JSFiddle - React, Tailwind, and code Playground

HTML

<!-- GAME WINDOW -->
<canvas id="canvas" width="800" height="600"></canvas>

JavaScript

function init(){
    window.box1 = new Box(50, 'red', 0, 0);
    window.box2 = new Box(50, 'blue', 400, 20);
    requestAnimationFrame(update);
}

function update() {
    window.box1.update();
    window.box2.update();
		document.getElementById('canvas').width = document.getElementById('canvas').width;
    requestAnimationFrame(update);
    requestAnimationFrame(render);
}

function render() {
    window.box1.render();
    window.box2.render();
}

// -------------------------------------------------------------------------------------------------------
// --
// -------------------------------------------------------------------------------------------------------
var Box = function(dimensions, color, x, y){
    this.width = dimensions;
    this.height = dimensions;
    this.x = x;
    this.y = y;
    this.velocityX = 10;
    this.velocityY = 10;
    this.color = color;
    this.context = document.getElementById('canvas').getContext('2d');

    this.update = function(){
        this.x += this.velocityX;
        this.y += this.velocityY;
        this.collisionCheck();
    };
    this.render = function(){
        this.context.fillStyle = this.color;
        //this.context.clearRect(this.x  - this.velocityX, this.y - this.velocityY, this.width, this.height);
        this.context.fillRect(this.x, this.y, this.width, this.height);
    };
    this.collisionCheck = function(){
        if(this.y > 600 - this.height || this.y < 0)
            this.velocityY *= -1;
        if(this.x > 800 - this.width || this.x < 0)
            this.velocityX *= -1;
    };
};

init();