[Home](./index.md) > [sip.js](./sip.js.md) > [Emitter](./sip.js.emitter.md) > [on](./sip.js.emitter.on.md) ## Emitter.on() method > Warning: This API is now obsolete. > > Use addListener. > Registers a listener. Signature: ```typescript on(listener: (data: T) => void): void; ``` ## Parameters | Parameter | Type | Description | | --- | --- | --- | | listener | (data: T) => void | Callback function. | Returns: `void`