Javascript

Try to evaluate

cell

.js
return 1+1

or

.js
const dom = document.createElement('span');
dom.innerText = "Hello World";
dom.style.color = 'lightblue';

return dom;

Github repo Javascript code is evaluated as a module, i.e. all defined variables are isolated to the cell.

tip

To define global variables, use window or core object.

window.variable = 1

Output cell

The returned value from the function can be a Javascript object or DOM element. The last one will be displayed in the output cell

.js
const canvas = document.createElement('canvas');
var ctx = canvas.getContext('2d');

canvas.width = 500;
canvas.height = 500;


const ballRadius = 10;
let x = canvas.width / 2;
let y = canvas.height - 30;
let dx = 2;
let dy = -2;

function drawBall() {
  ctx.beginPath();
  ctx.arc(x, y, ballRadius, 0, Math.PI * 2);
  ctx.fillStyle = "#0095DD";
  ctx.fill();
  ctx.closePath();
}

let uid;

function draw() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  drawBall();

  if (x + dx > canvas.width - ballRadius || x + dx < ballRadius) {
    dx = -dx;
  }
  if (y + dy > canvas.height - ballRadius || y + dy < ballRadius) {
    dy = -dy;
  }

  x += dx;
  y += dy;
  
  uid = requestAnimationFrame(draw);
}

uid = requestAnimationFrame(draw);

this.ondestroy = () => {
	cancelAnimationFrame(uid);
}



return canvas;

Handlers

There is a few quite useful built-in objects accesible from the cell.

this.ondestroy

This object is called when a cell has been destroyed. Assign any clean-up function to the given object

this.ondestroy = () => {
	//clean up the stuff
}

danger

Always clean up any timers using this.ondestroy variable. Otherwise those timers and animation loops will continue to work even after reevaluating the cell.

requestAnimationFrame

It is well-common method used in Javascript to synchronize with a framerate of the browser and render some graphics

danger

Do not forget to cancelAnimationFrame using this.ondestroy method

Communication with Wolfram Kernel

In general one can define any function for WLJS Interpreter using Javascript cells, please see guide here FrontSubmit and Integration with server / client via WebSockets

Also see Communication

For the most applications event-based system is used, see Dynamics