React useEffect doesn't trigger sometimes when I update my list as its dependency

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const mylist = [1,2,3,4,5];
useEffect(() => {
console.log(mylist)
}, [mylist])

This is part of my code. useEffect doesn't trigger when I append a new element to mylist, but it does when I delete an element.

How do I fix it so that it triggers when I append new elements to mylist

I have a button doing onClick(e => mylist.push(e)) and another one onClick(e => mylist.remove(e))

The array that you're creating isn't being stored in state, so every render a new array is being created. The solution is to use react state:

function MyComponent() {
  const [myList, setMyList] = useState([0,1,2,3,4])

  useEffect(() => {
    console.log(myList)
  }, [myList])

  return (
   <div>
     {JSON.stringify(myList)}
     <button onClick={() => setMyList([...myList, myList.length])}>Add</button>
   </div>);
}



Hooks API Reference – React, Use an effect for updating the title useEffect(function updateTitle() { document.title = name + ' ' + surname; }); // } So how does React know which state� useEffect hook is an extremely powerful an versatile tool, allowing you to even create your own, cust Tagged with javascript, webdev, react.

It worked when I change the dependency to mylist.toString() I thought useEffect does deep comparison on the second parameter

Rules of Hooks – React, Trying to take the old paradigm and apply it to hooks just doesn't work Ah, I see that useEffect with an empty dependency array does just that. I have a component in React where in useEffect I make two API calls and set two states. After still in useEffect I am trying to set another state with one of the states updated by the API. If I do this by keeping the square brackets parameter of useEffect empty it won't set the state. If I pass the state constants in the brackets it goes into

You could check the length of the array so if the array size changes the effect will be excecuted:

useEffect(() => {
  //Your effect  
}, [mylist.length])

useEffect(fn, []) is not the new componentDidMount(), Then React updates the DOM to match our latest render output. The key takeaway is that the count constant inside any particular render doesn't� That’s what allows React to correctly preserve the state of Hooks between multiple useState and useEffect calls. (If you’re curious, we’ll explain this in depth below.) Only Call Hooks from React Functions . Don’t call Hooks from regular JavaScript functions. Instead, you can: Call Hooks from React function components.

useEffect using strict comparison, but an array always comes up as false, so [1] === [1] is false and [1] === [1, 2] is still false.

It likely only runs on first render, that is why it's not updating it when you add or remove from list. Try putting the length of the array as a dependancy and it'll work as you intend it to.

So,

useEffect(() => {
//stuff on myList
}, [myList.length])

So if you add something to it, it'll be comparing integer to integer

A Complete Guide to useEffect — Overreacted, Hooks provide the useEffect() API. The call accepts a function as argument. The function runs when the component is first rendered, and on every� Try using useLayoutEffect like how currently the only Hook test executing on react-test-renderer is doing it. I think this is intentional behaviour, as far as I understand, useEffect executes after paint, but react-test-renderer never paints, because it doesn't use a DOM at all. EDIT: typo'd useLayoutEffect as asLayouteffect 😄

How to use the useEffect React hook, React does not do this – it will only re-render in response to state changes. useEffect, too, does not actively “watch” for changes. You can think� React is a focused component library. So it has no opinion on how to request remote data. If you’re requesting and sending data to web APIs via HTTP, here are four options to consider.

How the useEffect Hook Works (with Examples), If we want the effect to run in the same phase that componentDidUpdate does, we can use useLayoutEffect instead. Example. const { useState, useRef,� Next let's see how to create a React hook for the async action. Auto Effect Pattern. To simplify the usage of async action, we can create a hook named "useFetchData" to wrap the Redux action as a hook: In this hook, we guarantee the existence of the data. That is if it doesn't exist then fetch it, otherwise it does nothing.

Make React useEffect hook not run on initial render, That way if the App component receives any new props or if the state changes, the effect won't be triggered again. Replicating React useEffect on� You can’t use useEffect hook for this case because the effect might run after several renders asynchronously. For this case you should use useLayoutEffect hook which runs immediately after each render and in addition this logic doesn’t block painting then it’s fine to use it.

Comments
  • Show how you are appending and deleting to give some context
  • Right, if you converted your values to strings, it will work. String equality is much simpler than Array equality, as the arrays you're comparing are different objects even if their values are the same.
  • It may be possible to add and remove an item in the same update. This wouldn't work in that case.