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Controller Scripting

In order to support the features of many MIDI controllers, Mixxx offers what we call MIDI Scripting (introduced in Mixxx v1.7.0). It enables MIDI controls to be mapped to QtScript (also known as Javascript/EMCAScript) functions, allowing mappings to manage complex behaviors. These user-created functions can then do anything desired with the MIDI event info such as affect different controls depending on whether another button is pressed, adjust incoming control values to work better with Mixxx (i.e. for scratching), send messages to LED displays on the controller, or even turn a 2 deck controller into a 4 deck controller.

If you would like your mapping included in Mixxx, please see the coding guidelines on the Contributing Mappings page.

JavaScript is mostly used for programming complex functionality in Web pages. There are many tutorials online, such as W3Schools, aimed at people who have never programmed before. However, understanding them may require understanding HTML, the language used to write Web pages. HTML is fairly simple and easy to learn the basics. It is very similar to XML, the language used for Mixxx's MIDI controller mapping file format. The additional examples section at the bottom of this page is aimed at people with little or no programming experience. It has examples for common uses of MIDI scripting to help get you started writing code in an organized and maintainable way from the start. This will make it easier for you and other people to edit the code later.

If you have any programming experience, you can probably learn the basics of JavaScript quickly and easily. Mozilla Developer Network has helpful resources for JavaScript programming that focus on the language itself without regards to the Web, although these may not be very easy to understand for people without any programming experience:

Here are some resources for a deeper understanding of JavaScript:

If you are unfamiliar with MIDI, see the MIDI Crash Course page.

Tip: If there is already a Mixxx mapping for a controller made by the same manufacturer as your controller, you may want to look at that mapping for examples. The controllers likely send similar types of signals (although they could be very different).

Tip: When you're testing your scripts, you don't have to restart Mixxx. Every time you save your file, Mixxx will reload it immediately. This can make testing changes very fast.

Set up a JavaScript mapping

Specify the JS file

All JavaScript files need an accompanying XML mapping file. See the controller mapping file locations for where to put mapping files on your OS.

To specify script files to load, add the following section to the device's XML file inside the <controller> tag:

 <controller id="controller">
            <file filename="Manufacturer-model-scripts.js" functionprefix="MyController"/>

The functionprefix attribute specifies the name of the JavaScript object in the file that has init and shutdown methods called when the controller is opened and closed by Mixxx (typically when the user opens and closes Mixxx).

You can add as many <file> tags as you like, but be sure to specify the appropriate functionprefix in every one. These will all be loaded when the controller is activated.

There is a default script function file called common-controller-scripts.js which contains functions common to all controllers and is always loaded. See available common functions below for information on these functions.

Script file header

In your script file, you need to have a declaration of the controller's object. It looks like this:

var MyController = {};

…and you would replace MyController with whatever you entered for 'functionprefix' in the XML file above. This declares a new JavaScript variable representing your controller (in this example, called MyController) and assigns it to an empty object.

This object should have properties called “init” and “shutdown” defined and assigned to functions (in JavaScript, object methods are just properties whose value is a function). They can be empty, but are useful for putting controllers into known states and/or lighting certain LEDs before operation begins or the program exits. Some controllers can be sent a MIDI message that tells the controller to send back MIDI messages for the position of the controls on the device. If your controller can do this, it is helpful to send that message from the init function so the state of Mixxx's controls match the hardware when Mixxx opens. For controllers designed for Serato, this can be done by sending the Serato sysex message.

For example, if there are 40 LEDs on your controller that respond to MIDI note numbers 1 through 40 that turn on when sent value 0x7f and turn on when sent value 0x00, your script could start with:

var MyController = {};
MyController.init = function (id, debugging) {
    // turn on all LEDs
    for (var i = 1; i <= 40; i++) { // Repeat the following code for the numbers 1 through 40
                                // see
        midi.sendShortMsg(0x90, i, 0x7f);
MyController.shutdown = function() {
   // turn off all LEDs
   for (var i = 1; i <= 40; i++) {
        midi.sendShortMsg(0x90, i, 0x00);

The ID parameter of the init function is the controller id attribute from the XML file. This can be used to identify the particular controller instance in print statements. The debugging parameter is set to 'true' if the user specified the --mididebug parameter on the command line.

Note: Instead of using global variables, define properties of your controller object (MyController in this example) to avoid name collisions with other scripts that may be loaded.

To link a script function to an incoming MIDI message, put the full function name in the <key> tag of the MIDI message's <control> element in the XML file, with a <Script-Binding/> tag in the <options> block, like so:

            <control>    <!--    Pitch slider    -->

The value for <group> doesn't matter when using a script function, but it is available to the script function as an extra parameter. This can be useful so one script function can be used for manipulate decks.

