Is this how closure works in useState hook?

Question

I am trying to understand how to implement useState hook using pure Javascript. and I stuck at the part where the closure is used.

here is the code.

    const React = (function () {
      let hooks = [];
      let idx = 0;

      function useState(initVal) {
        const _idx = idx;
        console.log(_idx) // 0, 1

        const state = hooks[idx] || initVal;

        const setState = newVal => {
          hooks[_idx] = newVal;
        };

        idx++;
        return [state, setState];
      }

      function render(Component) {
        idx = 0;
        const C = Component();
        C.render();
        return C;
      }

      return { useState, render };
    })();

    function Component() {
      const [count, setCount] = React.useState(1);
      const [text, setText] = React.useState('apple');

      return {
        render: () => console.log({ count, text }),
        click: () => setCount(count + 1),
        type: (word) => setText(word),
      }
    }

    var App = React.render(Component); // {count: 1, text: "apple"}
    App.click();
    var App = React.render(Component); // {count: 2, text: "apple"}
    App.type('pear');
    var App = React.render(Component); // {count: 2, text: "pear"}

When setState function(click or type) is called, it updates value of hooks array according to their index which is 0 for count and 1 for text.

Which means setState function inside useState remembers the value of _idx of each state(count and text) by javascript closure?

user_15320126 · Answer

Which means setState function inside useState remembers the value of _idx of each state(count and text) by javascript closure?

Yes. When useState is called, it gets the next available index using idx and stores it in the constant _idx. The setState function returned forms a closure because of which it remembers the _idx corresponding to its state even though useState has finished execution.

I stuck at the part where the closure is used.

Other places where closure is used:

useState and render functions inside the React module form a closure over hooks and idx. Hence the functions are able to read/write to these variables even after React (an iife) has finished execution.
render, click and type functions form a closure. The render method closes over Component. Hence it is able to access count and text even after Component function has finished execution. Similarly, click and type form a closure and hence can invoke setCount and setText funtions which were defined inside Component scope.

Is this how closure works in useState hook?

1 Answers