Implementing a manifest property

Like functions and events, implementing a new manifest key requires writing a definition in the schema and extending the API’s instance of ExtensionAPI.

The contents of a WebExtension’s manifest.json are validated using a type called WebExtensionManifest defined in the namespace manifest. The first step when adding a new property is to extend the schema so that manifests containing the new property pass validation. This is done with the "$extend" property as follows:

  "namespace": "manifest",
  "types": [
      "$extend": "WebExtensionManifest",
      "properties": {
        "my_api_property": {
          "type": "string",
          "optional": true,

The next step is to inform the WebExtensions framework that this API should be instantiated and notified when extensions that use the new manifest key are loaded. For built-in APIs, this is done with the manifest property in the API manifest (e.g., ext-toolkit.json). Note that this property is an array so an extension can implement multiple properties:

"myapi": {
  "schema": "...",
  "url": "...",
  "manifest": ["my_api_property"]

The final step is to write code to handle the new manifest entry. The WebExtensions framework processes an extension’s manifest when the extension starts up, this happens for existing extensions when a new browser session starts up and it can happen in the middle of a session when an extension is first installed or enabled, or when the extension is updated. The JSON fragment above causes the WebExtensions framework to load the API implementation when it encounters a specific manifest key while starting an extension, and then call its onManifestEntry() method with the name of the property as an argument. The value of the property is not passed, but the full manifest is available through this.extension.manifest:

this.myapi = class extends ExtensionAPI {
  onManifestEntry(name) {
    let value = this.extension.manifest.my_api_property;
    /* do something with value... */