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HTML

              
                <body>
    <div id="root"></div>
</body>
              
            
!

CSS

              
                html {
  box-sizing: border-box;
}

html * {
  box-sizing: inherit;
}
body {
  background-color: grey;
}

#root {
  display: inline-block;
}
.main {
  position: relative;
  min-width: 300px;
  margin: 0 auto;
  border-radius: 10px;
  border: solid silver;
  display: grid;
  grid: "info info"
        "options board";
  grid-auto-columns: min-content;
  background-color: black;
}
.info {
  position: relative;
  color: greenyellow;
  grid-area: info;
  min-height: 50px;
  min-width: 400px;
}

.board {
  box-sizing: content-box;
  position: relative;
  border: solid green;
  border-radius: 5px;
  align-self: center;
  grid-area: board;
  display: flex;
  flex-wrap: wrap;
  min-height: 10px;
}

.filling {
  cursor: url("https://github.com/DavidKrpt/react-game-of-life/blob/master/src/images/pencil2.png?raw=true"), pointer
}
.erasing {
  cursor: url("https://github.com/DavidKrpt/react-game-of-life/blob/master/src/images/eraser2.png?raw=true"), pointer
}

.controls {
  color: greenyellow;
  align-self: start;
  width: 200px;
  grid-area: options;
  flex-basis: 200px;
  background-color: black;
  border: solid green;

}
.info__heading {
  max-width: 1000px;
  font-size: 30px
}
.heading {
  text-align: center;
}
.info__body {
  font-size: 20px;
  display: flex;
  max-width: 1000px;
  justify-content: space-around;
}

.controls__buttons {
  display: flex;
  height: 150px;
  padding: 0 20px;
  flex-direction: column;
  justify-content: space-evenly;
  box-shadow: none;
}

.button {
  border-radius: 7px;
  font-size: 20px;
  height: 40px;
  font-weight: bold;
  border: none;
  box-shadow: 2px 2px green;
  cursor: pointer;
}

.message {
  color: lime;
  background-color: #071305;
  position: absolute;
  max-width: 600px;
  max-height: 100px;
  left: 205px;
  top: 147px;
  transition-duration: 500ms;
  border-right: dotted lime;
  border-bottom: dotted lime;
}

.message_fading {
  opacity: 0;
}
.message_hidden {
  visibility: hidden;
}

.options {
  display: flex;
  flex-direction: column;
  height: 300px;
  justify-content: space-evenly;
} 

.stats__value {
  display: inline-block;
  font-family: monospace;
  font-weight: bold;
  width: 100px;
}

.option {
  margin-left: 10%;
}

.option__label {
  display: block;
  font-size: 20px;
}

.option__input {
  width: 80%;
  height: 30px;
  font-size: 25px;
}
.help__item {
  cursor: pointer;
}

.tool {
  font-size: 20px;
  margin-left: 20px;
}
.square {
  box-sizing: border-box;
  border-top: solid 1px green;
  border-left: solid 1px green;
}
.square_type-life {
  background-color: lime;
}

.square_type-death {
  background-color: #071305;
}

.link:visited {
  color: green;
}
              
            
!

JS

              
                class App extends React.Component {
  constructor() {
    super();
    this.state = {
      rows: 30,
      cols: 30,
      squareSize: 20,
      squares: makeFalsyArray(30, 30),
      intervalId: null,
      genNumber: 1,
      deathCount: 0,
      tool: "filler",
      message: {
        text: null, //2 timeouts for smooth messages
        timer1: null, //set opacity to 0
        timer2: null //remove from the DOM
      }
    };
    this.isDown = false;
  }

  displayMessage = (text, timer) => {
    const delay = this.hideMessage() ? 500 : 0; //remove previous message
    setTimeout(() => {
      this.setState({
        message: {
          text: text,
          timer1: setTimeout(this.hideMessage, timer),
          timer2: null
        }
      });
    }, delay);
  };

