0.4.0 • Public • Published


dancer.js is a high-level audio API, usable with both Mozilla's Audio Data API and Web Audio API with flash fallback, designed to make sweet visualizations.

v0.4.0 (2/2/2014)


  • Use real-time audio waveform and frequency data and map it to any arbitrary visualization
  • Use Dancer to get audio data from any preexisting audio source
  • Leverage kick detection into your visualizations
  • Simple API to time callbacks and events to any section of a song
  • Supports Web Audio (webkit/mozilla), Audio Data (mozilla) and flash fallback (v9+)
  • Extensible framework supporting plugins and custom behaviours

Dancer Instance Methods


  • load( source ) specifies the audio source for the dancer instance. source can either be an audio element, or an object with a src property and an optional codecs array. While the audio element source is recommended to use with other players, if you specify a config object, the src property can either be a string of the audio path, or a string of the audio path, without the file extension, if you specify a codec array to work across multiple audio implementations. Examples of input:
  var dancer = new Dancer();

  // Using an audio object
  var a = new Audio();
  a.src = 'somesong.mp3';
  dancer.load( a );

  // Using an audio element on the page
  dancer.load( document.getElementsByTagName('audio')[0] );

  // Using a config object and you only have one encoding
  dancer.load({ src: 'somesong.mp3' });

  // Using a config object, and you have an ogg and mp3 version
  dancer.load({ src: 'somesong', codecs: [ 'ogg', 'mp3' ]});


All controls return this. If provided an audio element as the source, one can also control the audio through that, or can access the audio element in the audio property on the dancer instance.

  • play() plays the audio and begins the dance.
  • pause() pauses the madness.
  • setVolume() sets the player's current volume.


  • getVolume() returns a normalized value (0 to 1) of the current volume.
  • getTime() returns the current time.
  • getProgress() returns the downloading progress as a float from 0 to 1.
  • getWaveform() returns the waveform data array (Float32Array(1024))
  • getSpectrum() returns the frequency data array (Float32Array(512)).
  • getFrequency( freq [, endFreq ] ) returns the magnitude of a frequency or average over a range of frequencies.
  • isLoaded() returns a boolean value for the dancer instance's song load state.
  • isPlaying() returns a boolean value indicating whether the dancer instance's song is currently playing or not.


All section methods return this (CHAIN IT UP) and callbacks executed with this referencing the dancer instance.

  • after( t, callback ) fires callback on every frame after time t.
  • before( t, callback ) fires callback on every frame before time t.
  • between( t0, t1, callback ) fires callback on every frame between time t0 and t1.
  • onceAt( t, callback ) fires callback once at time t.


Basic pub/sub to tie into the dancer instance. update and loaded are predefined events called within the framework that are published on every frame (update) and on audio file load (loaded). All callbacks executed with this referencing the dancer instance.

  • bind( name, callback ) subscribes a callback of name. Can call this method several times to bind several callbacks of the same name.
  • unbind( name ) unsubscribes all callbacks of name.
  • trigger( name ) calls all callbacks of name.


Kicks are detected when the amplitude (normalized values between 0 and 1) of a specified frequency, or the max amplitude over a range, is greater than the minimum threshold, as well as greater than the previously registered kick's amplitude, which is decreased by the decay rate per frame.

  • createKick( options ) creates a new kick instance tied to the dancer instance, with an options object passed as an argument. Options listed below.
    • frequency the frequency (element of the spectrum) to check for a spike. Can be a single frequency (number) or a range (2 element array) that uses the frequency with highest amplitude. Default: [ 0, 10 ]
    • threshold the minimum amplitude of the frequency range in order for a kick to occur. Default: 0.3
    • decay the rate that the previously registered kick's amplitude is reduced by on every frame. Default: 0.02
    • onKick the callback to be called when a kick is detected.
    • offKick the callback to be called when there is no kick on the current frame.

Dancer Static Methods

  • addPlugin( name, fn ) registers a plugin of name with initiation function fn -- described in more detail below
  • isSupported() returns a string of webaudio, audiodata or flash indicating level of support. Returns an empty string if the browser doesn't support any of the methods. Can also return null when browser does not support typed arrays.
  • canPlay( type ) returns either true or false indicating whether the browser supports playing back audio of type type, which can be a string of 'mp3', 'ogg', 'wav', or 'aac'.
  • setOptions( options ) takes a set of key-value pairs in an object for options. Options below.
  • version not a method, but a property of the Dancer object to return a string of the current Dancer version

Dancer Options

  • flashSWF The path to soundmanager2.swf. Required for flash fallback.
  • flashJS The path to soundmanager2.js. Required for flash fallback.