When Mixxx receives a MIDI signal with the first two bytes matching the `<status>` and `<midino>` elements, the named script function is called. That function then determines how to change the state of Mixxx and/or script variables.

For system exclusive messages, the status byte must be 0xF0 and there does not need to be a midino element. If the controller can send multiple different SysEx messages, the one script function specified by the <key> element is responsible for deciding which has been received then taking the appropriate action.

New in 2.1: The value of the <key> element can be any snippet of JavaScript code that evaluates to a function (when executed in the global context).

JavaScript mapping basics

MIDI input handling functions

Short 3-byte messages

Except for system exclusive message, most MIDI signals are 3 byte messages. The parameters passed to functions linked to MIDI input from the controller in the XML file are, in order:

  1. MIDI channel (0x00 = Channel 1..0x0F = Channel 16,)
  2. Control/note number (byte 2)
  3. Value of the control (byte 3)
  4. MIDI status byte (byte 1)
  5. MixxxControl group (from the <group> value in the XML file)

Therefore, function definitions should look like:

ControllerName.functionName = function (channel, control, value, status, group) {
    // your custom code goes here

Note that in JavaScript, everything is an object. This code does not define a function as is done in many other programming languages; it defines a property of the ControllerName object called functionName and assigns its value to the function expression on the right of the = assignment operator. So, MyController.functionName is a variable that could be reassigned to a different function by the script at any time to change how the mapping handles input for this MIDI signal.

You can leave off parameters you don't need from the end of the function declaration, but the parameters must stay in order. For example, if you don't need the status or group variables, you can define an input handling function like:

ControllerName.functionName = function (channel, control, value) {
    // your custom code goes here

System Exclusive messages

Data passed from SysEx messages to functions are, in order:

  1. an array of raw data bytes
  2. the length of that array

Therefore, function definition should look like:

ControllerName.inboundSysex = function (data, length) {

If the controller can send multiple different SysEx messages, this one script function is responsible for deciding which has been received then taking the appropriate action.

Note that some controllers may send bytes that violate MIDI standards, e.g. setting the high bit in a data byte or using undefined status bytes (like 0xF9.) On Linux, recent versions of ALSA (from November 2012 onward) automatically standardize these by breaking the bytes into two nybbles and sending two bytes for every one received from the controller. For example 0xF0 0x97 0x30 0xF7 would become 0xF0 0x09 0x07 0x03 0x00 0xF7. Consult the ALSA documentation for full details.

Interact with Mixxx

Scripts can access the state of Mixxx through the Mixxx Control system using the following functions:

engine.getParameter(string group, string key);
engine.setParameter(string group, string key, double newValue);
engine.getValue(string group, string key);
engine.setValue(string group, string key, double newValue);

To check a Mixxx control value, call engine.getParameter() with the “group” and “key” values for a particular Mixxx Control, a list of which can be found here. For example:

var currentValue = engine.getParameter("[Channel1]", "rate");

Values can be set by calling engine.setParameter() with the group and key as above, and the new value to set, like so:

engine.setParameter("[Channel1]", "rate", 0.5);

engine.getParameter and engine.setParameter work with values on a scale from 0 to 1. These should be used for Mixxx Controls with a continuous range like volume, rate, and parameterN. The older engine.getValue and engine.setValue functions work with values on the scale listed for each Control on the MixxxControls page. The engine.get/setValue functions should be used for Controls with discrete states like orientation.

Note that since this is a script, you can do calculations and use state variables so a single function can work for multiple cases, such as a single controller working with Mixxx's multiple virtual decks (assuming you've defined currentDeck and currentValue here):

engine.setValue("[Channel"+currentDeck+"]", "rate", (currentValue+10)/2);

Tip: For toggling the state of a binary Mixxx Control, the script.toggleControl(string group, string key) function can be used as a convenient shortcut.

Connect output callback functions

To keep the state of your controller in sync with the state of Mixxx, register callback functions that Mixxx will execute when the state of a Mixxx Control changes. Typically these callback functions will send MIDI output to the controller, but they can also be used to change the state of script variables.

Mixxx 2.1

Callback functions are registered with the engine.makeConnection function, which takes 3 parameters:

  1. group of the Mixxx Control (string)
  2. name of the Mixxx Control (string)
  3. JavaScript function to execute when the Mixxx Control changes. This function takes three parameters: the new value of the Mixxx Control, the group, and the Mixxx Control name. this in the context of the function refers to the value of this where engine.makeConnection was called.

engine.makeConnection returns an object that represents the callback connection. This object should be stored in a script variable. To switch the controller between different modes, such as controlling a different deck:

  1. disconnect the old connection object by calling its disconnect method (with no arguments)
  2. register the new connection with engine.makeConnection
  3. call the trigger method of the new connection object (with no arguments) to immediately execute the callback using the state of the new Mixxx Control.

