npm package: @rnx-kit/react-native-lazy-index
Description: Hermes/RAM bundle friendly index.js with on-demand loaded modules
Author: Microsoft Open Source
Latest version: 4.0.0

# @rnx-kit/react-native-lazy-index

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`react-native-lazy-index` is a Hermes/RAM bundle friendly, bundle-time generated
`index.js`. Improve your app startup time by only loading features you'll use on
demand.

For information on RAM bundles and inline requires, see
[React Native Performance](https://reactnative.dev/docs/ram-bundles-inline-requires).

If you use [Haul](https://github.com/callstack/haul), also take a look at their
[documentation](https://github.com/callstack/haul/blob/2c68e97766f9f6c2632c46e40631bd7aaacdc75b/docs/CLI%20Commands.md#haul-ram-bundle).

## Installation

```sh
npm install --save @rnx-kit/react-native-lazy-index
```

## Usage

`react-native-lazy-index` uses
[`babel-plugin-codegen`](https://github.com/kentcdodds/babel-plugin-codegen#configure-with-babel),
so you'll need to configure Babel to include it. The recommended way is to add
it to your `.babelrc`:

```diff
 {
   "presets": ["module:metro-react-native-babel-preset"],
   "plugins": [
+    "codegen"
   ]
 }
```

In your `index.js`, import `@rnx-kit/react-native-lazy-index` and pass to it the
features that should be lazy loaded. In the example below, we register four
entry points:

```js
// @codegen
module.exports = require("@rnx-kit/react-native-lazy-index")({
  SomeFeature: "@awesome-app/some-feature",
  "callable:AnotherFeature": "@awesome-app/another-feature",
  YetAnotherFeature: "@awesome-app/yet-another-feature",
  FinalFeature: "@awesome-app/final-feature",
});
```

By default, a call to `AppRegistry` is generated using the key as the app key,
and the value is the name of the module containing the app. If the key is
prefixed with `callable:`, a call to `BatchedBridge` will be generated.

That's it!

## Why

With a naive `index.js`, all features will be loaded when your app starts and
React Native is initialized for the first time.

```js
import "@awesome-app/some-feature";
import "@awesome-app/another-feature";
import "@awesome-app/yet-another-feature";
import "@awesome-app/final-feature";
```

By loading features on demand, we can improve app startup time.

With `react-native-lazy-index`, we no longer load all features up front.
Instead, `index.js` wraps calls to `AppRegistry.registerComponent` and
`BatchedBridge.registerCallableModule`, deferring the import of a feature until
it is used. Features that are never used, are never loaded.

When you import `react-native-lazy-index`, something similar to below is
generated:

```js
const { AppRegistry } = require("react-native");
const BatchedBridge = require("react-native/Libraries/BatchedBridge/BatchedBridge");

AppRegistry.registerComponent("SomeFeature", () => {
  // We'll import the module the first time "SomeFeature" is accessed.
  require("@awesome-app/some-feature");
  // "SomeFeature" is now overwritten and we can return the real component.
  // Subsequent calls to "SomeFeature" will no longer go through this wrapper.
  return AppRegistry.getRunnable("SomeFeature").componentProvider();
});

BatchedBridge.registerLazyCallableModule("AnotherFeature", () => {
  // We'll import the module the first time "AnotherFeature" is accessed.
  require("@awesome-app/another-feature");
  // "AnotherFeature" is now overwritten and we can return the real component.
  // Subsequent calls to "AnotherFeature" will no longer go through this
  // wrapper.
  return BatchedBridge.getCallableModule("AnotherFeature");
});

AppRegistry.registerComponent("YetAnotherFeature", () => {
  // We'll import the module the first time "YetAnotherFeature" is accessed.
  require("@awesome-app/yet-another-feature");
  // "YetAnotherFeature" is now overwritten and we can return the real
  // component. Subsequent calls to "YetAnotherFeature" will no longer go
  // through this wrapper.
  return AppRegistry.getRunnable("YetAnotherFeature").componentProvider();
});

AppRegistry.registerComponent("FinalFeature", () => {
  // We'll import the module the first time "FinalFeature" is accessed.
  require("@awesome-app/final-feature");
  // "FinalFeature" is now overwritten and we can return the real component.
  // Subsequent calls to "FinalFeature" will no longer go through this wrapper.
  return AppRegistry.getRunnable("FinalFeature").componentProvider();
});
```

## Contributing

This project welcomes contributions and suggestions. Most contributions require
you to agree to a Contributor License Agreement (CLA) declaring that you have
the right to, and actually do, grant us the rights to use your contribution. For
details, visit https://cla.opensource.microsoft.com.

When you submit a pull request, a CLA bot will automatically determine whether
you need to provide a CLA and decorate the PR appropriately (e.g., status check,
comment). Simply follow the instructions provided by the bot. You will only need
to do this once across all repos using our CLA.

This project has adopted the
[Microsoft Open Source Code of Conduct](https://opensource.microsoft.com/codeofconduct/).
For more information see the
[Code of Conduct FAQ](https://opensource.microsoft.com/codeofconduct/faq/) or
contact [opencode@microsoft.com](mailto:opencode@microsoft.com) with any
additional questions or comments.