let row = 10;

let gridWidth = 400;

let gridHeight = 400;

​

function setup() {

  createCanvas(gridWidth, gridHeight);

  noLoop();

}

function draw() {

  background(255);

  let cellWidth = gridWidth / col;

  let cellHeight = gridHeight / row;

  for (let i = 0; i < col; i++) {

    for (let j = 0; j < row; j++) {

      if ((i + j) % 2 == 0) {

        fill("black");

      } else {

        fill("white");

      }

      rect(i * cellWidth, j * cellHeight, cellWidth, cellHeight);

    }

  }

}

​