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Components JS Library

Components JS is a JavaScript library that makes it easier to code controller mappings for Mixxx. It lets you focus more on your controller and less on the details of MIDI signals and how Mixxx works. It is centered around JavaScript objects called Components that represent a physical component of a controller, such as a button, knob, fader, or encoder. Components provide generic functions that can be made to work for most use cases just by changing some attributes of the Component, without having to write many or any custom functions. The library also provides more specialized Components for common use cases. Components can be organized into ComponentContainer objects, making it easy to iterate over them and change their behavior to switch between different modes.

Components JS is new in Mixxx 2.1 and does not work with older versions of Mixxx. To use the library, in the <scriptfiles> element at the top of your mapping's XML file, load the Lodash library and the Components library:

<file filename="lodash.mixxx.js"/>
<file filename="midi-components-0.0.js"/>

Components JS uses a few functions from Lodash, which is why they both need to be loaded. Importing the midi-components-0.0.js file makes the library accessible by an object called components (plural, lower case).

If you are not familiar with object oriented programming in JavaScript, read Mozilla Developer Network's Object-oriented JavaScript introduction. If you are familiar with OOP in other languages but new to JavaScript, you can skip ahead to the Constructors and object instances section of that tutorial.


A Component represents a physical component of a controller, such as a button, knob, fader, or encoder. It encapsulates all the information needed to receive MIDI input from that component and send MIDI signals out to the controller to activate its LED(s). In general, you should not use the Component object directly; instead, use one of its subtypes (Button, Pot, or Encoder). If you do need to use Component directly, do not confuse it with the components object (plural, lower case) that contains all the objects for the library; access Component as components.Component.

Components should generally be properties of a ComponentContainer object. Most Components should be properties of a custom Deck object, which is a subtype of ComponentContainer.


The input function of each Component needs to be mapped to the incoming MIDI signals in the XML file. For example:

    <!-- MyController.leftDeck would be an instance of a custom Deck. -->

In the future Mixxx will be able to register MIDI inputs from JavaScript, so that will not be necessary. Until then, if you decide to rename a Component or map it to different MIDI input signals, you need to edit the XML file and reload the mapping in Mixxx's Preferences. The output does not need to be mapped in XML. It is handled by the library in JavaScript.

Create Components by calling the constructor with JavaScript's “new” keyword. The Component constructor takes a single object as an argument. Generally you should provide this as an object literal. Each property of that object passed to the constructor becomes a property of the new Component object, making it easy to customize the functionality of the Component. The constructors for all Component subtypes work the same way. Most Components need at least these properties defined:

  • midi (array with 2 numbers): the first two MIDI bytes that the controller sends/receives when the physical component changes state. Refer to the MIDI Crash Course if you do not understand what this means.
  • inKey (string): the key of the Mixxx ControlObject that this Component manipulates when it receives a MIDI input signal
  • outKey (string): when the Mixxx ControlObject specified by this key changes value, the output function will be called
  • group (string): the group of the Mixxx ControlObjects for both inKey and outKey, for example [Channel1] for deck 1

For example:

var quantizeButton = new components.Button({
    midi: [0x91, 0x01],
    group: '[Channel1]'
    inKey: 'quantize',
    outKey: 'quantize',

If inKey and outKey are the same, you can specify key in the options object for the constructor to set both the inKey and outKey properties of the new Component. For example:

var quantizeButton = new components.Button({
    midi: [0x91, 0x01],
    group: '[Channel1]'
    key: 'quantize'

Setting the key property after calling the constructor will not automatically set inKey and outKey; you would need to do that manually if necessary.


The following methods (in JavaScript, methods are just object properties that happen to be functions) must be defined for every Component, but in most cases the defaults will work so you do not need to define them yourself:

The following methods are called by the default Component input and output methods, as well as the default input functions of Button, Pot, and Encoder. If you do not need to implement complex custom behavior, you can overwrite these instead of the default input and output methods:

  • inValueScale: takes the third byte of the incoming MIDI signal as its first argument and returns the value to set group, inKey to
  • outValueScale: takes the value of group, outKey as its first argument and returns the third byte of the outgoing MIDI signal

Each Component also has these methods that you probably should not overwrite:

