Typings for `Form#on` and `FormEditor#on` are incorrect

[nikku]

Describe the Bug

We current expose type definitions for Form#on and FormEditor#on that require users to use a third priority=number argument. We should fix our type definitions to allow both forms, two arg and three argument invocations to the list:

form.on('change', function(event) { });
form.on('change', 500, function(event) { });

Expected Behavior

As a user I'd like to use the simple form of registering for events. That form is referenced and documented, too.

Environment

• Host (Browser/Node version), if applicable: any
• OS: Windows
• Library version: v0.4.1

---

Reported via Camunda Cloud Devs.

[barmac]

Related to microsoft/TypeScript#25590

[dfulgham]

Sorry if this is not a good way of handling this, but would something like this take care of this issue?

Add the following typedef's

/**
 * @typedef { (type:FormEvent,priorityOrHandler:number,handler:Function) => void } OnEventWithPriority;
 * @typedef { (type:FormEvent,priorityOrHandler:Function) => void } OnEventWithOutPriority;
 * @typedef { OnEventWithPriority & OnEventWithOutPriority} OnEventType
 * /

then create a private definition (#onEvent) for the on method and assign (this.on = #onEvent) in the constructor. This will result in the overload being typed correctly in Typescript and in the IDE

...

/**
   *  private OnEvent function definition
   * 
   */
  #onEvent = (type,priorityOrHandler,handler) => {
    if (typeof priorityOrHandler !== 'number' && typeof handler === 'undefined') {
      handler = priorityOrHandler;
      priorityOrHandler = 0;
    }
    this.get('eventBus').on(type, priorityOrHandler, handler);
  }

  /**
   * @constructor
   * @param {FormOptions} options
   */
  constructor(options = {}) {

    /**
     * @type {OnEventType}
    */
    this.on = this.#onEvent;

...
}

[dfulgham]

in my PR its not accepting the #onEvent private class method, I could just change it to _onEvent ? I have the same version of all libraries and it compiles and tests fine on my local machine, but the testing environment on CI is failing on the # character.

[nikku]

This would fix it, indeed.

Please change # to _. We try to avoid using the latest of JS but stick to certain evergreen patterns (_ for private) instead.