Kick Instance Methods

These methods can be called on a kick instance to turn on and off the registered callbacks

  • on() turns on the kick instance's callbacks and detections
  • off() turns off the kick instance's callbacks and detections
  • set( options ) can pass in an object literal with the 5 kick options, similar to creating a new kick option


For simple examples, check out the examples/ folder -- both the FFT and waveform examples are straight forward, leveraging the corresponding plugins for visualizations.

  // To enable flash fallback, specify the paths for the flashSWF and flashJS
    flashJS  : '../../lib/soundmanager2.js',
    flashSWF : '../../lib/soundmanager2.swf'
    audio  = document.getElementsByTagName('audio')[0],
    dancer = new Dancer(),
    kick = dancer.createKick({
      onKick: function ( mag ) {
      offKick: function ( mag ) {
        console.log('no kick :(');
  // Let's turn this kick on right away
  dancer.onceAt( 10, function() {
    // Let's set up some things once at 10 seconds
  }).between( 10, 60, function() {
    // After 10s, let's do something on every frame for the first minute
  }).after( 60, function() {
    // After 60s, let's get this real and map a frequency to an object's y position
    // Note that the instance of dancer is bound to "this"
    object.y = this.getFrequency( 400 );
  }).onceAt( 120, function() {
    // After 120s, we'll turn the kick off as another object's y position is still being mapped from the previous "after" method;
  }).load( audio ); // And finally, lets pass in our Audio element to load;


HTML5 Playback with Web Audio or Audio Data Chrome and Firefox are both supported out of the box -- other browsers will need to leverage the flash fallback until either of these APIs are implemented.

Safari 6 Web Audio API While Safari 6 does have the Web Audio API, it doesn't currently support processing audio from a media element source. Falls back to flash.

To enable flash You must set Dancer's defaults for flashSWF with the path to the soundmanager2.swf and flashJS to the path to soundmanager2.js, both found in lib/. Flash player 9 is required, and you must provide an mp3 option. Waveform data in Flash is a 1024 Float32Array, but only the first 512 elements have values due to flash's computeSpectrum method.

Uint32Array and Float32Array are required Include a shim if you'd like to support browsers that do not have these typed arrays.


  • dsp.js - A subset of dsp.js (fft) is used for Fast Fourier Transformations ( Included in packaged Dancer )
  • flash_detect - flash detect is used for immediate flash detection ( Included in packaged Dancer )
  • soundmanager2 - soundmanager2 is used for flash fallback ( found in lib/, asynchronously loaded )


You can extend the Dancer prototype by calling the static method addPlugin( name, fn ), which extends the Dancer prototype. A Dancer instance then can call the function provided in its context and subscribe to a preexisting event like update, or make your own. Look in the plugins/ directory for examples.


This project uses grunt to build and jasmine for testing. Run grunt from the project root to lint and build files. A CLI for testing would be awesome, but Mozilla and WebKit implementations differ greatly -- go to spec/index.html in Mozilla/WebKit browsers to test. All tests should pass in Chrome and Firefox (95% of the time) -- Flash implementations are much more annoying, need to have cleaned up tests.

Change Logs

v0.4.0 (1/28/2014)

  • Update to work with new Web Audio function names. Dancer now uses Web Audio in Firefox 25+.

v0.3.2 (9/29/2012)

  • Change build process to using grunt.js

v0.3.1 (8/13/2012)

  • Renamed beat to kick for future, true kick-detection
  • Added getProgress() to track progress of downloaded audio file (#20)
  • Added setVolume() and getVolume() as instance methods (#21)
  • Added set() method to kick instance (#16), fixed ability to assign 0 to a kick attribute

v0.3.0 (8/9/2012)

  • Added ability to provide an audio element as a source -- can control audio via the element, or accessed through instance's audio property, or through Dancer's helper controls (play, pause)
  • Pulled out loading from the constructor to the instance method load. Can use the above mentioned audio element, or a config object with path information.
  • Changed instance method stop to pause, to be more in line with audio elements
  • Added example of using the audio element in examples/audio\_element.

v0.2.1 (6/16/2012)

  • Added getWaveform() method and a corresponding visualization for waveforms

v0.2.0 (6/14/2012)

  • Added flash support with soundmanager2 -- flash_detect now included in build
  • Added static methods isSupported, canPlay and setOptions
  • Added multiple audio codecs support (#7)
  • Added a new simple FFT examples, both examples having feature detection and controls (#10)
  • Fixed several Webkit bugs (#4, #8)

v0.1.0 (6/3/2012)

  • Initial Web Audio/ Audio Data release




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