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Last active Oct 25, 2016
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// book keeping so that we can easily animate the two hands for the watchface
// .scale/.angle are updated by tween/event handler (see below)
var renderState = {
minute: {style: 'white', scale: 0.80, angle: 0},
hour: {style: 'red', scale: 0.51, angle: 0}
// helper function for the draw function (see below)
// extracted as a standalone function to satisfy common believe in efficient JS code
// TODO: verify that this has actually any effect on byte code level
var drawHand = function(handState, ctx, cx, cy, maxRadius) {
ctx.lineWidth = 8;
ctx.strokeStyle =;
ctx.moveTo(cx, cy);
ctx.lineTo(cx + Math.sin(handState.angle) * handState.scale * maxRadius,
cy + Math.cos(handState.angle) * handState.scale * maxRadius);
// the 'draw' event is being emitted after each call to rocky.requestDraw() but
// at most once for each screen update, even if .requestDraw() is called frequently
// the 'draw' event might also fire at other meaningful times (e.g. upon launch)
rocky.on('draw', function(drawEvent) {
var ctx = drawEvent.context;
var w = ctx.canvas.clientWidth;
var h = ctx.canvas.clientHeight;
// clear canvas on each render
ctx.fillStyle = 'black';
ctx.fillRect(0, 0, w, h);
// center point
var cx = w / 2;
var cy = h / 2;
var maxRadius = Math.min(w, h - 2 * 10) / 2;
drawHand(renderState.minute, ctx, cx, cy, maxRadius);
drawHand(renderState.hour, ctx, cx, cy, maxRadius);
// Draw a 12 o clock indicator
drawHand({style: 'white', scale: 0, angle: 0}, ctx, cx, 8, 0);
// overdraw center so that no white part of the minute hand is visible
drawHand({style: 'red', scale: 0, angle: 0}, ctx, cx, cy, 0);
// listener is called on each full minute and once immediately after registration
rocky.on('minutechange', function(e) {
// WatchfaceHelper will later be extracted as npm module
var wfh = new Rocky.WatchfaceHelper(;
renderState.minute.angle = wfh.minuteAngle;
renderState.hour.angle = wfh.hourAngle;
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HBehrens commented May 10, 2016

Unnoticed by the general public, Pebble FW 3.12 replaces the default watchface on Pebble Time with a version written in JavaScript. The goal was to produce exactly the same pixels and to launch the app as quickly as the native application so it's virtually indistinguishable from the previous version. But still, what you see is JavaScript executed directly on the Pebble smartwatch using a port of JerryScript! If you want to learn more about the reasoning behind it, please watch this recording of a presentation at last year's JSConf last call.


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