w3school drawrect code example

Example 1: js draw square to canvas

// canvas context.fillRect(x, y, height, width);
ctx.fillRect(20, 20, 150, 100);

Example 2: draw rectangle on canvas javascript

//Assuming you have canvas and context (ctx) set up:
ctx.fillStyle = "(enter color)";
ctx.fillRect(x, y, width, height);
//to set up canvas and context:
var canvas = document.getElementById("(enter canvas's id)");
var ctx = canvas.getContext("2d");