  hideMessage = () => {
    if (this.state.message.text) {
      clearTimeout(this.state.message.timer1);
      this.setState({
        message: {
          text: this.state.message.text,
          timer1: null,
          timer2: setTimeout(
            () =>
              this.setState({
                message: { text: null, timer1: null, timer2: null }
              }),
            500
          )
        }
      });
      return true;
    }
  };

  handleSizeChange = event => {
    const newSize = validateNumber(event.target.value) || 1;
    this.setState({
      squareSize: newSize
    });
  };

  handleHelpClick = () => {
    const text = `Click and drag or hit randomize to populate the board and press play.
    Bright green color represents live cells. If a cell has more than 3 or
    less than 2 neighbours it dies. If a dead cell has exactly 3 live
    neighbours, it becomes alive.`;
    if ((this.state.message.text === text)) return this.hideMessage();
    return this.displayMessage(text, 10000);
  };

  handleRowChange = event => {
    const rows = validateNumber(event.target.value) || 1;
    const newSquares = makeFalsyArray(rows, this.state.cols);
    this.handleReset();
    this.setState({
      rows: rows,
      squares: newSquares
    });
  };

  handleColChange = event => {
    const cols = validateNumber(event.target.value) || 1;
    const newSquares = makeFalsyArray(this.state.rows, cols);
    this.handleReset();
    this.setState({
      cols: cols,
      squares: newSquares
    });
  };

  handleReset = () => {
    const { rows, cols } = this.state;
    const newSquares = makeFalsyArray(rows, cols);
    clearInterval(this.state.intervalId);
    this.setState({
      squares: newSquares,
      genNumber: 1,
      deathCount: 0,
      intervalId: null
    });
  };
  handleToolChange = (event) => {
    this.setState({
      tool: event.target.value
    })
  }
  handleMouseMove = (i, j) => {
    if (!this.isDown) return;
    if ((this.state.squares[i][j] && this.state.tool === "filler")||
    (!this.state.squares[i][j] && this.state.tool === "eraser")) return;
    const squares = copyArray(this.state.squares);
    squares[i][j] = this.state.tool ==="filler";
    this.setState({
      squares: squares
    });
  };
  handleClick = (i,j) => {
    this.isDown = true;
    this.handleMouseMove(i,j);
    this.isDown = false

  }
  handleStartPause = () => {
    let intervalId;
    if (this.state.intervalId) {
      clearInterval(this.state.intervalId);
      intervalId = null;
    } else {
      if (arrIsFalsy(this.state.squares))
        return this.displayMessage(
          "There are no live cells, try randomizing or drawing some",
          2000
        );
      intervalId = setInterval(this.makeNextGeneration, 100);
    }
    this.setState({
      intervalId: intervalId
    });
  };

  handleRandomize = () => {
    const randomSquares = [];
    randomSquares.length = this.state.rows;
    for (let i = 0; i < this.state.squares.length; i++) {
      randomSquares[i] = [];
      randomSquares[i].length = this.state.cols;
      for (let j = 0; j < this.state.squares[0].length; j++) {
        randomSquares[i][j] = Math.random() > 0.7;
      }
    }
    this.setState({
      squares: randomSquares
    });
  };

  handleMouseDown = () => {
    this.isDown = true;
  };

  handleMouseUp = () => {
    this.isDown = false;
  };

