React function expression 'this' undefined

Question

Can someone briefly explain why this keyword is undefined in a function expression, but defined in an arrow function. I'm simply trying to follow the tutorial on the React website and they gloss over a seemingly important concept regarding confusing behavior of this but the link refers to a very in-depth article which is not at all friendly to beginners. After completing the tutorial I wanted to play around and try to understand what works and what doesn't and I tried replacing an arrow function with a function expression, and suddenly the onClick event failed to work.

This is the code as written in the tutorial:

class Board extends React.Component {
    renderSquare(i) {
        return (
            <Square
                value={this.props.squares[i]}
                onClick={() => {
                    this.props.onClick(i);
                }} />
        );
    }
//other stuff
}

And I wanted to change the arrow function to a traditional function expression, but had some problems

class Board extends React.Component {
    renderSquare(i) {
        return (
            <Square
                value={this.props.squares[i]}
                onClick={function() {
                    console.log(this);          //undefined
                    console.log(this.props);    //throws exception
                    this.props.onClick(i);      //throws exception
                }} />
        );
    }
}

Why is this undefined in the function expression? When I read the documentation for arrow function and function expressions on MDN it seems that the opposite should be true. What's going on here?

Here the codepen Here's the full project:

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';

function Square(props) {
    return (
        <button className="square" onClick={props.onClick}>
            {props.value}
        </button>
    );
}

class Board extends React.Component {
    renderSquare(i) {
        return (
            <Square
                value={this.props.squares[i]}
                onClick={() => this.props.onClick(i) } />
        );
    }

    render() {
        return (
            <div>
                <div className="board-row">
                    {this.renderSquare(0)}
                    {this.renderSquare(1)}
                    {this.renderSquare(2)}
                </div>
                <div className="board-row">
                    {this.renderSquare(3)}
                    {this.renderSquare(4)}
                    {this.renderSquare(5)}
                </div>
                <div className="board-row">
                    {this.renderSquare(6)}
                    {this.renderSquare(7)}
                    {this.renderSquare(8)}
                </div>
            </div>
        );
    }
}

class Game extends React.Component {
    constructor(props) {
        super(props);
        this.state = {
            isX: true,
            history: [{
                squares: Array(9).fill(null)
            }],
            stepNumber: 0
        }
    }

    handleClick(i) {
        const history = this.state.history.slice(0, this.state.stepNumber + 1);
        const current = this.state.history[history.length - 1];
        const squares = current.squares.slice();
        if (squares[i] == null) {
            squares[i] = this.state.isX ? 'X' : 'O';
        } else {
            console.log("Square [" + i + "] is already marked!");
            return;
        }
        this.setState({
            history: history.concat([{squares: squares}]),
            isX: !this.state.isX,
            stepNumber: history.length
        });
    }

    jumpTo(step) {
        this.setState({
            stepNumber: step,
            xIsNext: (step % 2) === 0,
        });
    }

    render() {
        const history = this.state.history;
        const current = this.state.history[this.state.stepNumber];
        const winner = calculateWinner(current.squares);

        const moves = history.map((step, move) => {
            const desc = move ?
                'Go to move #' + move :
                'Go to game start';
            return (
                <li key={move}>
                    <button onClick={() => this.jumpTo(move)}>{desc}</button>
                </li>
            );
        });

        let status;
        if (winner) {
            status = 'Winner: ' + winner;
        } else {
            status = 'Next player: ' + (this.state.isX ? 'X' : 'O');
        }
        return (
            <div className="game">
                <div className="game-board">
                    <Board
                        squares={current.squares}
                        onClick={(i) => this.handleClick(i)}/>
                </div>
                <div className="game-info">
                    <div>{status}</div>
                    <ol>{moves}</ol>
                </div>
            </div>
        );
    }
}

function calculateWinner(squares) {
    const lines = [
        [0, 1, 2],
        [3, 4, 5],
        [6, 7, 8],
        [0, 3, 6],
        [1, 4, 7],
        [2, 5, 8],
        [0, 4, 8],
        [2, 4, 6],
    ];
    for (let i = 0; i < lines.length; i++) {
        const [a, b, c] = lines[i];
        if (squares[a] && squares[a] === squares[b] && squares[a] === squares[c]) {
            return squares[a];
        }
    }
    return null;
}

// ========================================

ReactDOM.render(
    <Game />,
    document.getElementById('root')
);