For example:

var syncButtonOutputCallback = function (value, group, control) {
    midi.sendShortMsg(byte 1, byte 2, value * 127); // see below section for an explanation of this example line
var syncConnection = engine.makeConnection('[Channel1]', 'sync_enabled', syncButtonOutputCallback);
// when switching to deck 3:
syncConnection = engine.makeConnection('[Channel3]', 'sync_enabled', syncButtonOutputCallback);

Mixxx 2.0 and older

Callback functions take the same 3 arguments as those passed to the new engine.makeConnection function described above, but the connections are registered with these functions:

  • engine.connectControl(control group, control name, script function) - Connects the specified Mixxx control signal to the script function. The script function can either be a JavaScript function or a string of JavaScript code that evaluates to a function.
  • engine.connectControl(control group, control name, script function name, true) - Tacking a , true on to the list of parameters disconnects the specified Mixxx Control from the specified script function. However, this only works when the script function is specified as a string of JavaScript code that evaluates to a function, not when an actual JavaScript function is passed.
  • engine.trigger(control group, control name) - Cause the specified Mixxx Control signal to fire so the connected script function is called with the updated value even if it hasn't changed, such as when forcing LEDs to update on a mode change. If multiple callbacks are connected, they will all be executed. This will only trigger connected JavaScript functions and will not refresh outputs connected in XML.

These functions are deprecated because there is no way to individually disconnect or trigger specific callback functions when multiple callbacks were connected to the same Mixxx Control.

Send MIDI output to the controller

To light LEDs and change other states of the controller, send MIDI messages back to the controller from your script. There are different functions for typical three-byte “short” messages and less common system-exclusive “long” messages:

midi.sendShortMsg(byte1, byte2, byte3);
midi.sendSysexMsg([array of bytes], length);

Generally, buttons have their LEDs controlled by sending a 3 byte short message with the same first two bytes as when the controller sends a signal for a button press. For example, if the controller sends a 0x91, 0x11, 0x7F message when a button is pressed, calling midi.sendShortMsg(0x91, 0x11, 0x7F) will light the LED and calling midi.sendShortMsg(0x91, 0x11, 0x00) will turn the LED off. If the LED has multiple colors, typically the color is determined by the third byte.

An explanation of the MIDI signals that your controller sends and receives should be available from the controller manufacturer. This is likely in a document on the product page for your controller on the manufacturer's website. If it is not in a separate document, it is likely at the end of the manual. If you cannot find this documentation, contact the manufacturer to ask for it. If they do not provide it, you will have to intercept the MIDI messages other DJ software uses or guess and check to figure out the messages to send to your controller. To intercept MIDI messages from other programs, you can use MIDIOX on Windows or MIDI Monitor on Mac OS X.

For system-exclusive messages, call midi.sendSysexMsg() with:

  • An array of data bytes to send, always leading with 0xF0 and ending with 0xF7
  • The number of bytes in the array, including the 0xF0 and 0xF7 (start counting with 1 or just Array.prototype.length property as below)
var byteArray = [ 0xF0, byte2, byte3, ..., byteN, 0xF7 ];
midi.sendSysexMsg(byteArray, byteArray.length);

Generally, you should not call midi.sendShortMsg or midi.sendSysexMsg directly from functions that handle MIDI input. Instead, the input function should change the state of a Mixxx Control and you should call midi.sendShortMsg/midi.sendSysexMsg in a callback function that reacts to changes in that Mixxx Control. Refer to the section above for details. This way, the state of the controller will always be in sync with what Mixxx is actually doing, even if the user manipulates Mixxx with the keyboard, mouse, or another controller. If the MIDI input handling function only changes the state of script variables but not Mixxx Controls, then it would be appropriate to call midi.sendShortMsg/midi.sendSysexMsg from the input handling function.

Components library

Now that you understand the basics, it is suggested to use the Components JS library for new mappings for Mixxx 2.1.


To prevent sudden wide parameter changes when the on-screen control diverges from a hardware control, use soft-takeover. While it's active on a particular parameter, manipulating the control on the hardware will have no effect until the position of the hardware control is close to that of the software, at which point it will take over and operate as usual. You can enable and disable it at any point, and it operates on each MixxxControl independently. Typically, for each control that has physical limits (typically, knobs and sliders) on your controller, you would enable soft-takeover in the init() script function and just leave it enabled.

