let numCols = 10; // Number of columns

let numRows = 10; // Number of rows

let w, h; // Width and Height of each cell

​

function setup() {

  createCanvas(400, 400);

  // Calculate w and h of each cell in relation to width and height of canvas

  w = width/numCols;

  h = height/numRows;

}

​

function draw() {

  background(220);

  // For every column

  for(let c = 0; c < numCols; c++) {

    // Calculate x-position of column: How many pixels is it from the left-side of the canvas?

    let x = c * w;

    // For every row

    for(let r = 0; r < numRows; r++) {

      // Calculate y-position of row: How many pixels is it from the top-side of the canvas?

      let y = r * h;

      // Check left-right, top-bottom boundaries of this cell

      if(mouseX > x && mouseX < x + w && mouseY > y && mouseY < y + h) fill('red');

      else fill('white');

      // Draw the cell

      rect(x, y, w, h);

    }

  }

}