How to get data from parsedJSON reactJS

I'm setting up a basic WebApp here using ReactJS. Right now I can add data to my database using POST request no problem, the thing, my backend response sends me a JSON with all the data that I passed and the _id in the database. I need to get this _id and save it in my state so I can pass it along to the next URL in my WebApp. That's the code for my POST request :

  SubmitClick(){

    if (this.state.password !== this.state.reTypepassword){
      alert('Passwords do not match. Please check your data !');
    } else {
      //console.log(this.state); //debug only
      fetch('http://localhost:4000/users/', {
        method: 'POST',
        headers: {
          'Accept': 'application/json',
          //'Authorization': 'Basic YWRtaW46c3VwZXJzZWNyZXQ=',
          'Content-Type': 'application/json',
        },
        body: JSON.stringify({
          email: this.state.email,
          first_name: this.state.first_name,
          last_name: this.state.last_name,
          personal_phone: this.state.personal_phone,
          password: this.state.password
        })
      })
      .then(response => response.json())
      .then(parsedJSON => console.log(parsedJSON._id))
      .catch(error => alert('Check your data', error))
      .then(this.props.history.push('/get')) // change page layout and URL
    }
    console.log(this.state.id); //debug to see if the _id is saved in my state
  }

Here is my constructor:

  constructor(props){
    super(props);
    this.state={
      email:'',
      first_name:'',
      last_name:'',
      personal_phone:'',
      password:'',
      reTypepassword:'',
      id:''
    }
  }

I tried calling a function after parsedJSON that used this.setState(), using function(parsedJSON){this.state.id : parsedJSON._id}. I tried with a new function like this:

  changeID(parsedJSON){
    this.setState({id : parsedJSON._id})
  }

and changing the .then(parsedJSON => console.log(parsedJSON._id)) to .then(parsedJSON => this.cahngeID(parsedJSON)). But none of then worked...

I left the code with .then(parsedJSON => console.log(parsedJSON._id)) so can make sure that I can see this value, and in my console it`s printed perfectly.

Here is a example of the response send by my backend: {"email":"[email protected]","first_name":"TESTER","last_name":"Testing","personal_phone":"(55) 2020-5252","password":"12345","_id":"5a27f511cd7d7a0db8ab65b9"}

How can I get the "_id" from my response?

Solution

You should not touch the this.state property directly. React will throw an error if you do this because React needs to know when this.state has been updated so that it can keep track of the changes. It cannot do this if you manipulate the property directly. That's why we have React.Component#setState. The "shallow" version is the most commonly used, where you pass an object that will be merged into your state. For example, the following:

.then(parsedJSON => this.setState({ id: parsedJSON._id }))

is equivalent to:

Object.assign(this.state, { id: parsedJSON._id });

except that React gets to keep track of when the state is updated.

Note that setState is also asynchronous and takes a callback as the second parameter. You can read more about that here.

As per the comment below, this other Stack Overflow question was helpful to the OP (Why calling react setState method doesn't mutate the state immediately?)