It's very simple to use:

engine.softTakeover(string group, string key, bool enable);

So to enable soft-takeover for the pitch control on channel 1:

engine.softTakeover("[Channel1]", "rate", true);

…and to disable it:

engine.softTakeover("[Channel1]", "rate", false);

Note that this only works for controls manipulated through engine.setValue() in a script. It does not work for controls mapped in an XML file.

There is a bug in Mixxx 2.0 that prevented this from working with engine.setParameter(). This has been fixed in the master git branch and the fix will be in Mixxx 2.1.

New for 2.0: If you change the function of an absolute control (one that has hard stops at max and min positions) that is controlling MixxxControls with soft-takeover enabled, you will need to reset the soft-takeover status each time you change what the physical control is manipulating to avoid an abrupt jump when switching the physical control back. Do this with the following function, supplying the MixxxControl you're switching control away from:

engine.softTakeoverIgnoreNextValue("[Channel1]", "rate");

Scratching and jog wheels

Typically jog wheels are mapped so they control scratching when touched from the top and temporarily bend the pitch (speed up/slow down playback) when touched from the side, like a turntable. We have an easy way to scratch with jog wheels that send relative (+1/-1) signals. (Others can be scaled to work as well.) The applicable functions are:

engine.scratchEnable(int deck, int intervalsPerRev, float rpm, float alpha, float beta, bool ramp);
engine.scratchTick(int deck, int interval);
engine.scratchDisable(int deck, bool ramp);
bool engine.isScratching(int deck);

Here is how to use them:

  1. When you want to start scratching (such as when the wheel is touched,) call engine.scratchEnable() with:
    • the virtual deck number you want to scratch
    • the resolution of the MIDI control (in intervals per revolution, typically 128.)
    • the speed of the imaginary record at 0% pitch (in revolutions per minute (RPM) typically 33+1/3, adjust for comfort)
    • the alpha-beta filter coefficients (together these affect responsiveness and looseness of the imaginary slip mat)
      • the alpha value for the filter (start with 1/8 (0.125) and tune from there)
      • the beta value for the filter (start with alpha/32 and tune from there)
    • whether you want Mixxx to ramp the deck speed down or to stop instantly. (TRUE for ramping, which is the default.)
  2. Each time the MIDI control is moved, call engine.scratchTick() with:
    • the virtual deck number this control is currently scratching
    • the movement value (typically 1 for one “tick” forwards, -1 for one “tick” backwards)
  3. When you're done scratching (like when the wheel is released,) just call engine.scratchDisable() with the number of the virtual deck to stop scratching and whether you want Mixxx to ramp up to the play speed or jump to it instantly. (Default is to ramp which also allows spin-backs with wheels.)

Here is an example for the two most common types of wheels. Click the tab labeled 'scratchingExample.js' below to open this example as a file in your text editor.

// The button that enables/disables scratching
MyController.wheelTouch = function (channel, control, value, status, group) {
    if ((status & 0xF0) === 0x90) {    // If button down
  //if (value === 0x7F) {  // Some wheels send 0x90 on press and release, so you need to check the value
        var alpha = 1.0/8;
        var beta = alpha/32;
        engine.scratchEnable(MyController.currentDeck, 128, 33+1/3, alpha, beta);
    } else {    // If button up
// The wheel that actually controls the scratching
MyController.wheelTurn = function (channel, control, value, status, group) {
    // --- Choose only one of the following!
    // A: For a control that centers on 0:
    var newValue;
    if (value < 64) {
        newValue = value;
    } else {
        newValue = value - 128;
    // B: For a control that centers on 0x40 (64):
    var newValue = value - 64;
    // --- End choice
    // In either case, register the movement
    if (engine.isScratching(MyController.currentDeck)) {
        engine.scratchTick(MyController.currentDeck, newValue); // Scratch!
    } else {
        engine.setValue('[Channel'+MyController.currentDeck+']', 'jog', newValue); // Pitch bend

And that's it! Just make sure to map the button/touch sensor and wheel to these script functions as described above and you'll be ready to tear up some tracks.

Timed reactions

Sometimes you need to be able to do things at certain time intervals regardless of whether the controller is manipulated or something changes in Mixxx. Timed reactions let you do just that with 20ms resolution. Here are the functions:

  • engine.beginTimer(milliseconds, function, one-shot) - Starts a timer that will call the specified script function (with parameters if desired) repeatedly every time (if one-shot is false or not present) or just once (if one-shot is true) the given number of milliseconds (1/1000 second) pass. It returns an ID number for the timer (0 on failure) that you'll want to store in a variable so you can stop it later if it's a repeating timer. Note that the function must be enclosed in quotes.
  • engine.stopTimer(timer ID) - Stops the specified timer.

