What is metro bundler in react-native?

I am learning React Native.

I can't find a proper documentation for metro bundler. So, I have few questions on it. As the name suggest it creates a bundle.

1. But where is the bundle file located ?
2. Is this same as webpack ?
3. What is the use of that bundle file ?

A React Native app is a compiled app that is running some Javascript. Whenever you build and run your React Native project, a packager starts up called Metro. You’ve probably seen this output in your terminal before, letting your know the packager is running.

The packager does a few things:

Combines all your Javascript code into a single file, and translates any Javascript code that your device won’t understand (like JSX or some of the newer JS syntax).

Converts assets (e.g. PNG files) into objects that can be displayed by an Image component.

reference: https://hackernoon.com/understanding-expo-for-react-native-7bf23054bbcd

Metro is a JavaScript bundler which takes in options, an entry file, and gives you a JavaScript file including all JavaScript files back. Every time you run a react native project, a compilation of many javascript files are done into a single file. This compilation is done by a bundler which is called Metro.

Answers to your questions:

1> Bundled file is located on the device itself on which you are building your app and is stored in different formats like in case of Android Plain bundling in which .bundle is created. Another format is of Indexed RAM bundle in which file is stored as binary file.

2> Webpack is also a similar type of module bundler which does bundling to ReactJS web platform and its modules are accessible through browser. Bundling process is while similar to metro.

3> These bundled files are indexed and stored in a particular numerical format and thus its easy at the run time to arrange JS files in order.

There are multiple functions of Metro bundler and you can read about the role of Metro in React Native here : https://medium.com/@rishabh0297/role-of-metro-bundler-in-react-native-24d178c7117e

Hope it helps.

Metro team keeps improving its documentation, now you can find some really good explanations at https://facebook.github.io/metro/docs/concepts (link updated):

Metro is a JavaScript bundler. It takes in an entry file and various options, and gives you back a single JavaScript file that includes all your code and its dependencies.

So yes, it is a sort of Webpack, but for React Native apps :)

But where is the bundle file located?

Once the bundler is started, you can check its contents at http://localhost:8081/index.bundle?platform=ios&dev=true&minify=false (like webpack, it is served from memory, so it is not being written on your project's folder).

What is the use of that bundle file ?

This file is installed in the device for its code to be executed there. Remember that when you are writing code for a React Native application, your code is not "translated" to Java / Swift / whatever. The Native Modules will send events to a Javascript thread, and the JS thread will execute your bundled React Native code.