pxt.json Manual Page

A PXT extension lives in its own directory, locally under libs/ in a PXT target. A extension is described by the pxt.json file. To show a real example, here is the pxt.json file for the pxt-neopixel extension.

The pxt.json is described by the interface PackageConfig in pxtpackage.d.ts:

Packages are now referred to as extensions. The use of the package name in identifiers implies extension.

interface PackageConfig {
    name: string;                 // public:true -> name must match ^[a-z][a-z0-9\-_]+
    description?: string;         // longer description of extension
    license?: string;             // name of license (as found on github)
    authors?: string[];      

    files: string[];              // files to be included and compiled in the extension
    additionalFilePath?: string;  // another directory to find files from

    dependencies: Map<string>;    // extension dependencies (see below for more)
    public?: boolean;             // set true to enable the extension to be published (to cloud),
                                    // in support of publishing user scripts

    icon?: string;                // url to icon -- support for built-in extensions only
    card?: CodeCard;
    documentation?: string; // doc page to open when loading project

    // semver description for support target version
    version?: string;
    installedVersion?: string;
    targetVersions?: TargetVersions; // versions of the target/pxt the extension was compiled against

    fileDependencies?: Map<string>; // exclude certain files if dependencies are not fulfilled

    testFiles?: string[];
    testDependencies?: Map<string>;
    simFiles?: string[];

    cppDependencies?: Map<string>;

    binaryonly?: boolean;
    platformio?: PlatformIOConfig;
    yotta?: YottaConfig;

    gistId?: string;

dependencies (on other extensions)

Simple extensions generally just depend on their own target’s core extension:

    "dependencies": {
        "core": "file:../core"

A number of targets use pxt-common-packages and specialize them to fit their target’s needs. These are a base set of extensions for use in a target. For example, the Adafruit Circuit Playground Express extension is the union of a number of extensions.

    "dependencies": {
        "base": "file:../base",
        "core": "file:../core",
        "buttons": "file:../buttons",
        "accelerometer": "file:../accelerometer",
        "lightsensor": "file:../lightsensor",
        "thermometer": "file:../thermometer",
        "music": "file:../music",
        "light": "file:../light",
        "switch": "file:../switch",
        "infrared": "file:../infrared",
        "microphone": "file:../microphone",
        "touch": "file:../touch"

Each of the above extensions is local to the target but inherits code from pxt-common-packages, which it can then override or specialize, as the target needs. For example, the button extension in the target pxt-adafruit is defined in terms of the button extension from pxt-common-packages using the additionalFilePath field:

    "description": "Button A and B drivers",
    "additionalFilePath": "../../node_modules/pxt-common-packages/libs/buttons"

The additionalFilePath field refers to the node_modules directory of the target. The pxt.json file need to only specify what’s changed (in the example above description) with respect to the pxt.json in additionalFilePath.

The additionalFilePath is recursive or multi-level - the pxt.json in the referenced directory might have another additionalFilePath and it will work as expected.

Test files

The files listed under testFiles are only included when the extension is compiled as the top-level program and not just imported into some other program. Typically this happens when you run pxt from command line in the extension directory, or when you hit Download when editing extension itself in the online editor. They usually contain unit tests for extension.

Similarly, dependencies from testDependencies are only included when compiled as top-level. The testDependencies can be added for multiple targets and will only be added if they can be resolved.

File dependencies

While not very common, in some extensions certain functionality should be only enabled when another extension is already present in the project. For example, a weather sensor package may have code for streaming weather data over radio, but that should be only enabled when there’s already the radio extension in the project (to avoid problems on boards without radio, or when Bluetooth disables radio). Another solution to this problem is to create a new package weather-radio, which depends on weather and radio. This is advisable, when the additional functionality is sizable, otherwise it’s better to keep the number of packages down.

Example configuration:

  "files": [
  "fileDependencies": {
      "weather-radio.ts": "radio",
      "weather-jacdac.ts": "jacdac",
      "jd-helper.ts": "jacdac"

Here, the file weather-radio.ts will be only included when radio is referenced in the project, and files weather-jacdac.ts and jd-helper.ts will be only included when jacdac is present.

Typically, you would add radio and jacdac as testDependencies, so you can see the entire extension in the editor. There is no point in adding them as regular dependencies - that would negate the effects of fileDependencies and always include both the dependencies and files.

In future, we may allow things like "radio >= 1.2.3", but for now the package identifier is the only thing supported.

C++ dependencies

Dependencies under cppDependencies are only considered when generating code for the C++ compiler. Usually, one would list all optional packages which contain C++ code in cppDependencies of your core package. Then, when user actually adds any of these optional packages, the C++ code doesn’t change and re-compilation (and thus cloud round-trip) is not required.