Defining blocks

This section describes how to annotate your PXT APIs to expose them in the Block Editor.

All the //% annotations are found in TypeScript library files. They can optionally be auto-generated from C++ library files or from TypeScript simulator files.


Each top-level javascript namespace is used to populate a category in the Block Editor toolbox. The name will automatically be capitalized in the toolbox.

namespace basic {

You can also provide a JsDoc comment, color and weight for the namespace, as well as a friendly name (in Unicode). We strongly recommend carefully picking colors as it dramatically impacts that appearance and readability of your blocks. All blocks within the same namespace have the same color so that users can find the category easily from samples.

 * Provides access to basic micro:bit functionality.
//% color=190 weight=100 icon="\uf1ec" block="Basic Blocks"
namespace basic {
  • icon icon Unicode character from the icon font to display. The Semantic UI icon set has been ported from Font Awesome (v4.5.6 at the time of writing), and a full list can be found at
  • color should be included in a comment line starting with //%. The color takes a hue value or a HTML color.

To have a category appear under the “Advanced” section of the Block Editor toolbox, add the annotation advanced=true.


All exported functions with a block attribute will be available in the Block Editor.

//% block
export function showNumber(v: number, interval: number = 150): void
{ }

If you need more control over the appearance of the block, you can specify the blockId and block parameters.

//% blockId=device_show_number
//% block="show|number %v"
export function showNumber(v: number, interval: number = 150): void
{ }
  • blockId is a constant, unique id for the block. This id is serialized in block code so changing it will break your users.
  • block contains the syntax to build the block structure (more below).

Other optional attributes can also be used:

  • blockExternalInputs= forces External Inputs rendering
  • advanced=true causes this block to be placed under the parent category’s “More…” subcategory. Useful for hiding advanced or rarely-used blocks by default

Block syntax

The block attribute specifies how the parameters of the function will be organized to create the block.

block = field, { '|' field }
field := string
    | string `%` parameter [ `=` type ]
parameter = string
type = string
  • each field is mapped to a field in the block editor
  • the function parameter are mapped in order to %parameter argument. The loader automatically builds a mapping between the block field names and the function names.
  • the block will automatically switch to external inputs when enough parameters are detected
  • A block type =type can be specified optionally for each parameter. It will be used to populate the shadow type.

Supported types

The following types are supported in function signatures that are meant to be exported:

  • number (TypeScript) or int (C++)
  • string (TypeScript) or StringData* (C++)
  • enums (see below)
  • custom classes that are also exported
  • arrays of the above

Callbacks with Parameters

APIs that take in a callback function will have that callback converted into a statement input. If the callback in the API is designed to take in parameters, the best way to map that pattern to the blocks is by passing the callback a single parameter with a class type that contains all the other values. For example:

export class ArgumentClass {
    argumentA: number;
    argumentB: string;

//% mutate=objectdestructuring
//% mutateText="My Arguments"
//% mutateDefaults="argumentA;argumentA,argumentB"
// ...
export function addSomeEventHandler((a: ArgumentClass) => void) { };

In the above example, setting mutate=objectdestructuring will cause this API to use Blockly “mutators” to let users change what parameters appear in the blocks. Each parameter will be given an optional variable field in the block that defines a variable that can be used within the callback. The variable fields compile to object destructuring in the TypeScript code. For example:

addSomeEventHandler(({argumentA, argumentB}) => {


For an example of this pattern in action, see the radio.onDataPacketReceived block in the microbit target.

In some cases it can be useful to change the runtime behavior of the API based on the properties selected by the user. To enable that behavior, create an enum with entries that have the same names as the argument object’s properties and add an extra parameter taking in an enum array to the API. For example:

export class ArgumentClass {
    argumentA: number;
    argumentB: string;

enum ArgNames {

//% mutate=objectdestructuring
//% mutateText="My Arguments"
//% mutateDefaults="argumentA;argumentA,argumentB"
//% mutatePropertyEnum="argNames"
// ...
export function addSomeEventHandler(args: ArgNames[], (a: ArgumentClass) => void) { };

Note the mutatePropertyEnum attribute added to the comment annotations. The block for this API will look the same as the previous example but the compiled code will also include the arguments passed:

addSomeEventHandler([ArgNames.argumentA, ArgNames.argumentB], ({argumentA, argumentB}) => {


The other attributes related to object destructuring mutators include:

  • mutateText - defines the text that appears in the top block of the Blockly mutator dialog (the dialog that appears when you click the blue gear)
  • mutateDefaults - defines the versions of this block that should appear in the toolbox. Block definitions are separated by semicolons and property names should be separated by commas


Enum are supported and will automatically be represented by a dropdown in blocks.

enum Button {
    A = 1,
    B = 2,
    //% blockId="ApB" block="A+B"
    AB = 3,
  • the initializer can be used to map the value
  • the blockId attribute can be used to override the block id
  • the block attribute can be used to override the rendered string

Tip: dropdown for non-enum parameters

It’s possible to provide a drop-down for a parameter that is not an enum. It involves the following step:

  • create an enum with desired drop down entry
    enum Delimiters {
      //% block="new line"
      NewLine = 1,
      //% block=","
      Comma = 2
  • a function that takes the enum as parameter and returns the according value
    //% blockId="delimiter_conv" block="%del"
    export function delimiters(del : Delimiters) : string {
      switch(del) {
          case Delimiters.NewLine: return "\n";
          case Delimiters.Comma:  return ",";
  • use the enum conversion function block id (delimiter_conv) as the value in the block parameter of your function
    //% blockId="read_until" block="read until %del=delimiter_conv"
    export function readUntil(del: string) : string {

Docs and default values

The JSDoc comment is automatically used as the help for the block.

