React JS : this.setState and JSON.parse

Question

I have a problem, I don't know how I can fix it. I am doint a request to my api and it responds with an object like this :

{"_id":"5a806648300caf0072c7254a","mail":"[email protected]"}

I want to print to my react component the mail. In my "componentDidMount()" function I get the mail, so there is no problem with my api. But when I want to set the state of my "users" variable, I can not print the variable in my component... It says that it's undefined.

Here is my code :

class UserList extends Component {
    constructor(props){
        super(props);
        this.state = {users:[]}
    }
    componentDidMount() {
        //here my function that store the response from my api
        GetData('http://localhost:3333/users', 'username=admin', 
        (res) => 
        {
            this.setState({users: JSON.parse(res)});
            //Here I can log the mail
            console.log(this.state.users[0].mail);
        })
    }
    render() {
        return (
            <table>
                <thead>
                <tr>
                    <th>EMAIL</th>
                </tr>
                </thead>
                <tbody>
                    <tr>
                        //Here it is 'undefined'
                        <td>{this.state.users[0].mail}</td>
                    </tr>
                </tbody>
            </table>
        );
    }
}

Thank you for your help !

Answer 1

React mounting component lifecycle is:

• constructor
• componentWillMount
• render
• componentDidMount

See more here: https://reactjs.org/docs/react-component.html#mounting

When your component is mounting this.state.users is an empty array.

You can change the render method:

class UserList extends Component {
    constructor(props){
        super(props);
        this.state = {users:[]}
    }
    componentDidMount() {
        //here my function that store the response from my api
        GetData('http://localhost:3333/users', 'username=admin', 
        (res) => 
        {
            // After setState react runs updating lifecycle methods (https://reactjs.org/docs/react-component.html#updating)
            this.setState({users: JSON.parse(res)});
            //Here I can log the mail
            console.log(this.state.users[0].mail);
        })
    }
    render() {
        // if users are not loaded, we return null from render function
        if (this.state.users.length === 0) {
            return null;
        }
        
        return (
            <table>
                <thead>
                <tr>
                    <th>EMAIL</th>
                </tr>
                </thead>
                <tbody>
                    <tr>
                        <td>{this.state.users[0].mail}</td>
                    </tr>
                </tbody>
            </table>
        );
    }
}