Game.js

import React, { Component } from "react";
import Board from "./Board";

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

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

  currentSquares() {
    const history = this.state.history;
    const current = history[this.state.stepNumber];
    return current.squares;
  }

  handleClick(i, isComputer) {
    const history = this.state.history.slice(0, this.state.stepNumber + 1);
    const current = history[history.length - 1];
    const squares = current.squares.slice();
    const winner = calculateWinner(squares);
    //stops player from picking a chosen square
    if (winner || squares[i]) {
      return;
    }
    squares[i] = this.state.xIsNext ? "X" : "0";
    this.setState({
      history: history.concat({
        squares: squares,
      }),
      xIsNext: !this.state.xIsNext,
      stepNumber: history.length,
    });
  }

  render() {
    const history = this.state.history;
    const current = history[this.state.stepNumber];
    const winner = calculateWinner(current.squares);
    const moves = history.map((step, move) => {
      const desc = move
        ? "Go to #" + move
        : "Choose a square to begin the game";
      return (
        <li key={move}>
          <button
            onClick={() => {
              this.jumpTo(move);
            }}
          >
            {desc}
          </button>
        </li>
      );
    });
    let status;
    if (winner) {
      status = "Winner is " + winner;
    } else {
      status = "Next Player is " + (this.state.xIsNext ? "X" : "0");
    }

    return (
      <div className="game">
        <div className="game-board">
          <Board
            onClick={(i) => {
              // Players presses button.
              this.handleClick(i, false);

              // Computer presses button.
              //   const randomIndex = getRandomIndex();
              //   console.log(randomIndex);

              //   console.log(JSON.stringify(this.currentState()));
              //   this.handleClick(randomIndex, true);

              const randomIndex = getRandomIndexNotOccupied(
                this.currentSquares(),
                i
              );
              this.handleClick(randomIndex, true);
              //   console.log(randomIndex);
            }}
            squares={current.squares}
          />
        </div>
        <div>{status}</div>
        <ul>{moves}</ul>
      </div>
    );
  }
}

//sets win condition
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[b] === squares[c]) {
      return squares[a];
    }
  }

  return null;
}

function getRandomIndex() {
  const min = Math.ceil(0);
  const max = Math.floor(9);
  return Math.floor(Math.random() * (max - min)) + min; //The maximum is exclusive and the minimum is inclusive
}

function getRandomIndexNotOccupied(state, index) {
  var randomIndex = null;
  //   console.log(JSON.stringify(state));
  // state[randomIndex]
  console.log("selected ", index);

  do {
    randomIndex = getRandomIndex();
    // console.log(
    //   "state under random index ",
    //   randomIndex,
    //   " is: ",
    //   state[randomIndex]
    // );
    // console.log("getRandom randomIndex ", randomIndex);
    // console.log("whats in the state ", state[randomIndex]);
  } while (randomIndex !== index && state[randomIndex]);

  return randomIndex;
}