  makeNextGeneration = () => {
    const currentSquares = this.state.squares;
    if (
      currentSquares.filter(
        row => row.filter(square => square === true).length > 0
      ).length === 0
    ) {
      this.displayMessage("Everyone died on day " + this.state.genNumber, 2000);
      return this.handleStartPause();
    }
    const nextSquares = makeFalsyArray(this.state.rows, this.state.cols);
    let deathCount = this.state.deathCount;
    for (let i = 0; i < currentSquares.length; i++) {
      for (let j = 0; j < currentSquares[i].length; j++) {
        let neighbourNum = currentSquares[i][j] ? -1 : 0;
        for (let m = i - 1; m <= i + 1; m++) {
          if (m >= 0 && m < currentSquares.length) {
            for (let n = j - 1; n <= j + 1; n++) {
              if (n >= 0 && n < currentSquares[i].length) {
                if (currentSquares[m][n]) {
                  neighbourNum++;
                }
              }
            }
          }
        }
        if (currentSquares[i][j]) {
          if (neighbourNum > 1 && neighbourNum < 4) {
            nextSquares[i][j] = true;
          } else {
            deathCount++;
          }
        } else if (neighbourNum === 3) {
          nextSquares[i][j] = true;
        }
      }
    }
    this.setState({
      squares: nextSquares,
      genNumber: this.state.genNumber + 1,
      deathCount: deathCount
    });
  };

  render() {
    return (
      <main className="main">
        <Info
          onHelpClick={this.handleHelpClick}
          genNumber={this.state.genNumber}
          deathCount={this.state.deathCount}
        />
        <Controls
          rows={this.state.rows}
          cols={this.state.cols}
          intervalId={this.state.intervalId}
          squareSize={this.state.squareSize}
          tool = {this.state.tool}
          onStartPause={this.handleStartPause}
          onRandomize={this.handleRandomize}
          onReset={this.handleReset}
          onRowChange={this.handleRowChange}
          onColChange={this.handleColChange}
          onSizeChange={this.handleSizeChange}
          onToolChange = {this.handleToolChange}
        />
        <Board
          tool = {this.state.tool}
          squareSize={this.state.squareSize}
          squares={this.state.squares}
          onMouseUp={this.handleMouseUp}
          onMouseMove={this.handleMouseMove}
          onClick={this.handleClick}
          onMouseDown={this.handleMouseDown}
        />
        <Message message={this.state.message} />
      </main>
    );
  }
}

function copyArray(array) {
  //deep copy array of arrays
  const copy = [];
  const [rows, cols] = [array.length, array[0].length];
  copy.length = rows;
  for (let i = 0; i < rows; i++) {
    copy[i] = [];
    copy[i].length = cols;
    for (let j = 0; j < cols; j++) {
      copy[i][j] = array[i][j];
    }
  }
  return copy;
}

function validateNumber(num) { // 101>num>0
  if (!Number(num)) return false;
  num = Math.round(Number(num));
  return Math.max(Math.min(Number(num), 100), 1);
}

function arrIsFalsy(arr) {
  //checks if there are no true values in nested arrays
  for (let i = 0; i < arr.length; i++) {
    for (let j = 0; j < arr.length; j++) {
      if (arr[i][j]) return false;
    }
  }
  return true;
}

