> A Deferred object is returned by the obsolete Promise.defer() method to provide a new promise along with methods to change its state. > Starting from Gecko 3

> A Deferred object is returned by the obsolete Promise.defer() method to provide a new promise along with methods to change its state. > Starting from Gecko 30, this object is obsolete and should not be used anymore. Use the new Promise() constructor instead (or use the above backwards/forwards compatible Deferred function given below). For example, the equivalent of Seems like it's obsolete. [0] If you want to write async code, use async / await. const run = async () => { const response = await new Promise((resolve, reject) => methodOne(data, (e, res) => e ? reject(e) : resolve(res))); }; If you spend enough time with fat arrow, it's as easy to read as `function` is. On top of that, IMO it looks cleaner. It also allows you to do scope binding in a different manor which in React is much better. This: <a onClick={this.onClickEvent.bind(this)}>Click Me</a> becomes this: <a onClick={(event) => this.onClickEvent(event)}>Click Me</a> [0] https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_code_modules/Promise.jsm/Deferred https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_...