Annotated example

var SomeBlocks = function (runtimeProxy) {
    /**
     * A proxy to communicate with the Scratch 3.0 runtime across a worker boundary.
     * @type {Runtime}
     */
    this.runtime = runtimeProxy;
};

/**
 * @return {object} This extension's metadata.
 */
SomeBlocks.prototype.getInfo = function () {
    return {
        // Required: the machine-readable name of this extension.
        // Will be used as the extension's namespace.
        id: 'someBlocks',

        // Optional: the human-readable name of this extension as string.
        // This and any other string to be displayed in the Scratch UI may either be
        // a string or a call to `formatMessage`; a plain string will not be
        // translated whereas a call to `formatMessage` will connect the string
        // to the translation map (see below). The `formatMessage` call is
        // similar to `formatMessage` from `react-intl` in form, but will actually
        // call some extension support code to do its magic. For example, we will
        // internally namespace the messages such that two extensions could have
        // messages with the same ID without colliding.
        // See also: https://github.com/yahoo/react-intl/wiki/API#formatmessage
        name: formatMessage({
            id: 'extensionName',
            defaultMessage: 'Some Blocks',
            description: 'Extension name'
        }),

        // Optional: URI for a block icon, to display at the edge of each block for this
        // extension. Data URI OK.
        // TODO: what file types are OK? All web images? Just PNG?
        blockIconURI: 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAkAAAAFCAAAAACyOJm3AAAAFklEQVQYV2P4DwMMEMgAI/+DEUIMBgAEWB7i7uidhAAAAABJRU5ErkJggg==',

        // Optional: URI for an icon to be displayed in the blocks category menu.
        // If not present, the menu will display the block icon, if one is present.
        // Otherwise, the category menu shows its default filled circle.
        // Data URI OK.
        // TODO: what file types are OK? All web images? Just PNG?
        menuIconURI: 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAkAAAAFCAAAAACyOJm3AAAAFklEQVQYV2P4DwMMEMgAI/+DEUIMBgAEWB7i7uidhAAAAABJRU5ErkJggg==',

        // Optional: Link to documentation content for this extension.
        // If not present, offer no link.
        docsURI: 'https://....',

        // Required: the list of blocks implemented by this extension,
        // in the order intended for display.
        blocks: [
            {
                // Required: the machine-readable name of this operation.
                // This will appear in project JSON.
                opcode: 'myReporter', // becomes 'someBlocks.myReporter'

                // Required: the kind of block we're defining, from a predefined list:
                // 'command' - a normal command block, like "move {} steps"
                // 'reporter' - returns a value, like "direction"
                // 'Boolean' - same as 'reporter' but returns a Boolean value
                // 'hat' - starts a stack if its value is truthy
                // 'conditional' - control flow, like "if {}" or "if {} else {}"
                // A 'conditional' block may return the one-based index of a branch to
                // run, or it may return zero/falsy to run no branch.
                // 'loop' - control flow, like "repeat {} {}" or "forever {}"
                // A 'loop' block is like a conditional block with two differences:
                // - the block is assumed to have exactly one child branch, and
                // - each time a child branch finishes, the loop block is called again.
                blockType: 'reporter',

                // Required for conditional blocks, ignored for others: the number of
                // child branches this block controls. An "if" or "repeat" block would
                // specify a branch count of 1; an "if-else" block would specify a
                // branch count of 2.
                // TODO: should we support dynamic branch count for "switch"-likes?
                branchCount: 0,

                // Optional, default false: whether or not this block ends a stack.
                // The "forever" and "stop all" blocks would specify true here.
                terminal: true,

                // Optional, default false: whether or not to block all threads while
                // this block is busy. This is for things like the "touching color"
                // block in compatibility mode, and is only needed if the VM runs in a
                // worker. We might even consider omitting it from extension docs...
                blockAllThreads: false,

                // Required: the human-readable text on this block, including argument
                // placeholders. Argument placeholders should be in [MACRO_CASE] and
                // must be [ENCLOSED_WITHIN_SQUARE_BRACKETS].
                text: formatMessage({
                    id: 'myReporter',
                    defaultMessage: 'letter [LETTER_NUM] of [TEXT]',
                    description: 'Label on the "myReporter" block'
                }),