  • disconnect: disconnect the output function from being called when group, outKey changes
  • trigger: manually call output with the same arguments as if group, outKey had changed
  • send: send a 3 byte (short) MIDI message out to the controller. The first two bytes of the MIDI message are specified by the Component's midi property. The third MIDI byte is provided as the first argument to the send function.
  • inGetParameter: returns the value of group, inKey normalized to a 0-1 scale
  • inSetParameter: sets the value of group, inKey to the function's first argument, normalized to a 0-1 scale
  • inGetValue: returns the value of group, inKey
  • inSetValue: sets the value of group, inKey to the function's first argument
  • inToggle: sets group, inKey to its inverse (0 if it is >0; 1 if it is 0)
  • outGetParameter: returns the value of group, outKey normalized to a 0-1 scale
  • outSetParameter: sets the value of group, outKey to the function's first argument, normalized to a 0-1 scale
  • outGetValue: returns the value of group, outKey
  • outSetValue: sets the value of group, outKey to the function's first argument
  • outToggle: sets group, outKey to its inverse (0 if it is >0; 1 if it is 0)

Shift layers

Components can be used to manage alternate behaviors in different conditions. The most common use case for this is for shift buttons. If your controller sends different MIDI signals depending on whether shift is pressed, map both the shifted and unshifted input signals to the Component's input function in XML. Assign functions to the shift and unshift properties that manipulate the Component appropriately.

In some cases, using the shift/unshift functions to change the Component's inKey, outKey, and/or group properties will be sufficient. As an example, here is the source code for PlayButton:

var PlayButton = function (options) {, options);
PlayButton.prototype = new Button({
    unshift: function () {
        this.inKey = 'play';
    shift: function () {
        this.inKey = 'start_stop';
    outKey: 'play_indicator',

In more complex cases, overwriting the input and output functions may be required. Refer to the source code of SamplerButton for an example.

For convenience, the Component constructor will automatically call the unshift function if it exists (otherwise, in the above example for PlayButton, inKey would have to be specified as a property of the prototype object and set in the unshift function). The shift and unshift functions of ComponentContainer will call the corresponding function of all the Components within it that have that function defined and will recursively descend into ComponentContainers that are properties of the parent ComponentContainer.

To use separate output callback functions in shifted and unshifted modes, the Component's shift and unshift functions need to call disconnect/connect and trigger. ComponentContainer's shift/unshift methods will not do this automatically. Generally, you should avoid making LEDs change when a shift button is pressed. This is distracting if the user is pressing shift to use the shifted functionality of a different part of the controller. For layers that stay activated after a button is released, this is not as much of an issue.

To handle the interaction of shifted and unshifted states with another layer, you can create another system of methods for each Component that changes properties of the Component when a layer is activated, and within those methods, you can change the shift and unshift of the Component. Refer to the source code of EffectUnit for an example.

Optional properties

The following properties can be specified in the options object passed to the Component constructor to customize the Component's initialization. Changing their value after creating the Component does not do anything.

  • outConnect (boolean, default true): whether to call connect in the constructor (assuming group and outKey were specified in the options object)
  • outTrigger (boolean, default true): whether to call trigger in the constructor (assuming group and outKey were specified in the options object)

Some controllers send and receive two sets of MIDI signals for most physical components, one for when the shift button is pressed and one for when the shift button is not pressed. To avoid defining two Components for every physical component of your controller, set the following options as appropriate:

  • sendShifted (boolean, default false): whether to send a second, shifted MIDI message for every call to send
  • shiftChannel (boolean, default false): whether the shifted MIDI message changes the MIDI channel (second nybble of the first byte of the MIDI signal)
  • shiftComponent (boolean, default false): whether the shifted MIDI message changes the MIDI control number (second byte) of the MIDI signal
  • shiftOffset (number, default 0): how much to shift the MIDI channel or control number by

To avoid having to define those properties for every Component, you can change the properties of components.Component.prototype in your controller's init function. For example:

components.Component.prototype.shiftOffset = 3;
components.Component.prototype.shiftChannel = true;
components.Component.prototype.sendShifted = true;

Syntactic sugar

Components JS provides convenient shortcuts for common situations.

To avoid typing out the group for the constructor of each Component, Components that share a group can be part of a ComponentContainer and the ComponentContainer's reconnectComponents method can assign the group to all of them. Refer to the Deck ComponentContainer documentation for an example.

