Posted on Mar 24, 2022 • Edited on Apr 2, 2023 • Originally published at w3courses.org

How to validate form in ReactJS?

In this tutorial we will use props, state, constructor, events in reactjs.

First of all create your component like Signup and extend it from Component Class (Import Component From react).

Now, Add Constructor to our component.
Create a function handleChange that will trigger on change of form field.
Create a function handleSubmit that will trigger on form submit.
Show the errors on Template inside render method and Create Form.

Here below is complete code.

import { Component } from "react"; class Signup extends Component { constructor(props) { super(props); this.state = { fullName: null, email: null, password: null, errors: { fullName: '', email: '', password: '', } }; } handleChange = (event) => { event.preventDefault(); const validEmailRegex = RegExp(/^(([^<>()\[\]\.,;:\s@\"]+(\.[^<>()\[\]\.,;:\s@\"]+)*)|(\".+\"))@(([^<>()[\]\.,;:\s@\"]+\.)+[^<>()[\]\.,;:\s@\"]{2,})$/i); const { name, value } = event.target; let errors = this.state.errors; switch (name) { case 'fullName': errors.fullName = value.length < 5 ? 'Full Name must be 5 characters long!' : ''; break; case 'email': errors.email = validEmailRegex.test(value) ? '' : 'Email is not valid!'; break; case 'password': errors.password = value.length < 8 ? 'Password must be 8 characters long!' : ''; break; default: break; } this.setState({ errors, [name]: value }, () => { console.log(errors) }) } handleSubmit = (event) => { event.preventDefault(); const validateForm = (errors) => { let valid = true; Object.values(errors).forEach( // if we have an error string set valid to false (val) => val.length > 0 && (valid = false) ); return valid; } if (validateForm(this.state.errors)) { console.info('Valid Form') } else { console.error('Invalid Form') } } render() { console.log(this.props); return ( <div> <form onSubmit={(event) => this.handleSubmit(event)}> <input type="text" name="fullName" onChange={(event) => this.handleChange(event)} />  {errors.fullName.length > 0 && <span className='error'>{errors.fullName}</span>}  <input type="email" name="email" onChange={(event) => this.handleChange(event)}/>  {errors.email.length > 0 && <span className='error'>{errors.email}</span>}  <input type="password" name="password" onChange={(event) => this.handleChange(event)}/>  {errors.password.length > 0 && <span className='error'>{errors.password}</span>}  <input type="submit" name="submit" /> </form>  </div>  ); } } export default Signup;