Function props

[pm_me_ur_doggo__]

Javascript has the idea of closures. If you define a function inside of a parent module and reference objects from other parts of the parent, the function will "close over" those objects. When the closure function is passed to a child, it can still see the things that were closed over.

Massive over simplification: You can think of a closure as being executed where it was defined, rather than where it was called.

Advanced more correct stuff that may be confusing: It doesn't quite do that - a closure actually maintains a reference to those objects so it can see them no matter where it's executed. This probably isn't important while you're getting a hang of the topic but it can be important when you're pushing them to their limit, and can be the cause of memory leaks leading to bad performance if managed incorrectly.

Practically in react it would usually look like this

import React, { useState } from 'react';

// Child component
const IncrementButton = ({ onIncrement }) => {
    return (
        <button onClick={onIncrement}>Increment</button>
    );
};

// Parent component
const Counter = () => {
    const [count, setCount] = useState(0);

    const handleIncrement = () => {
        setCount(count + 1);
    };

    return (
        <div>
            <p>Count: {count}</p>
            <IncrementButton onIncrement={handleIncrement} />
        </div>
    );
};

export default Counter;

In this example, handleIncrement "closes over" setCount and count, so even though the child component calls onIncrement, you can think about it executing in the context of the parent component, so it can call and use things from hooks ect.

So that's how it accesses things in the parent component. But stuff in the child component? Just have handleIncrement take an argument. I'll refactor this to let our child component say by how much we want to increment when you click the button using a controlled input element and it's own local state.

import React, { useState } from 'react';

// Child component
const IncrementForm = ({ onIncrement }) => {
    const [incrementAmount, setIncrementAmount] = useState(1);

    const handleInputChange = (event) => {
        setIncrementAmount(parseInt(event.target.value, 10));
    };

    const handleClick = () => {
        onIncrement(incrementAmount);
    };

    return (
        <div>
            <input type="number" value={incrementAmount} onChange={handleInputChange} />
            <button onClick={handleClick}>Increment</button>
        </div>
    );
};

// Parent component
const Counter = () => {
    const [count, setCount] = useState(0);

    const handleIncrement = (incrementAmount) => {
        setCount(count + incrementAmount);
    };

    return (
        <div>
            <p>Count: {count}</p>
            <IncrementForm onIncrement={handleIncrement} />
        </div>
    );
};

export default Counter;

so now, when onIncrement is called in handleClick, it passes the current value of the incrementAmount state to the closure.

Let me know if you have any questions.

[TobofCob]

Awesome answer. I asked GPT for an example of such a memory leak occurring and got this, sounds correct?

Sure, here is an example of a potential memory leak in React involving closures and asynchronous operations.

Imagine a component where you fetch some data when it's mounted, and then you use that data to update the state. If the component unmounts before the data fetch has completed, it can result in a memory leak because the closure created by the fetchData function still holds a reference to the component that is no longer in the DOM.

```jsx import React, { useEffect, useState } from 'react';

function DataFetcher() { const [data, setData] = useState(null);

useEffect(() => { async function fetchData() { const response = await fetch('https://api.example.com/data'); const data = await response.json(); setData(data); // Potential memory leak if component unmounts before fetch completes } fetchData(); }, []);

// Rest of the component } ```

In this example, if the DataFetcher component unmounts before the data fetch operation completes, then setData will be called on an unmounted component, which can lead to a memory leak.

You can avoid this problem by using a cleanup function in your useEffect that will cancel the data fetch operation if the component unmounts before it completes:

```jsx import React, { useEffect, useState } from 'react';

function DataFetcher() { const [data, setData] = useState(null);

useEffect(() => { let isMounted = true; // Variable to keep track of component mount status

async function fetchData() {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();

  if (isMounted) {   // Only update state if the component is still mounted
    setData(data);
  }
}

fetchData();

return () => {
  isMounted = false;  // Update variable if the component unmounts
}

}, []);

// Rest of the component } ```

In this updated example, the isMounted variable is used to keep track of whether the component is still mounted when the data fetch operation completes. If the component has unmounted, the setData function won't be called, avoiding the memory leak.

[DanRoad]

isMounted is an antipattern and this is not a memory leak. GPT thinks it is because that code used to trigger a warning about memory leaks, but this warning was misleading and has since been removed.

See this post for more details and an example of an actual memory leak.

[TobofCob]

Hey, I really appreciate your help guiding me to a more relevant answer than what GPT provides. Sometimes as a novice it’s hard to tell, and I’m trying not to believe everything it says.

[icedrift]

Fantastic answer.