Tweak values in JavaScript without having to reload.
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JavaScript source transformation to support Bret Victor-style value-scrubbing interfaces with the information without having to recompile code every time the value changes.

Try it out!


To use scrubby in your project, just include scrubby.all.js in your page and mark any scripts you want to scrub with type=text/scrubby.

<!DOCTYPE html>
<script src=''></script>
  .ball {
    position: absolute;
    left: 100px; top: 100px;
    width: 50px; height: 50px;
    border-radius: 25px;
    background: green;
<div class='ball'></div>
<script type='text/scrubby'>
var t = 0;
var lastFrame =;
setInterval(function() {
  var ball = document.querySelector('.ball');
  var now =, dt = (now - lastFrame) / 1000 * 40;
  t += dt; lastFrame = now; = 100 + 50 * Math.sin(t * 0.2) + 'px'; = 100 + 50 * Math.sin(t * 0.2 + 0.4 * Math.PI*2) + 'px';
}, 1000/60);

(See this example live.)

Scrubby adds a button to the top-left of the page for each script you specify that's scrubbable. Clicking on that button will open a new window with the code in it, and you can scrub live immediately.

Note that in this code, scrubbing the framerate (1000/60) does nothing, because the setInterval has already been called by the time you start scrubbing. Scrubbing will only work on values that are referenced again after you edit them.

If you want to re-run a piece of code when a value is scrubbed, you can listen for the 'scrubbed' event:

scrubby.on('scrubbed', function() {

(the demo code does this to update the canvas when you edit a value.)

How it works

Scrubby transforms this JavaScript:

function draw() {
  var x = 20, y = 32
  ctx.lineTo(x+50, y)

into this:

var $values = {
  '1': 20,
  '2': 32,
  '3': 50
function draw() {
  var x = $values['1'], y = $values['2'];
  ctx.moveTo(x, y);
  ctx.lineTo(x + $values['3'], y);

When you scrub a value, scrubby updates the global $values object with the edited value. When the code runs again, it automatically uses the new value.


  • It would be nice to be able to type in a value, as well as to scrub it.
  • Editing strings would be good.
  • Strings like 'hsla(260,40%,40%,0.1)' could have a color picker attached instead of a text editor.
  • I'd like to explore indirecting not just values, but whole functions, allowing you to edit the text of a function. I'm worried that it'll break in non-obvious ways, though. The current method is very straightforward, and I'd like to preserve that.