If a Component only needs its midi property specified for its constructor, this can be provided simply as an array without wrapping it in an object. For example:

var playButton = new components.PlayButton([0x90 + channel, 0x0A]);

instead of

var playButton = new components.PlayButton({
    midi: [0x90 + channel, 0x0A]


A Button is a subtype of Component for buttons/pads. Subtypes of Button are provided for many common use cases, documented in the subsections below, making it easy to map those buttons without having to worry about particularities of Mixxx's ControlObjects. To use the Button subtypes, you only need to specify their midi and group properties, except for HotcueButton and SamplerButton.

A generic Button toggles the state of a binary inKey and sends outgoing MIDI messages indicating whether a binary outKey is on or off. Button adds the following properties to Component:

  • onlyOnPress (boolean, default true): whether to toggle group, inKey only when the button is pressed (when onlyOnPress is true), or respond both when the button is pressed and released (when onlyOnPress is false)
  • on (number, default 127): number to send as the third byte of outgoing MIDI messages when group, outKey is on (its value is > 0)
  • off (number, default 0): number to send as the third byte of outgoing MIDI messages when group, outKey is off (its value is 0)
  • isPress (function): function that takes the same first 4 arguments as a MIDI input function (channel, control, value, status) and returns a boolean indicating whether the button was pressed.

For buttons/pads with multicolor LEDs, you can change the color of the LED by defining the on and off properties to be the MIDI value to send for that state. For example, if the LED turns red when sent a MIDI value of 127 and blue when sent a value of 126:

MyController.padColors = {
    red: 127,
    blue: 126
MyController.quantize = new components.Button({
    midi: [0x91, 0x01],
    group: '[Channel1]',
    key: 'quantize',

The default isPress function works for controllers that indicate whether a button is pressed or released by sending a different third MIDI byte (value). Some controllers distinguish between a button press and release by changing the first nybble (hexadecimal digit) of the first byte of the MIDI message, also known as an opcode. These controllers typically use an opcode of 9 to indicate a button press and 8 to indicate a button release. Both the press and release signals need to be mapped in the XML file to the Button's input method. To make Button work for such a controller, overwrite the prototype isPress function in your mappings's init function:

components.Button.prototype.isPress = function (channel, control, value, status) {
    return (status & 0xF0) === 0x90;


Default behavior: play/pause
Shift behavior: go to start of track and stop

LED behavior depends on cue mode selected by the user in the preferences Refer to the manual for details.


Behavior depends on cue mode configured by the user in the preferences
Refer to the manual for details.


Default behavior: momentary sync without toggling sync lock
Shift behavior: toggle sync lock (master sync)


Toggle a loop on/off


Default behavior: set hotcue if it is not set. If it is set, jump to it.
Shift behavior: delete hotcue

The LED indicates whether the hotcue is set.

Pass the number of the hotcue as the number property of the options argument for the constructor. For example:

var hotcues = [];
for (var i = 1; i <= 8; i++) {
    hotcues[i] = new components.HotcueButton({
        number: i,
        group: '[Channel1]',
        midi: [0x91, 0x26 + i],


Default behavior: Press the button to load the track selected in the library into an empty sampler. Press a loaded sampler to play it from its cue point. Press again while playing to jump back to the cue point.
Shift behavior: If the sampler is playing, stop it. If the sampler is stopped, eject it.

Specify the sampler number as the number property of the object passed to the constructor. There is no need to manually specify the group. For example:

var samplerButtons = [];
for (var n = 1; n <= 8; n++) {
    samplerButtons[n] = new components.SamplerButton({
        number: n,
        midi: [0x91, 0x02],

When the sampler is loaded, the LED will be sent a MIDI message with the value of the on property (default 127) When the sampler is empty, the LED will be sent a MIDI message with the value of the off property (default 0). If your controller's pads have multicolor LEDs, specify the value to send for a different LED color with the playing property to set the LED to a different color while the sampler is playing. For example:

MyController.padColors = {
// These values are just examples, consult the MIDI documentation from your controller's
// manufacturer to find the values for your controller. If that information is not available,
// guess and check to find the values.
    red: 125,
    blue: 126,
    purple: 127,
    off: 0
var samplerButton = [];
var samplerButton[1] = new components.SamplerButton(
    midi: [0x91, 0x02],
    number: 1,
    // off is inherited from Button.prototype


A Pot is a Component subtype for potentiometers (faders and knobs) with finite ranges. Pot's connect and disconnect methods take care of soft takeover when switching layers with ComponentContainer's reconnectComponents or applyLayer methods. Soft takeover is not activated until the first input signal is received, so it does not interfere with setting initial values for controllers that can report that information.

