By the time the user clicks on the button and your handleSubmit fires, the React component has long since finished rendering, so returning a redirect JSX from the handler won't really have any effect.

If your app architecture supports it (i.e. this isn't an SPA) consider just doing a plain old window.location.reload to send the user to the right place.

If not (and I guess you said this is an SPA), you will have to be clever about how to get the <Redirect> to get acted upon in the next render. React state might be a good start here... in your else clause, instead of returning the redirect, set some state variable that your render method could check for, and render the <Redirect> instead. (Your component is guaranteed to re-render if you change any of its state.)

In other words, modify your render method to either return what it already does, or a <Redirect>, depending on the value of a state variable. You can initialize the state to null and then have your handleSubmit method alter it to, say, the URL you want to redirect to. Then the flow is: render the login page, user provides bad data, your handleSubmit sets state saying so, you re-render now as a <Redirect> component.

Since you are using react-router I suggest you use the history api to push to the next route.

You can use the withRouter HOC to provide the history api to your props.

import { withRouter } from "react-router";


handleSubmit() {
  let usernameInput = document.getElementById('customerLoginUsername');
  let passwordInput = document.getElementById('customerLoginPassword');

  if (usernameInput.value !== 'customer' || passwordInput.value !== 'customer') {
    // alert("Wrong username or password!");
  } else {
    this.props.history.push('/customerhomepage')
  }
}

render() {
  return (
    <HashRouter>
      <Switch>
        <Route exact path="/main" component={Main}></Route>
        <Route exact path="/customerhomepage" component={CustomerHomePage}></Route>
      </Switch>
      <div class="App-header">
        <NavLink to="/main">
          <button id="backButton">Go Back</button>
        </NavLink>
        <h4>Login with your customer account</h4>
        <br />
        <input id="customerLoginUsername" placeholder=" Username"></input>
        <input id="customerLoginPassword" type="password" placeholder=" Password"></input>
        <br />
        <button id="customerLoginButton" onClick={this.handleSubmit}>
          Login
        </button>

        <div className="content"></div>
      </div>
    </HashRouter>
  );
}

export default withRouter(CustomerLoginPage);

Edit:

As other have pointed out, you will need to raise the HashRouter component to a higher component, in order for withRouter to work.

Issue

1. You can't return JSX from an asynchronous callback and expect it to be rendered and take effect. The Redirect must be rendered in the return of the component.

2. The reason using the history object wasn't working is because the handleSubmit handler is outside the router providing any routing context to navigate against.

Solution

1. Use the history object to issue an imperative redirect.
2. Move the HashRouter to wrap App and provide a routing context for the history object. In fact, you should have only one router wrapping your app to provide the routing context, so remove all other extraneous routers you may have in your app.

Example:

index.js

import { HashRouter as Router } from 'react-router-dom';

...

ReactDOM.render(
  <React.StrictMode>
    <Router>
      <App/>
    </Router>
  </React.StrictMode>,
  document.getElementById('root')
);

CustomerHomePage

handleSubmit() {
  const usernameInput = document.getElementById("customerLoginUsername");
  const passwordInput = document.getElementById("customerLoginPassword");

  if (usernameInput.value !== "customer" || passwordInput.value !== "customer") {
    // alert("Wrong username or password!");
  } else {
    this.props.history.replace('/customerhomepage');
  }
}