How do you insert arrays into a state variable of a nested array? - React

Question

I have a state variable like this:

const [todos, setTodos] = useState([])

I want to put a lot of arrays into it (in an amount unknown to me). This is the code I wrote:

data.forEach(x => {
           const userId = x.userId;
           const to_to_user = data.filter(d=>d.userId===userId)
           setTodos({...todos, [userId]:to_to_user});
           });

The problem is that each round of a loop overruns the previous index and at the end of the array there is only the last index. Does anyone have a solution?

Samathingamajig · Accepted Answer · 2022-11-24 23:24:43Z

3

Technically, you can do this:

data.forEach(x => {
  const userId = x.userId;
  const to_to_user = data.filter(d=>d.userId===userId)
  setTodos((oldTodos) => {...oldTodos, [userId]:to_to_user}));
});

so that you always have the most up-to-date version of todos.

But instead of calling setTodos multiple times, you should instead create the object first and then call setTodos once.

const newTodos = data.reduce((obj, x) => {
  const userId = x.userId;
  const to_to_user = data.filter(d=>d.userId===userId);
  obj[userId] = to_to_user;
}, {});

setTodos((oldTodos) => {...oldTodos, ...newTodos});

Either way, the original line should be

const [todos, setTodos] = useState({});

because you're storing an object of arrays, not an array of objects or and array of arrays

answered Nov 24, 2022 at 23:24

Samathingamajig

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4 Comments

Maxima Over a year ago

Thank you for the answer! What is obj? What does he represent to me?

Samathingamajig Over a year ago

obj is short for object. Just a generic name like key and val (value) that I use when there's not really a good name to use there.

Maxima Over a year ago

Yes, I understood that it is short for object, but I didn't understand, is it an array? object? I get an error like this:

main.js:26 Uncaught (in promise) TypeError: Cannot set properties of undefined (setting '0')     at main.js:26:1     at Array.reduce (<anonymous>)     at main.js:23:1

Maxima Over a year ago

It turns out that the problem was in the obj name... I changed obj1 and everything worked out. Thank you for your quick response!

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