Publish Block Libraries

Plugins that provide libraries of block definitions are a great way to share your reusable blocks with the Blockly community. To make your block library as versatile and useful as possible, we've developed these guidelines.


  • Make it easy for users to install all of your blocks, and make it possible for users to install only certain blocks or pieces of blocks they choose.
    • Make it easy to install everything: You can do this by providing a function that installs every piece a single block definition requires (such as mutators, extensions, mixins, fields, etc.). You can also provide a function that will install all of the blocks offered by your plugin at once.
    • Make it possible to choose specific parts: You should export all of the pieces of a block definition separately, so that it is possible for a user to import only the pieces they need in order to create their own similar custom block.
  • Avoid using side effects in your plugin.
    • Blocks, fields, extensions, and other pieces shouldn't be installed as a side effect of loading your plugin. Users should maintain control over which things are installed and when. This allows users to import the pieces they need without worrying that pieces they don't will be installed.
  • Use the JSON field registry instead of instantiating new fields directly.

    • Not Recommended - Instantiating a new field directly:

        const myCustomBlock = {
          init: function() {
                .appendField(new Blockly.FieldNumber(123), 'NAME');
    • Recommended - JSON field registry:

        export const myCustomBlock = {
          init: function() {
                    name: 'field_number',
                    value: 123,
                  }), 'NAME');
    • Using the field registry makes it easier for a user to replace the implementation of the field used in your block without having to change the block definition.

  • Don't make assumptions about what the user has already installed.

    • If your plugin requires a custom field or another plugin, register those fields yourself in your provided install function.
    • Soon, Blockly will provide tools that let you register already-registered items without error. Until then, you may want to check what has already been registered before registering an extension, mutator, mixin, or field yourself.
    • Be clear about any prerequisites or dependencies that are required by your plugin or block definitions.
  • Consider providing generator functions for each of the blocks you provide.

    • Providing generator functions that work out of the box makes it easier for users to use your blocks without having to understand their structure and design. If they have to write their own generator functions, this may lead to redundant work being done by each user.
    • JavaScript is the most commonly used language in Blockly, so if you only pick one language to provide, we recommend JavaScript, unless your blocks are built for a specific language such as implementing a Python library.
    • Consider posting 'help wanted' issues for languages for which you are unable to implement generator functions, and accept pull requests for these if a user contributes them.
    • If you provide an install function for your block, you can accept an optional generators parameter. If a user passes a generator instance that you support, you can automatically install the block-code generator function and do related work such as adding reserved words:

        // Your plugin's install function
        export const installMyCustomBlock(generators = {}) {
          Blockly.defineBlocks({my_custom_block: myCustomBlock});
          if (generators.javascript) {
            generators.javascript.forBlock['my_custom_block'] = myCustomGeneratorFunction;
        // How a user may install your block
        import {javascriptGenerator} from 'blockly/javascript';
        import {installMyCustomBlock} from 'blockly-cool-blocks-plugin';
        // installs the block definition and the javascript block-code generator
        installMyCustomBlock({javascript: javascriptGenerator});


If you have questions about how to best follow these guidelines in your plugin, let us know in the forum! We'd love to see your block libraries and provide feedback on them.

Note that not all first-party plugins that provide block definitions currently follow these guidelines, but new plugins will and we plan to migrate existing plugins.