function makeFalsyArray(rows, cols) {
  //nested arrays of false values
  const arr = [];
  arr.length = rows;
  for (let i = 0; i < rows; i++) {
    arr[i] = [];
    arr[i].length = cols;
    for (let j = 0; j < cols; j++) {
      arr[i][j] = false;
    }
  }
  return arr;
}
const Board = props => {
  const squares = props.squares;
  const [rows, cols] = [squares.length, squares[0].length];
  const squareElements = Array(rows * cols);
  for (let i = 0; i < rows; i++) {
    for (let j = 0; j < cols; j++) {
      const className =
        "square square_type-" + (squares[i][j] ? "life" : "death");
      squareElements[i * cols + j] = (
        <div
          key={"square" + i + "-" + j}
          className={className}
          style={{ width: props.squareSize, height: props.squareSize }}
          onMouseMove={() => props.onMouseMove(i, j)}
          onClick={() => props.onClick(i, j)}
        />
      );
    }
  }
  return (
    <div
      className={"board " + (props.tool === "eraser" ? "erasing" : "filling")}
      style={{ width: props.squares[0].length * props.squareSize + "px" }}
      onMouseDown={event => {
        event.preventDefault();
        return props.onMouseDown();
      }}
      onMouseUp={props.onMouseUp}
      onMouseLeave={props.onMouseUp}
    >
      {squareElements}
    </div>
  );
};
const Buttons = props => {
  return (
    <div className="controls__buttons">
      <button className="button" onClick={props.onStartPause}>
        {props.playOrPause}
      </button>
      <button className="button" onClick={props.onRandomize}>Randomize</button>
      <button className="button" onClick={props.onReset}>Reset</button>
    </div>
  );
};
const Controls = props => {
  return (
    <section className="controls">
      <Buttons
        playOrPause = {props.intervalId ? "Pause" : "Play"}
        onStartPause={props.onStartPause}
        onRandomize={props.onRandomize}
        onReset = {props.onReset}
      />
      <Options
        rows = {props.rows}
        cols = {props.cols}
        squareSize = {props.squareSize}
        onRowChange={props.onRowChange}
        onColChange={props.onColChange}
        onSizeChange={props.onSizeChange}
      />
      <Tools tool = {props.tool} onToolChange = {props.onToolChange} />
    </section>
  );
};
const Info = props => {
  return (
    <section className="info">
      <h1 className="heading info__heading">Conway's Game Of Life</h1>
      <div className="info__body">
        <div className="info__section stats">
          <div className="stats__label">
            Day: <span className="stats__value">{props.genNumber}</span>{" "}
          </div>
          <div className="stats__label">
            Death count:{" "}
            <div className="stats__value">{props.deathCount}</div>{" "}
          </div>
        </div>
        <div className="info__section help">
          <span onClick={props.onHelpClick} className="help__item rules">
            Help
          </span>
          <div className="help__item more">
            <a
              className="link"
              target="_blank"
              href="https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life"
            >
              More Info (Wikiedia)
            </a>
          </div>
        </div>
      </div>
    </section>
  );
};
const Message = props => {
    let className = "";
    console.log(props.message)
    if (!props.message.timer1) {
        if (!props.message.timer2) className = " message_hidden"
        className = " message_fading"
    }
  return (
    <div className={"message" + className}>
      {props.message.text}
    </div>
  );
};
const Options = props => {
  return (
    <section className="options">
    <h2 className="heading options__heading">Options</h2>
      <div className="option">
        <label className="option__label" htmlFor="rowNum">
          Rows
        </label>
        <input
          className="option__input"
          id="rowNum"
          type="number"
          name="rows"
          min="1"
          max="100"
          value={props.rows}
          onChange={props.onRowChange}
        />
      </div>
      <div className="option">
        <label className="option__label" htmlFor="colNum">
          Columns
        </label>
        <input
          className="option__input"
          id="colNum"
          type="number"
          name="cols"
          min="1"
          max="100"
          value={props.cols}
          onChange={props.onColChange}
        />
      </div>
      <div className="option">
        <label className="option__label" htmlFor="squareSize">
          Cell size (px)
        </label>
        <input
          className="option__input"
          id="squareSize"
          type="number"
          name="size"
          min="1"
          value={props.squareSize}
          onChange={props.onSizeChange}
        />
      </div>
    </section>
  );
};
const Tools = props => {
  return (
    <div className="tools">
      <h2 className="heading options__heading">Tools</h2>
      <div className="tool">
        <div className="tool__option">
          <input
            id="filler"
            type="radio"
            name="tool"
            value="filler"
            checked={props.tool==="filler"}
            onChange={props.onToolChange}
          />
          <label htmlFor="filler">Filler</label>
        </div>
        <div className="tool__option">
          <input
            id="eraser"
            type="radio"
            name="tool"
            value="eraser"
            checked={props.tool==="eraser"}
            onChange={props.onToolChange}
          />
          <label htmlFor="eraser">Eraser</label>
        </div>
      </div>
    </div>
  );
};

ReactDOM.render(<App />, document.getElementById("root"))
              
            
!
999px

Console