# Hermes Profile Transformer

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Visualize Facebook's [Hermes JavaScript runtime](https://github.com/facebook/hermes) profile traces in Chrome Developer Tools.

![Demo Profile](https://raw.githubusercontent.com/MLH-Fellowship/hermes-profile-transformer/master/assets/convertedProfile.png)

## Overview

The Hermes runtime, used by React Native for Android, is able to output [Chrome Trace Events](https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/preview) in JSON Object Format.

This TypeScript package converts Hermes CPU profiles to Chrome Developer Tools compatible JSON Array Format, and enriches it with line and column numbers and event categories from JavaScript source maps.

## Usage

If you're using `hermes-profile-transformer` to debug React Native Android applications, you can use the [React Native CLI](https://github.com/react-native-community/cli) `react-native profile-hermes` command, which uses this package to convert the downloaded Hermes profiles automatically.

### As a standalone package

```js
const transformer = require('hermes-profile-transformer').default;
const { promises } = require('fs');

const hermesCpuProfilePath = './testprofile.cpuprofile';
const sourceMapPath = './index.map';
const sourceMapBundleFileName = 'index.bundle.js';

transformer(
  // profile path is required
  hermesCpuProfilePath,
  // source maps are optional
  sourceMap,
  sourceMapBundleFileName
)
  .then(events => {
    // write converted trace to a file
    return promises.writeFile(
      './chrome-supported.json',
      JSON.stringify(events, null, 2),
      'utf-8'
    );
  })
  .catch(err => {
    console.log(err);
  });
```

## Creating Hermes CPU Profiles

## Opening converted profiles in Chrome Developer Tools

Open Developer Tools in Chrome, navigate to the **Performance** tab, and use the **Load profile...** feature.

![Loading the Profile](https://raw.githubusercontent.com/MLH-Fellowship/hermes-profile-transformer/master/assets/loading.png)

## API

### transformer(profilePath: string, sourceMapPath?: string, bundleFileName?: string)

#### Parameters

| Parameter      | Type   | Required | Description                                                                                                                                                               |
| -------------- | ------ | -------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| profilePath    | string | Yes      | Path to a JSON-formatted `.cpuprofile` file created by the Hermes runtime                                                                                                 |
| sourceMapPath  | string | No       | Path to a [source-map](https://www.npmjs.com/package/source-map) compatible Source Map file                                                                               |
| bundleFileName | string | No       | If `sourceMapPath` is provided, you need to also provide the name of the JavaScript bundle file that the source map applies to. This file does not need to exist on disk. |

#### Returns

`Promise<DurationEvent[]>`, where `DurationEvent` is as defined in [EventInterfaces.ts](src/types/EventInterfaces.ts).

## Resources

- [Using Hermes with React Native](https://reactnative.dev/docs/hermes).
- [Chrome Trace Event Format](https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/preview). Hermes uses the JSON Object format.
- [Measuring JavaScript performance in Chrome](https://developers.google.com/web/tools/chrome-devtools/evaluate-performance/reference)

## LICENSE

[MIT](LICENSE)