You can create and stop timers as much as you like but be aware that the operating system has limits on the number of timers it will allow, so remember to stop them as soon as you're done with them. (Not to mention that overall performance decreases as the number and/or frequency of timers increase.)

NEVER use busy-wait loops! (Loops that do nothing but delay. They can cause Mixxx to stutter.) Always use a timer instead!

See the script timers page for more details on best practices for using timers.

Spinback, brake and soft start effect

A forwards or backwards brake effect can be enabled/disabled using engine.brake() or engine.spinback(). engine.spinback() just calls engine.brake() with default settings to make it behave like a spinback.

To achieve a forward acceleration effect from standstill to normal speed use engine.softStart().

When disabled while active, all three functions would jump to normal playback speed.

Both engine.softStart() and engine.brake()/engine.spinback() can interrupt each other: slow down a track with engine.brake() and (even before track has stopped) get it back to normal speed with engine.softStart(). See last example how to map this to press/release of just one button.

brake(int deck, bool activate, [float factor], [float rate])
spinback(int deck, bool activate, [float factor], [float rate])
softStart(int deck, bool activate, [float factor])
  • deck - the deck number to use, e.g: 1
  • activate - true to activate or false to disable.
  • factor (optional) - how quickly the deck should come to a stop, normal playback rate respectively. start with a value of 1 and increase to increase the acceleration/deceleration. Be aware that brake/spinback called with low factors (about 0.5 and lower) would keep the deck running altough the resulting very low sounds are not audible anymore. Accordingly, softStart with low factors would take a while until sound is audible.
  • rate (optional) - the initial speed of the deck when enabled. “1” (default) means normal speed forwards, “-10” means 10x speed in reverse


Activate brake on button press, jump to normal speed on button release

    MyControllerPrefix.brake_button = function(channel, control, value, status, group) {
        var deck = parseInt(group.substring(8,9)); // work out which deck we are using 
        var activate = value > 0;
        if (activate) {
            engine.brake(deck, true); // enable brake effect
        } else {
            engine.brake(deck, false); // disable brake effect

Activate brake on button press, jump to normal speed on button release (short version)

    MyControllerPrefix.spinback_button = function(channel, control, value, status, group) {
        var deck = parseInt(group.substring(8,9)); // work out which deck we are using 
        engine.brake(deck, value > 0, 1.2, -10); // start at a rate of -10 and decrease at a factor of 1.2

Activate spinback on button press, jump to normal speed on button release

    MyControllerPrefix.spinback_button = function(channel, control, value, status, group) {
        var deck = parseInt(group.substring(8,9)); // work out which deck we are using
        engine.spinback(deck, value > 0, 2.5); // use default starting rate of -10 but decrease speed more quickly

Activate softStart on button press, jump to normal speed on button release

    MyControllerPrefix.softStart_button = function(channel, control, value, status, group) {
        var deck = parseInt(group.substring(8,9)); // work out which deck we are using
        engine.softStart(deck, value > 0, 2.0); // double the default acceleration

Brake on button press, softStart on button release

    MyControllerPrefix.brake_SoftStart_button = function(channel, control, value, status, group) {
        var deck = parseInt(group.substring(8,9)); // work out which deck we are using
        var activate = value > 0;
        if (activate) { // act on button press
            engine.brake(deck, true); // slow down the track
        } else { // act on button release
            engine.softStart(deck, true);

The effects can also be mapped directly via XML using either script.spinback, script.brake or script.softStart:


Helper functions

Here is a list of functions available to you from the always-loaded common-controller-scripts.js file:

  • nop() - Does nothing (No OPeration.) Empty function you can use as a place-holder while developing to avoid errors.
  • secondstominutes(seconds) - Returns the given quantity of seconds in MM:SS format.
  • msecondstominutes(milliseconds) - Returns the given quantity of milliseconds in format.
  • script.toggleControl(group, control) - toggles the state of a binary control
  • script.midiDebug(channel, control, value, status, group)2 - Prints the values as passed to it. Call this from anywhere in your function to see what the current values of these variables are. You can also of course put it in the <key/> tag of your XML to make sure the values being passed to the script are what you expect.
  • script.midiPitch(LSB, MSB, status) - Intended to be called from another script function, pass this the values from a MIDI Pitch control and it will return a corresponding value suitable for Mixxx's pitch sliders (“rate” controls.) So if you just want to set those controls, the calling function need only have the single line:
    engine.setValue("[Channel"+deck+"]","rate",script.midiPitch(control, value, status));
  • script.crossfaderCurve(value, min, max)1 - Sets the cross-fader's curve based on a value from an absolute control (0..127 by default, customize with min and max.)
  • script.absoluteLin(value, low, high, min, max)2 - Takes a value from an absolute control (0..127 by default, customize with min and max) and returns the proportionate value between low and high for a linear Mixxx control like deck volume or LFO depth. You can then use this returned value to set the desired Mixxx control.
  • script.absoluteLinInverse(value, low, high, min, max) - The inverse of the above function. This is useful for sending MIDI values back to controllers.
  • script.absoluteNonLin(value, low, mid, high, min, max) - Takes a value from an absolute control (0..127 by default, customize with min and max) and returns the proportionate value between low, mid and high for a non-linear Mixxx control such as EQ or master volume. You can then use this returned value to set the desired Mixxx control.
  • script.absoluteNonLinInverse(value, low, mid, high, min, max) - The inverse of the above function. This is useful for sending MIDI values back to controllers. New in 1.12
  • bpm.tapButton(deck) - Call this every time the desired tap button is pressed. It takes the progressive average of the last 8 taps and sets the bpm of the specified deck to that value, assuming the pitch range is large enough to reach it. (This depends on the track having the correct original BPM value.) If more than two seconds pass between taps, the history is erased.
  • String.prototype.toInt - returns an ASCII byte array for all the characters in any string. Use like so: “Test string”.toInt()

1 Introduced in 1.11.0 2 Renamed in 1.11.

Additional examples

Here are some examples to get you started. These examples start simple and get progressively more complex.

Button presses

MIDI buttons usually send a signal with a value of 0x7f (127) when the button is pressed and 0x00 (0) when the button is released. Thus, JavaScript functions mapped to button presses will be called both when the button is pressed and released. To make the function only do something when the button is pressed, wrap the function in an if statement checking the value parameter:

MyController.someButton = function (channel, control, value, status, group) {
    if (value === 127) {
        // do something when this button is pressed

Rescale incoming values

To reduce the sensitivity of a relative-mode (touch strip) pitch slider: (assuming <group> is specified appropriately in the XML file)

MyController.pitchSlider = function (channel, control, value, status, group) {   // Lower the sensitivity of the pitch slider
    var currentValue = engine.getValue(group,"rate");

IMPORTANT NOTE: You must always declare variables with “var” when you first use them inside a function since it establishes scope. If you omit this, the variable becomes global and will clobber anything else with the same name even if it's in another script file.

Storing commonly used MIDI codes in JS objects

Putting codes you need to reference many times throughout your script into a JavaScript object makes your code more organized, readable, and easier to modify later. A JavaScript object is a container for variables of any other type (including functions and other objects), referred to as the object's attributes. In this case, they are used similar to hash tables in other programming languages. For example, you could store the MIDI notes for buttons and the MIDI values for LED colors:

MyController.buttons = {
    '[Channel1]': { // an object within another object
        'play': 0x01,
        'cue': 0x02,
        'sync': 0x03
    '[Channel2]': {
        'play': 0x04,
        'cue': 0x05,
        'sync': 0x06
MyController.colorCodes = {
    'off': 0x00,
    'red': 0x01,
    'green': 0x02,
    'blue': 0x03

Now, when writing code to change an LED to green, instead of typing the note number for the LED and the value for green directly, you can reference the object's properties through your code. In addition to being easier to write, this makes your code easier to read. Easier to read code helps you remember what it does when you look at it again later. It also helps other people who may modify the code later. For example, for a function that automatically reacts to changes of the play state of a track (through the play_indicator Mixxx Control), you can write:

MyController.playButtonLED = function (value, group, control) {
                      MyController.buttons[group].play, // an object's properties can be referenced through either or SomeObject[property]
                                                        // but with the [] brackets, property can be a variable or other code
                      (value === 1) ? :
                      // The above line is a shortcut that means: "If value is 1, then send; otherwise, send"
                      // see

rather than:

MyController.playButtonLED = function (value, group, control) {
    if (group === '[Channel1]') {
        if (value === 0x7F) {
            midi.sendShortMsg(0x90, 0x01, 0x02)
        } else {
            midi.sendShortMsg(0x90, 0x01, 0x00)
    } else if (group === '[Channel2]') {
        if (value === 0x7F) {
            midi.sendShortMsg(0x90, 0x04, 0x02)
        } else {
            midi.sendShortMsg(0x90, 0x04, 0x00)

The two examples above have the same effects, but the first one is more intuitive because the code more clearly and concisely represents what it does. The MyController.buttons and MyController.colorCodes objects can help you refer to numbers by what they do rather than having a bunch of numbers throughout your code. If you decide to change which MIDI controls do what later, it will be easier to edit the one line in the object declaration than go through and find each MIDI code you need to change.