For example:

var eqKnobs = [];
for (var i = 1; i <= 3; i++) {
    eqKnobs[i] = new components.Pot({
        midi: [0xB1, 0x02 + i],
        group: '[EqualizerRack1_[Channel1]_Effect1]',
        inKey: 'parameter' + i,


Encoder is a Component for infinitely turning encoders. The default input function assumes the encoder sends MIDI signals on a continous scale from 0 to 127 (0x7F). If the encoder sends relative MIDI signals to indicate whether it turns right or left, you will need to define your own input function. For example, for an encoder that sends a value of 1 when it is turned left and a value of 127 when it is turned right:

MyController.SomeEncoder = new components.Encoder({
    midi: [0xB1, 0x03],
    group: '[Channel1]',
    inKey: 'pregain',
    input: function (channel, control, value, status, group) {
        if (value === 1) {
            this.setParameterIn(this.getParameterIn() - .05);
        } else if (value === 127) {
            this.setParameterIn(this.getParameterIn() + .05);

To map an Encoder with an LED ring around it that receives MIDI signals on a continuous 0-127 scale, define an outKey property in the options object for the constructor. Similar to input, if the LEDs do not respond to a continuous 0-127 scale, define your own output function. If outKey and inKey are the same, you can just specify one key property for the constructor.

Encoders can often be pushed like a button. Usually, it is best to use a separate Button Component to handle the MIDI signals from pushing it.


A ComponentContainer is an object that contains Components as properties. It has methods to help iterate over those Components:

  • forEachComponent: Iterate over all Components in this ComponentContainer and perform an operation on them. The operation is a function provided as the first argument to forEachComponent. The operation function takes each Component as its first argument. In the context of the operation function, this refers to the ComponentContainer. forEachComponent iterates recursively through the Components in any ComponentContainers that are properties of this ComponentContainer. If you do not want forEachComponent to operate recursively, pass false as the second argument to forEachComponent.
  • reconnectComponents: Disconnect and reconnect output callbacks for each Component. Optionally perform an operation on each Component between disconnecting and reconnecting the output callbacks. Arguments are the same as forEachComponent.
  • shift: Call each Component's shift method if it exists. This iterates recursively on any Components in ComponentContainers that are properties of this ComponentContainer, so there is no need to call shift on each child ComponentContainer. This function takes no arguments.
  • unshift: same as shift, but call each Component's unshift method
  • applyLayer: Activate a new layer of functionality. Layers are merely objects with properties to overwrite the properties of the Components within this ComponentContainer. Layer objects are deeply merged. If a new layer does not define a property for a Component, the Component's old property will be retained.

In the most common case, for providing alternate functionality when a shift button is pressed, using applyLayer is likely overcomplicated and may be slow. Use shift/unshift instead. applyLayer may be useful for cycling through more than two alternate layers.

For example:

    someButton: { inKey: 'alternate inKey' },
    anotherButton: { outKey: 'alternate outKey' }

By default, applyLayer disconnects old layer's output callbacks and the new layer's output callbacks are connected. To avoid this behavior, which would be desirable if you are not changing any output functionality, pass false as the second argument to applyLayer.


Deck is a ComponentContainer with methods for conveniently changing the group attributes of contained Components to switch the deck that a set of Components is manipulating. The setCurrentDeck method takes the new deck as a string and sets the Components' group property appropriately, including for equalizer knobs and QuickEffect (filter) knobs.

The Deck constructor takes one argument, which is an array of deck numbers to cycle through with the toggle method. Typically this will be [1, 3] or [2, 4].

To map your own controller, create a custom subtype of Deck and create instances of your custom Deck objects in your controller's init function. Use a constructor function to create all the Components you need for your particular controller and assign your custom subtype's prototype to components.Deck. For example:

