }else{
    listener?.({
      path:[...path,key as string],
      value
    })
  }
    target[key] = value;
    return true;
  },
});

this method takes a string array and the listener as arguments and returns an object with get/set properties [You can extend this further to handle more cases ]
Now let's break this down !!!!!

in the get property, it takes the target and the key as an argument, and keep in mind this target is the nearest root from the property it's being accessed so no matter how much deep you go into a nested object you'll only get the nearest parent reference.
here if the expected property is of type object and not null we would create a new Proxy of that object with a recursive call and return that Proxy object instead of the actual object when the user tries to access a nested property and in this listener, we also add the key so that we can track the path when we eventually call the listener.

This way we don't create multiple nested proxies every time a state update needs to happen instead we would create proxies only when someone tries to access these properties, and this would be much faster than going over all the nodes in that object since we know the path, to begin with (this means travel depth of O(log(n)) base
n)
.

In the set property to handle an edge case we first check if the property is an array (because when we do any array operation like push and pop apart from the item itself 'length' property is also changed and we could handle this edge case either in our handler or when we update the state, I choose to do that here, Do keep in mind there will be other data structures in javascript that will probably have their edge case. I created a very simple hook to demonstrate the overall architecture).
and at each time we set the value we also call the listener callback with a target object that would have the value and the path of the property that needs to be updated.

Now we also need a function that takes the path array and the value to update our source object, which is easy to come up with

const updateNestedValue =<StateType>(object:StateType,path:string[],value:any):StateType=>{
  const keys = path
  let refernce: any = object
  for(let i=0;i<keys.length-1;i++){
    if(keys[i] in refernce){
      refernce = refernce[keys[i]] 
    }     
  }
  if(keys[keys.length-1] in refernce){
    refernce[keys[keys.length-1]] = value
  }
  return object
}

And in our listener callback, we just update the old state and call the setValue method with the updated value

 const listener =useCallback(debounce((target:Target) => {
    setValue((prevValue)=>({...updateNestedValue(prevValue,target.path,target.value)}))
  },100),[])

Notice I have used a debounce here, because as I said there are data structures like an array that have multiple properties changing with the proxy, but we only want to update the state once,(You can assume this is more of a batching technique )

Here is a working Sandbox for you guys, but do keep in mind this hook is in no way production ready or will be in the future (nor does it handle any edge case ), Immer.js has a very extensible and battle-tested design , this post is just a way to appreciate the way things work under the hood. And the reason is that there are a lot of data types (Maps, Set ...) supported by native javascript for which implementing the proxy handler would be a chore (Honestly that's too much work for me right now ), if you think you can extend the current version and handle those types i'm happy review any open PRs.
but if you are planning for only JSON serializable state then this might just work for you .

And the Github link

https://github.com/Akashdeep-Patra/use-custom-immer

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