ReactJS cannot fetch with state, error: "Cannot read property 'state' of null"

Question

I can access this.state outside of fetch,but when trying to access state inside fetch, I get this error:

Uncaught TypeError: Cannot read property 'state' of null

I looked through other posts on StackOverflow, but didn't find a working solution to use the state properties in the fetch call. Please help.

class SignIn extends React.Component {
  constructor(props){
    super(props);
    this.state = {
      email: '',
      password: ''
    }
    this.handleValueChange = this.handleValueChange.bind(this);
  }

  handleValueChange(event) {
    const property = event.target.name;
    this.setState({[property]: event.target.value});
  }

  handleLogin(event){
    event.preventDefault();

    fetch("http://localhost:3001/login.json", {
        method: "POST",
        headers: {
          'Accept': 'application/json',
          'Content-Type': 'application/json'
        },
        body: JSON.stringify({
          email: this.state.email,  //this is where the error occurs
          password: '123456'
        })
    })
    .then(function(res){return res.json();})
    .then(function(data){console.log(data)});
  }

  render() {
    return(
      <header>
        <NavigationBar />

        <div className="form-box">
          <form onSubmit={this.handleLogin}>
            <h2>Login</h2>
            <br />
            <div className="row">
              <div>
                <label>Email</label>
                <input type="text" name="email" ref='emailInputField' 
                  onChange={(event) => this.handleValueChange(event)}
                  required
                />
              </div>
            </div>
            <br />
            <div className="row">
              <div>
                <label>Password</label>
                <input type="text" name="password"
                  onChange={(event) => this.handleValueChange(event)}
                  required
                />
              </div>
            </div>
            <br />
            <div className="btn-submit">
              <input type="submit" value="Let's go!" />
            </div>
          </form>
          <button onClick={() => console.log("this.state", this.state)} >Show state</button>
        </div>
      </header>
    )
  }
}

NOTE: some code not shown for brevity

Answer 1

Have you tried something like this:

handleLogin = (event) => {
    event.preventDefault();

    fetch("http://localhost:3001/login.json", {
        method: "POST",
        headers: {
          'Accept': 'application/json',
          'Content-Type': 'application/json'
        },
        body: JSON.stringify({
          email: this.state.email,  //this is where the error occurs
          password: '123456'
        })
    })
    .then((res) => {return res.json();})
    .then((data) => {console.log(data)});
  }

Or just bind your function to your component like suggested in the comments and like you already did with your other function. Arrow functions are a lot smoother though.

Answer 2

Add this.handleLogin in the constructor like below.

 constructor(props){
    super(props);
    this.state = {
      email: '',
      password: ''
    }
    this.handleValueChange = this.handleValueChange.bind(this);
    this.handleLogin = this.handleLogin.bind(this)
  }

Answer 3

I think the following should work.

In the line above fetch(...) add const self = this; and call self.state.email.

Let me know if it does.