Javascript - How to asynchronously update state using useState before calling a function in React?

nz_19
August 15, 2023
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2 Answers

I’m working on a React application, and I have a situation where I need to update a state variable using useState and then call a function that depends on the updated state. Specifically, I’m looking for a solution where the state update happens asynchronously, and I want to ensure that the state is updated before the function call.

As an example, let’s consider a common use case where I have a login button. When the button is pressed, I need to first update the email and password states and then call a handleLogin function that uses these credentials for authentication.

I’ve tried using async/await with the state update functions, but it doesn’t seem to guarantee that the state updates are completed before the function call.

Here’s what I’ve attempted:

const handleLoginButtonPress = async () => {
  await setEmail(userInfo.email);
  await setPassword(userInfo.password);
  handleLogin();
};
// ...
<button onClick={handleLoginButtonPress}>Login</button>

This approach doesn’t seem to work reliably.

In this case, I want to load the credentials from local storage (though it’s not ideal), then call handleLogin (API call) on the button press event. However, I wanted to know, in general, how to update the state and then perform an action in a single event.

Answers

To ensure asynchronous state updates are completed before calling a function in React:

Use the useState hook to update state variables (email and password
in this case).
Utilize the useEffect hook to listen for changes in the state
variables.
Call the desired function (handleLogin) within the useEffect hook to
ensure it runs after state updates.

By utilizing the useEffect hook, you ensure that the state updates are processed before the subsequent function call.

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

function YourComponent() {
  const [email, setEmail] = useState('');
  const [password, setPassword] = useState('');

  const handleLogin = () => {
    // Authentication logic using email and password
  };

  useEffect(() => {
    handleLogin(); // Executes after email or password changes
  }, [email, password]);

  const handleLoginButtonPress = () => {
    setEmail(userInfo.email);
    setPassword(userInfo.password);
  };

  return (
    <div>
      <input
        type="email"
        value={email}
        onChange={(e) => setEmail(e.target.value)}
      />
      <input
        type="password"
        value={password}
        onChange={(e) => setPassword(e.target.value)}
      />
      <button onClick={handleLoginButtonPress}>Login</button>
    </div>
  );
}

export default YourComponent;

React state updates don’t take effect until the next render. You can set up a useEffect hook to conditionally call your handleLogin function after setting email and password.

You could pull this off with a ref instead of state for the doLogin condition, but here’s a proof-of-concept implementation. When you click the button, email and password state get set, and on the next render you can see via the console that the new state is available for the handleLogin call.

function App () {
  const [email, setEmail] = React.useState('');
  const [password, setPassword] = React.useState('');
  const [doLogin, setDoLogin] = React.useState(false);
  
  React.useEffect(() => {
    if (doLogin) {
      console.log(`do login with "${email}", "${password}"`);
      setDoLogin(false);
    }
  }, [doLogin])
  
  const doLoginStuff = () => {
    setEmail('[email protected]');
    setPassword('correct horse battery staple');
    setDoLogin(true);
  }
  
  return (
      <button onClick={doLoginStuff}>Do Login Stuff</button>
  )
}

const domNode = document.getElementById('app');
const root = ReactDOM.createRoot(domNode);
root.render(<App />);

<script crossorigin src="https://unpkg.com/react@18/umd/react.production.min.js"></script>
<script crossorigin src="https://unpkg.com/react-dom@18/umd/react-dom.production.min.js"></script>

<div id="app"></div>

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Javascript – How to asynchronously update state using useState before calling a function in React?

Answers