What is the purpose of emptyObject in React’s fbjs module

Question

I was reading through React’s source code which led me to the fbjs npm module which just looks like a bunch of helpful js utilities. I came across a file called emptyObject.js which currently looks like this:

/** * Copyright (c) 2013-present, Facebook, Inc. * * This source code is licensed under the MIT license found in the * LICENSE file in the root directory of this source tree. * * @providesModule emptyObject */ 'use strict'; const emptyObject = {}; if (__DEV__) { Object.freeze(emptyObject); } module.exports = emptyObject;

Two questions:

Why use Object.freeze(...) only in dev mode?
What’s the purpose of this functionality in the first place?

Per the second question above, why not just do something like this:

const myObj = {};

What’s the benefit of

import emptyObject from ‘fbjs/lib/emptyObject’; ... const myObj = emptyObject;

trixn · Accepted Answer · 2018-04-13 00:03:19Z

emptyObject is a singleton. It is only created once thus only using memory once. Also you can compare two occurrences of emptyObject and they will be equal (===) while having two distinct empty objects that were created separate they will not be equal. The react code uses emptyObject in multiple places where e.g. the default value should be an empty object or a function should return an empty object. Instead of always creating a new empty object they use a singleton.

I guess they use Object.freeze() in development to receive an error when you accidentally try to mutate that object while writing code for the react library. In production you usually try to suppress errors related to internals of the library.

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