Using useEffect in React to change API request

I am learning React hooks and made a project to display a random Pokemon name through https://pokeapi.co/docs/v2#pokemon by calling a random Pokemon ID. I'm not quite sure how to trigger the useEffect to fetch after clicking on the button.

import './App.css';
import { useState, useEffect } from 'react';


  function getNewPokemonId(pokemon){
  pokemon =  Math.floor(Math.random() * 1011);
  console.log(pokemon)
  return pokemon;
}

function App() {
  const [pokemon , setNewPokemon] = useState(1);


  useEffect(() => {
    console.log("render");
    const pokemonID = getNewPokemonId()
    fetch(`https://pokeapi.co/api/v2/pokemon/${pokemonID}`)
    .then((response) => response.json())
    .then((json) =>
      setNewPokemon(json)
  );
  
  }, []);


  return (
    <main>
      <h1>
        Random Pokemon Generator
      </h1>
      <section>
        <button onClick={() => setNewPokemon(getNewPokemonId(pokemon))}>
          New Pokemon
        </button>
        <h3>{pokemon?.name}</h3>
      </section>

    </main>
  );
}

export default App;

After clicking on the button I don't get a new Pokemon name instead it is blank.

You can define a function to fetch the data for a Pokemon by id and call it on mount and in the click event handler.

function App() {
  const [pokemon, setNewPokemon] = useState();
  const getPokemon = id => fetch(`https://pokeapi.co/api/v2/pokemon/${id}`).then(res => res.json()).then(d => setNewPokemon(d));
  useEffect(() => {
    console.log("render");
    getPokemon(getNewPokemonId());
  }, []);

  return (
    <main>
      <h1>Random Pokemon Generator</h1>
      <section>
        <button onClick={() => getPokemon(getNewPokemonId())}>
          New Pokemon
        </button>
        <h3>{pokemon?.name}</h3>
      </section>
    </main>
  );
}

I created a sandbox with this app working:

Sandbox https://codesandbox.io/s/holy-morning-tkwtpv

import "./styles.css";
import { useState, useEffect } from "react";

function getNewPokemonId() {
  const pokemon = Math.floor(Math.random() * 1011);
  console.log(pokemon);
  return pokemon;
}

export default function App() {
  const [pokemon, setNewPokemon] = useState(1);

  const fetchPokemon = async () => {
    const response = await fetch(
      `https://pokeapi.co/api/v2/pokemon/${getNewPokemonId()}`
    );
    const data = await response.json();
    setNewPokemon(data);
  };

  useEffect(() => {
    fetchPokemon();
  }, []);

  const handleOnClick = () => {
    fetchPokemon();
  };

  return (
    <div className="App">
      <h1>Random Pokemon Generator</h1>
      <section>
        <button onClick={handleOnClick}>New Pokemon</button>
        <h3>{pokemon?.name}</h3>
      </section>
    </div>
  );
}

I hope that this could help

having [] at the end of the useEffect will mean it will only trigger once at the start of the page load.
having this: [pokemon] will make the useEffect run every time the variable changes.

It' not fool-proof, but it's a start. You should store the Pokémon data into the state, not the ID. Make sure you properly set the state as well.

Note

If you notice, the IDs jump from 1010 to 10001.

So the random range should technically be: 1-1010, 10001-10271.

Example

Here is an basic working example of a Pokémon randomizer. It's fairly responsive.

<!-- begin snippet: js hide: false console: true babel: true -->

<!-- language: lang-js -->

const { useCallback, useEffect, useState } = React;
// Better PRNG than Math.random
// Source: https://stackoverflow.com/a/47593316/1762224
const mulberry32 = (seed) =>
() => {
let t = seed += 0x6D2B79F5;
t = Math.imul(t ^ t >>> 15, t | 1);
t ^= t + Math.imul(t ^ t >>> 7, t | 61);
return ((t ^ t >>> 14) >>> 0) / 4294967296;
}
const rand = mulberry32(performance.now()); // Seeded with current time
const randomInt = (min, max) => Math.floor(rand() * (max - min + 1)) + min;
const toTitleCase = (str = '') =>
str.replace(/\w\S*/g, (txt) =>
txt.charAt(0).toUpperCase() + txt.substr(1).toLowerCase());
// Refer to: https://pokeapi.co/api/v2/pokemon?offset=0&limit=2000
const ranges = [[1, 1010], [10001, 10271]]; // Distribution: (1010, 271)
const getRandomId = () => {
const which = randomInt(1, 5);
const range = ranges[which === 5 ? 1 : 0]; // 20% for range #2
return randomInt(...range);
};
const apiBaseUrl = 'https://pokeapi.co/api/v2';
const App = () => {
const [pokemonData, setPokemonData] = useState({ id: 0, name: '', sprites: [] });
const [loading, setLoading] = useState(false);
const [errorText, setErrorText] = useState('');
const randomize = useCallback(() => {
setErrorText('');
setLoading(true);
const randId = getRandomId();
fetch(`${apiBaseUrl}/pokemon/${randId}`)
.then(res => res.json())
.then(data => {
setPokemonData(data);
})
.catch(() => {
setErrorText(`Pokemon with ID of ${randId} not found!`);
})
.finally(() => setLoading(false));
}, []);
useEffect(() => {
randomize();
}, []);
return (
<main>
<h1>Random Pokemon Generator</h1>
<section>
<button disabled={loading} onClick={randomize}>New Pokemon</button>
{errorText !== '' && <p style={{color:'red'}}>{errorText}</p>}
<h3>[{pokemonData.id}] {toTitleCase(pokemonData.name)}</h3>
<img src={pokemonData.sprites.front_default} />
</section>
</main>
);
};
ReactDOM.createRoot(document.getElementById("root")).render(<App />);

<!-- language: lang-html -->

<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/18.2.0/umd/react.development.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/18.2.0/umd/react-dom.development.js"></script>

<!-- end snippet -->