Tip: An explanation of the MIDI signals that your controller sends to computers and how it reacts to MIDI signals that computers send to it should be available from the controller manufacturer. This is likely in a document on the product page for your controller on the manufacturer's website. If it is not in a separate document, it is likely at the end of the manual.

Modifier (shift) buttons and layered mappings

Many controllers send different MIDI signals while a shift button is held down. In that case, JavaScript may not be needed and using XML may be sufficient. However, if this is not the case, to have controls do something different depending on whether a shift button or layer switching button is active, you need to use JavaScript. There are multiple ways this can be accomplished.

Tracking the state of the modifier in a variable

One approach is to declare a boolean (true/false) variable to keep track of whether the modifier button is currently pressed. In the XML file, map the modifier button to a function that toggles the state of this variable and map other controls to functions that check the state of the variable. For example:

MyController.shift = false;
MyController.shiftButton = function (channel, control, value, status, group) {
    // Note that there is no 'if (value === 127)' here so this executes both when the shift button is pressed and when it is released.
    // Therefore, MyController.shift will only be true while the shift button is held down
    MyController.shift = ! MyController.shift; // '!' inverts a boolean (true/false) value 
MyController.someButton = function (channel, control, value, status, group) {
    if (value === 127) { // only do stuff when the button is pressed, not when it is released
        if (MyController.shift) {
            // do something when this button and the shift button are both pressed
        } else {
            // do something else when this button is pressed but the shift button is not pressed

While this approach can work well for simple cases, if you are checking the value of MyController.shift in many functions, it can get cumbersome. Also, it can get difficult to keep track of everything that happens in each mode, especially if you have more than two modes for a control.

Reassigning MIDI input functions

An alternative approach is to define different functions for each layer. Because MIDI input handling functions are variables that can be reassigned, the function executed when a shift button or modifier is activated can reassign the variables to different values. If you have multiple controls you want to change the behavior of in different conditions, it helps to group the MIDI input handling functions for each layer within an object.

// A container for the functions of the active layer.
// In the XML file, map the MIDI input handling functions to
// properties of this object, for example, MyController.activeButtons.buttonA
MyController.activeButtons = {};
MyController.unshiftedButtons = {
    buttonA = function (channel, control, value, status, group) {
        //code to be executed when buttonA is pressed without shift
    buttonB = function (channel, control, value, status, group) {
        //code to be executed when buttonB is pressed without shift
MyController.shiftedButtons = {
    buttonA = function (channel, control, value, status, group) {
        //code to be executed when buttonA is pressed with shift
    buttonB = function (channel, control, value, status, group) {
        //code to be executed when buttonB is pressed with shift
MyController.init = function(id, debugging) {
    MyController.activeButtons = MyController.unshiftedButtons;
MyController.shiftButton = function (channel, control, value, status, group) {
    // This function is mapped to the incoming MIDI signals for the shift button in the XML file
    if (value === 127) { // shift button pressed
        engine.connectControl(group, key, true); // disconnect callbacks for unshifted layer
        // see "Automatic reactions to changes in Mixxx" section above
        MyController.activeButtons = MyController.shiftedButtons;
        engine.connectControl(group, key); // connect callbacks for shifted layer
    } else { // shift button released
        engine.connectControl(group, key, true); // disconnect callbacks for shifted layer
        MyController.activeButtons = MyController.unshiftedButtons;
        engine.connectControl(group, key); // connect callbacks for unshifted layer

Turning a 2 deck controller into a 4 deck controller

With the magic of MIDI scripting, you can turn a 2 deck controller into a 4 deck controller by setting up your script following the example below. This example is complex, so if you are new to programming, it is recommended that you read the examples above before trying to understand this one. If you have trouble understanding this code, please ask for help on the forum. There are many different ways to organize the code and this is just one example.

For every MIDI control that you want to map to a value that changes something about a specific deck, in the XML mapping file, map it to a function like the playButton function in the script below that starts with 'group = MyController.deck[group]'. Use [Channel1] as the value for the <group> element in the XML file for controls that manipulate decks 1/3 and [Channel2] for decks 2/4. Map the buttons you want to change between decks 1/3 and decks 2/4 to the deckToggleButton function.