 * Scroll a number on the screen. If the number fits on the screen (i.e. is a single digit), do not scroll.
 * @param interval speed of scroll; eg: 150, 100, 200, -100
//% help=functions/show-number
export function showNumber(value: number, interval: number = 150): void
{ }
  • If @param annotation is available with an eg: section, the first value is used as the shadow value.
  • An optional help attribute can be used to point to an specific documentation path.
  • If the parameter has a default value (interval in this case), it is not exposed in blocks.
  • If you want to include minimum and maximum value range for a numeric parameter, you can use square brackets with the range [min-max] after the parameter name in the @param annotation. It is important to include the shadow value if you are using range
    • @param power [0-7] a value in the range 0..7, where 0 is the lowest power and 7 is the highest. eg: 7

Objects and Instance methods

It is possible to expose instance methods and object factories, either directly or with a bit of flattening (which is recommended, as flat, C-style APIs map best to blocks).


class Message {
    //% blockId="message_get_text" block="%this|text"
    public getText() { ... }
  • when annotating an instance method, you need to specify the %this parameter in the block syntax definition.

You will need to expose a factory method to create your objects as needed. For the example above, we add a function that creates the message:

//% blockId="create_message" block="create message|with %text"
export function createMessage(text: string) : Message {
    return new Message(text);


If object has a reasonable default constructor, and it is harmless to call this constructor even if the variable needs to be overwritten later, then it’s useful to designate a parameter-less function as auto-create, like this:

namespace images {
    export function emptyImage(width = 5, height = 5): Image { ... }
//% autoCreate=images.emptyImage
class Image {

Now, when user adds a block referring to a method of Image, a global variable will be automatically introduced and initialized with images.emptyImage().

In cases when the default constructor has side effects (eg., configuring a pin), or if the default value is most often overridden, the autoCreate syntax should not be used.

Fixed Instance Set

It is sometimes the case that there is only a fixed number of instances of a given class. One example is object representing pins on an electronic board. It is possible to expose these instances in a manner similar to an enum:

//% fixedInstances
class DigitalPin {
    //% blockId=device_set_digital_pin block="digital write|pin %name|to %value"
    //% blockNamespace=pins
    digitalWrite(value: number): void { ... }

namespace pins {
    //% fixedInstance
    let D0: DigitalPin;
    //% fixedInstance
    let D1: DigitalPin;

This will result in a block digital write pin [D0] to [0], where the first hole is a dropdown with D0 and D1, and the second hole is a regular integer value. The variables D0 and D1 can have additional annotations (eg., block="D#0"). Currently, only variables are supported with fixedInstance (let or const).

Fixed instances also support inheritance. For example, consider adding the following declarations.

//% fixedInstances
class AnalogPin extends DigitalPin {
    //% blockId=device_set_analog_pin block="analog write|pin %name|to %value"
    //% blockNamespace=pins
    analogWrite(value: number): void { ... }

namespace pins {
    //% fixedInstance
    let A0: AnalogPin;

The analog write will have a single-option dropdown with A0, but the optionals on digital write will be now D0, D1 and A0.

Variables with fixedInstance annotations can be added anywhere, at the top-level, even in different libraries or namespaces.

This feature is often used with indexedInstance* attributes.

It is also possible to define the instances to be used in blocks in TypeScript, for example:

namespace pins {
    //% fixedInstance whenUsed
    export const A7 = new AnalogPin(7);

The whenUsed annotation causes the variable to be only included in compilation when it is used, even though it is initialized with something that can possibly have side effects. This happens automatically when there is no initializer, or the initializer is a simple constant, but for function calls and constructors you have to include whenUsed.


All blocks have a default weight of 50 that is used to sort them in the UI with the highest weight showing up first. To tweak the ordering, simply annotate the function with the weight macro:

//% weight=10

If given API is only for Blocks usage, and doesn’t make much sense in TypeScript (for example, because there are alternative TypeScript APIs), you can use //% hidden flag to disable showing it in auto-completion.


Use the blockGap macro to specify the distance to the next block in the toolbox. Combined with the weight parameter, this macro allows to define groups of blocks. The default blockGap value is 8.

//% blockGap=14

Testing your Blocks

We recommend to build your block APIs iteratively and try it out in the editor to get the “feel of it”. To do so, the ideal setup is:

  • run your target locally using pxt serve
  • keep a code editor with the TypeScript opened where you can edit the APIs
  • refresh the browser and try out the changes on a dummy program.

Interestingly, you can design your entire API without implementing it!

Deprecating Blocks

To deprecate an existing API, you can add the deprecated attribute like so:

//% deprecated=true

This will cause the API to still be usable in TypeScript, but prevent the block from appearing in the Blockly toolbox. If a user tries to load a project that uses the old API, the project will still load correctly as long as the TypeScript API is present. Any deprecated blocks in the project will appear in the editor but not the toolbox.

API design Tips and Tricks

A few tips gathered while designing various APIs for the Block Editor.

  • Design for beginners: the block interface is for beginners. You’ll want to create a specific layer of C-like function for that purpose.
  • Anything that snaps together will be tried by the user: your runtime should deal with invalid input with graceful degradation rather than abrupt crashes. Some users will try to snap anything together - get ready for it.
  • OO is cumbersome in blocks: we recommend using a C-like APIs – just function – rather than OO classes. It maps better to blocks.
  • Keep the number of blocks small: there’s only so much space in the toolbox. Be specific about each API you want to see in Blocks.