sketch.js

var rects = [];

function setup() {
  createCanvas(800, 800);
  noLoop();
  
  var iterations = 6; // the number of times we'll do the split
  rects.push([30, 30, width-30*2, width-30*2]); // initial square
  
  for(var i = 0; i < iterations; i++){ // this can be done with recursion, but it's simple enough this way
    var temp = []; // at each iteration we go through all current rectangles, split them, and put in a temp list
    for(var k = 0; k < rects.length; k++){
      var parts = splitRect(rects[k]);
      temp.push(parts[0]);
      temp.push(parts[1]);
    }  
    rects = temp; // we'll now set our rects to the newly created temp list 
  }
}

function draw(){
  background(20);
  noFill();
  stroke(220);
  for(var k = 0; k < rects.length; k++){
    rect(rects[k][0], rects[k][1], rects[k][2], rects[k][3]);
  }
}

function splitRect(rec){
  var x = rec[0];
  var y = rec[1];
  var w = rec[2];
  var h = rec[3];
  
  var p = random(.3, .7);
  var r1, r2;
  // I decided to make the split vertical if w>h, and vice-versa
  // it can also be random or whatever
  if(w > h){ 
    r1 = [x, y, w*p, h];
    r2 = [x+w*p, y, w*(1-p), h];
  }
  else{
    r1 = [x, y, w, h*p];
    r2 = [x, y+h*p, w, h*(1-p)];
  }
  return [r1, r2]
}