Need to Execute Function before render() in ReactJS

I've created a login system with React which stores a session when the user logs in. When the page is reloaded, I have added a function which should check if the session exists and then either setState() to true or to false.

As I'm new to React, I'm not sure how to execute this function. Please see my code below for App.js:

import React from 'react';
import './css/App.css';
import LoginForm from "./LoginForm";
import Dashboard from "./Dashboard";

class App extends React.Component {

    constructor(props) {
        super(props)
        this.state = {
            renderLoginForm: true
        };
        this.handleLoginFormMount = this.handleLoginFormMount.bind(this);
    }

    handleLoginFormMount() {
        this.setState({
            renderLoginForm: false
        });
    }

    // Check session function.
    checkSession() {
        fetch('/check-session', {
            credentials: 'include'
        })
        .then((response) => {
            return response.json();
        })
        .then((sessionResult) => {
            if (sessionResult.username) {
                console.log('false');
                this.setState({
                    renderLoginForm: false
                });
            } else {
                console.log('true');
                this.setState({
                    renderLoginForm: true
                });
            }
        })
        .catch((error) => {
            console.log('Error: ', error);
        });
    }

    render() {

        checkSession();

        return (
            <div className="App">
                {this.state.renderLoginForm ? <LoginForm mountLoginForm={this.handleLoginFormMount} /> : null}
                {this.state.renderLoginForm ? null : <Dashboard />}
            </div>
        );
    }
}

export default App;

Having checkSession() in this position outputs the following in the console when loading the page:

Line 50: 'checkSession' is not defined no-undef

If I put the function outside of the class App extends React.Component {}, then it tells me that I cannot set the state of undefined.

Having checkSession() in this position outputs the following in the console when loading the page:

Line 50:  'checkSession' is not defined  no-undef

That's because it's a method, but you're calling it like a freestanding function. The call should be this.checkSession();. But keep reading.

Separately:

The render function must be pure, it cannot have side-effects like changing state. Instead, put any side-effects code in componentDidMount; from the documentation for that lifecycle method:

If you need to load data from a remote endpoint, this is a good place to instantiate the network request.

Be sure that your component renders correctly for the original state (before the session check), as well as for the updated state (after the session check).

More about lifecycle methods and such in the documentation.

Alternately, if this component can't do anything useful without the session, you might move the session check to its parent component, and have the parent only render this child component when it has the session check results.