function Command() {
  /**
   * Gets whether this command can currently be executed.  This property is observable.
   * @type {boolean}
   * @default undefined
   */
  this.canExecute = undefined;

  /**
   * Gets an event which is raised before the command executes, the event
   * is raised with an object containing two properties: a <code>cancel</code> property,
   * which if set to false by the listener will prevent the command from being executed, and
   * an <code>args</code> property, which is the array of arguments being passed to the command.
   * @type {Event}
   * @default undefined
   */
  this.beforeExecute = undefined;

  /**
   * Gets an event which is raised after the command executes, the event
   * is raised with the return value of the command as its only parameter.
   * @type {Event}
   * @default undefined
   */
  this.afterExecute = undefined;

  DeveloperError.throwInstantiationError();
}
A Command is a function with an extra <code>canExecute</code> observable property to determine
whether the command can be executed.  When executed, a Command function will check the
value of <code>canExecute</code> and throw if false.

This type describes an interface and is not intended to be instantiated directly.
See {@link createCommand} to create a command from a function.

@alias Command
@constructor