                // Required: describe each argument.
                // Argument order may change during translation, so arguments are
                // identified by their placeholder name. In those situations where
                // arguments must be ordered or assigned an ordinal, such as interaction
                // with Scratch Blocks, arguments are ordered as they are in the default
                // translation (probably English).
                arguments: {
                    // Required: the ID of the argument, which will be the name in the
                    // args object passed to the implementation function.
                    LETTER_NUM: {
                        // Required: type of the argument / shape of the block input
                        type: 'number',

                        // Optional: the default value of the argument
                        default: 1
                    },

                    // Required: the ID of the argument, which will be the name in the
                    // args object passed to the implementation function.
                    TEXT: {
                        // Required: type of the argument / shape of the block input
                        type: 'string',

                            // Optional: the default value of the argument
                        default: formatMessage({
                            id: 'myReporter.TEXT_default',
                            defaultMessage: 'text',
                            description: 'Default for "TEXT" argument of "myReporter"'
                        })
                    }
                },

                // Required: the function implementing this block.
                func: 'myReporter',

                // Optional: list of target types for which this block should appear.
                // If absent, assume it applies to all builtin targets -- that is:
                // ['sprite', 'stage']
                filter: ['someBlocks.wedo2', 'sprite', 'stage']
            },
            {
                // Another block...
            }
        ],

        // Optional: define extension-specific menus here.
        menus: {
            // Required: an identifier for this menu, unique within this extension.
            menuA: [
                // Static menu: list items which should appear in the menu.
                {
                    // Required: the value of the menu item when it is chosen.
                    value: 'itemId1',

                    // Optional: the human-readable label for this item.
                    // Use `value` as the text if this is absent.
                    text: formatMessage({
                        id: 'menuA_item1',
                        defaultMessage: 'Item One',
                        description: 'Label for item 1 of menu A'
                    })
                },

                // The simplest form of a list item is a string which will be used as
                // both value and text.
                'itemId2'
            ],

            // Dynamic menu: returns an array as above.
            // Called each time the menu is opened.
            menuB: 'getItemsForMenuB'
        },

        // Optional: translations
        translation_map: {
            de: {
                'extensionName': 'Einige Blöcke',
                'myReporter': 'Buchstabe [LETTER_NUM] von [TEXT]',
                'myReporter.TEXT_default': 'Text',
                'menuA_item1': 'Artikel eins',

                // Dynamic menus can be translated too
                'menuB_example': 'Beispiel',

                // This message contains ICU placeholders (see `myReporter()` below)
                'myReporter.result': 'Buchstabe {LETTER_NUM} von {TEXT} ist {LETTER}.'
            },
            it: {
                // ...
            }
        },

        // Optional: list new target type(s) provided by this extension.
        targetTypes: [
            'wedo2', // automatically transformed to 'someBlocks.wedo2'
            'speech' // automatically transformed to 'someBlocks.speech'
        ]
    };
};

/**
 * Implement myReporter.
 * @param {object} args - the block's arguments.
 * @property {string} MY_ARG - the string value of the argument.
 * @returns {string} a string which includes the block argument value.
 */
SomeBlocks.prototype.myReporter = function (args) {
    // This message contains ICU placeholders, not Scratch placeholders
    const message = formatMessage({
        id: 'myReporter.result',
        defaultMessage: 'Letter {LETTER_NUM} of {TEXT} is {LETTER}.',
        description: 'The text template for the "myReporter" block result'
    });

    // Note: this implementation is not Unicode-clean; it's just here as an example.
    const result = args.TEXT.charAt(args.LETTER_NUM);

    return message.format({
        LETTER_NUM: args.LETTER_NUM,
        TEXT: args.TEXT,
        LETTER: result
    });
};

Extension initialization

Each extension in Scratch 3.0 is sandboxed in its own web worker along with a small amount of support code. Below is a rough depiction of the sequence of events between the request to load an extension and that extension being ready to run blocks. For the sake of this example, assume  ext1.js  is an extension file.
Main thread
 
Extension worker
User requests to load an extension
 
 
GUI asks extension manager to load  ext1.js
 
 
Extension manager puts  ext1.js  into the extension-load queue
 
 
Extension manager starts a new worker
->
 
 
 
Extension worker loads support code
Central dispatch handshakes with new extension worker
->
 
 
 
Worker dispatch system returns handshake
 
Extension worker asks to be allocated an extension
Extension manager allocates the next ID (say,  0 ) and the next extension in the queue ( ext1.js ) to the extension worker as one composite action
->
 
 
 
Extension worker loads  ext1.js
 
Extension worker registers service  extension0  with dispatch system