Click the tab below labeled 'deckToggleExample.js' to download this example as a file to open in your text editor.

function MyController() {}
MyController.init = function () {
    // Set up the controller to manipulate decks 1 & 2 when this script is loaded (when Mixxx starts or you save an edited script file)
    // The MyController.initDeck function is defined below.
MyController.shutdown = function() {}
MyController.deck = {
    // a hash table (technically an object) to store which deck each side of the controller is manipulating
    // The keys (object properties) on the left represent the <group> elements in the XML mapping file.
    // The values on the right represent which deck that set of mappings in the XML file is currently controlling.
    // These values are toggled between [Channel1]/[Channel3] and [Channel2]/[Channel4] by the deckToggleButton function below.
    // see
    '[Channel1]': '[Channel1]',
    '[Channel2]': '[Channel2]'
MyController.buttons = { // a hash table that stores the MIDI notes that correspond to LED backlit buttons
    '[Channel1]': {
        'deckToggle': 0x01
        // Add any other LEDs for decks 1/3 here
     '[Channel2]': {
        'deckToggle': 0x02
        // Add any other LEDs for decks 2/4 here
MyController.buttons['[Channel3]'] = MyController.buttons['[Channel1]'] // Copy [Channel1] to [Channel3]
MyController.buttons['[Channel4]'] = MyController.buttons['[Channel2]'] // Copy [Channel2] to [Channel4]
MyController.channelRegEx = /\[Channel(\d+)\]/ // a regular expression used in the deckToggleButton function below
// This extracts the number from the strings '[Channel1]' ... '[Channel4]' so we can do math with that number
// see
MyController.deckToggleButton = function (channel, control, value, status, group) {
    if (value) { // only execute the below code when the button is pressed but not when it is released
        // First, get the number out of the string '[Channel1]' ... '[Channel4]'
        var deckNumber = parseInt( // convert string to an integer number variable
                             MyController.channelRegEx.exec( // execute the regular expression
                                 MyController.deck[group] // on this string
                             )[1] // Get the string that matches the part of the regular expression inside the first group of parentheses in the regular expression
                                  // which is (\d+)
                                  // this matches any number of digits
        if (deckNumber <= 2) {
            deckNumber += 2 // This is a shortcut for 'deckNumber = decknumber + 2'
        } else {
            deckNumber -= 2 // This is a shortcut for 'deckNumber = decknumber - 2'
        MyController.deck[group] = '[Channel' + deckNumber + ']'
        MyController.initDeck(MyController.deck[group]) // Initialize the new deck. This function is defined below.
MyController.initDeck = function (group) { // This function is not mapped to a MIDI signal; it is only called by this script in the init and deckToggleButton functions
    // Execute code to set up the controller for manipulating a deck
    // Putting this code in a function allows you to call the same code from the script's init function and the deckToggleButton function without having to copy and paste code
    // Figure out which deck was being controlled before so automatic reactions to changes in Mixxx (see above) can be disabled for that deck
    var disconnectDeck = parseInt(MyController.channelRegEx.exec(group)[1])
    if (disconnectDeck <= 2) {
        disconnectDeck += 2
    } else {
        disconnectDeck -= 2
    MyController.connectDeckControls('[Channel'+disconnectDeck+']', true) // disconnect old deck's Mixxx controls from LEDs. This function is defined below.
    MyController.connectDeckControls(group) // connect new deck's Mixxx controls to LEDs
    // Toggle LED that indicates which deck is being controlled
        (disconnectDeck > 2) ? 0x7f : 0x00 // If the condition in parentheses is true, send 0x7f; otherwise, send 0x00
                                           // see
MyController.connectDeckControls = function (group, remove) { // This function is not mapped to a MIDI signal; it is only called by this script in the initDeck function below
    // This function either connects or disconnects automatic reactions to changes in Mixxx (see wiki section above), depending on the value of the 'remove' parameter
    // Putting this in its own function allows the same code to be reused for both connecting and disconnecting
    // This is particularly helpful when the list of Mixxx controls connected to LEDs is long
    remove = (typeof remove !== 'undefined') ? remove : false // If the 'remove' parameter is not passed to this function, set remove = false
    var controlsToFunctions = { // This hash table maps Mixxx controls to the script functions (not shown in this example) that control LEDs that react to changes in those controls
        'play': 'MyController.playButtonLED',
        'sync_enabled': 'MyController.syncLED',
        'pfl': 'MyController.headphoneLED'
        // ... and any other functions that react to changes in Mixxx controls for a deck
    for (var control in controlsToFunctions) { // For each property (key: value pair) in controlsToFunctions, control = that property of controlsToFunctions
                                               // see
        engine.connectControl(group, control, controlsToFunctions[control], remove)
        if (! remove) { // '!' means "not"; it inverts the value of a boolean (true/false)
            engine.trigger(group, control)
MyController.playButton = function (channel, control, value, status, group) {
    group = MyController.deck[group] // Change the value of the group variable to the deck we actually want to manipulate based on the state of the deck toggle button
    if (value) {
        // toggle whether the deck is playing
        engine.setValue(group, 'play', ! (engine.getValue(group, 'play')
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midi_scripting.txt · Last modified: 2017/04/07 11:35 by ronso