SOURCE

function setup(){
   createCanvas(500,500);
   background(255);
   noStroke();
}

function draw(){
   for(let x=0; x<500; x+=100){
       for(let y=0; y<500; y+=100){
           fill(0);
           rect(x,y,50,50);
        }
    }
   for(let x=50; x<500; x+=100){
       for(let y=50; y<500; y+=100){
       fill(0);
       rect(y,x,50,50);
       }
    }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.1.9/p5.min.js"></script>