MyController.init = function () {
    this.leftDeck = new MyController.Deck([1, 2], 1);
    this.rightDeck = new MyController.Deck([2, 4], 2);
MyController.Deck = function (deckNumbers, midiChannel) {
    // Call the Deck constructor to setup the currentDeck and deckNumbers properties., deckNumbers);
    this.playButton = new components.PlayButton([0x90 + midiChannel, 0x01]);
    this.CueButton = new components.CueButton([0x90 + midiChannel, 0x02]);
    this.hotcueButtons = [];
    for (var i = 1; i <= 8; i++) {
        this.hotcueButtons[i] = new components.HotcueButton({
            midi: [0x90 + midiChannel, 0x10 + i],
            number: i
    // ... define as many other Components as necessary ...
    // Set the group properties of the above Components and connect their output callback functions
    // Without this, the group property for each Component would have to be specified to its
    // constructor.
    this.reconnectComponents(function (c) {
        if ( === undefined) {
            // 'this' inside a function passed to reconnectComponents refers to the ComponentContainer.
   = this.currentDeck;
MyController.Deck.prototype = new components.Deck();


EffectUnit is a ComponentContainer that contains Components designed to be mapped to the common arrangement of 4 knobs and 4 buttons for controlling effects. If your controller's effects section has fewer components, the EffectUnit object provided by Components JS probably will not be very helpful. You may want to read the source code for the library's EffectUnit to get an idea for how to map your controller though.

3 knobs are used for controlling effect metaknobs or parameters, depending on whether the effects' parameters are shown. The other knob is used for the dry/wet knob of the whole chain or the superknob when shift is pressed. 3 buttons are used for enabling effects and the other button toggles the effect unit between hiding and showing effect parameters. The Components provided are:

When the effect unit is showing the metaknobs of the effects but not each parameter, the knobs control the metaknobs. The enableButtons control whether each effect is enabled. Pressing an enableButton with shift switches to the next available effect.

When the effect unit is showing all the parameters, the knobs behave differently depending on whether an effect is focused. When there is no focused effect (the default state), the knobs control the effect metaknobs like they do when parameters are not showing. When an effect is focused, the knobs control the first 3 parameters of the focused effect. An effect can be focused by pressing shift + its enableButton or clicking the focus button on screen. Pressing shift + the enableButton for the focused effect again unfocuses the effect.

The enableOnChannelButtons allow assigning the effect unit to different channels and are named after the Mixxx channel they affect. Not all controllers have buttons to map these. The following Buttons are provided by default:

  • Channel1
  • Channel2
  • Channel3
  • Channel4
  • Headphones
  • Master
  • Microphone
  • Auxiliary1

You can easily add more, for example for additional microphones, auxiliary inputs, or samplers by calling enableOnChannelButtons.addButton('CHANNEL_NAME') (do not put brackets around the CHANNEL_NAME).


To map an EffectUnit for your controller, call the constructor with the unit number of the effect unit as the only argument. Then, set the midi attributes for the showParametersButton, enableButtons[1-3], and optionally enableOnChannelButtons. After the midi attributes are set up, call the EffectUnit's init method to set up the output callbacks. For example:

MyController.effectUnit = new components.EffectUnit(1);
MyController.effectUnit.enableButtons[1].midi = [0x90, 0x01];
MyController.effectUnit.enableButtons[2].midi = [0x90, 0x02];
MyController.effectUnit.enableButtons[3].midi = [0x90, 0x03];
MyController.effectUnit.knobs[1].midi = [0xB0, 0x01];
MyController.effectUnit.knobs[2].midi = [0xB0, 0x02];
MyController.effectUnit.knobs[3].midi = [0xB0, 0x03];
MyController.effectUnit.dryWetKnob.midi = [0xB0, 0x04];
MyController.effectUnit.showParametersButton.midi = [0x90, 0x04];
MyController.effectUnit.enableOnChannelButtons.Channel1 = [0x90, 0x05];
MyController.effectUnit.enableOnChannelButtons.Channel2 = [0x90, 0x06];

Controllers designed for Serato and Rekordbox often have an encoder instead of a dry/wet knob (labeled “Beats” for Serato or “Release FX” for Rekordbox) and a button labeled “Tap”. If the encoder sends a MIDI signal when pushed, it is recommended to map the encoder push to the EffectUnit's showParametersButton, otherwise map that to the “Tap” button. To use the dryWetKnob Pot with an encoder, replace its input function with a function that can appropriately handle the signals sent by your controller. Refer to the Encoder documentation for an example.

For the shift functionality to work, the shift button of your controller must be mapped to a function that calls the shift/unshift methods of the EffectUnit on button press/release. If the EffectUnit is a property of another ComponentContainer (for example a Deck), calling shift and unshift on the parent ComponentContainer will recursively call it on the EffectUnit too (just like it will for any other ComponentContainer).

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components_js.txt · Last modified: 2017/02/12 15:06 by