node package manager



Simple event scheduler for Web Audio API.

This library is based on Chris Wilson's scheduling method used in his metronome example.


You can install wa-metro from npm.

npm install wa-metro

Or grab the latest version from build folder and include it in your html.

<script src='wa-metro.min.js'></script>

Example Usage

var context = new AudioContext();
var callback = function (time, step) {
    //schedule audio events with time and step number 
var metro = new Metro(context, callback);


The callback function will have time, step and timeFromScheduled parameters. You can use these values to create dynamic loops and animations.

Callback Parameters

  • time - the audio time for the step which can be used to schedule audio events.
  • step - the number of the scheduled step.
  • timeFromScheduled - the time (in seconds) that the event will be triggered from the schedule event (time - context.currentTime). This time can be used to schedule animations and other events as shown in the example below.
var metro = new Metro(context, callback);
function callback(time, step, timeFromScheduled) {
    var osc = context.createOscillator();
    if(step === 1) {
        osc.frequency.value = 880;
    osc.stop(time + 0.1);
    // trigger some js event 
  }, timeFromScheduled * 1000);


var metro = new Metro(context, callback);
metro.start(); // starts the clock and schedules events 
metro.stop(); // stops the clock and resets the step to 1 
metro.pause(); // pauses the clock and keeps the current step number 


var metro = new Metro(context, callback);
metro.tempo = 100; // in BPM, defaults to 120 
metro.steps = 8; // number of steps in a bar. defaults to 16 
metro.look_ahead = 0.5; // look ahead time in Seconds. defaults to 1.0 
metro.callback = function(time, step){}; // change the callback