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There are cases where prettier inserts spaces in between detached nodes.
Mainly this impacted function arguments as we'd do things like:
[].map(foo(a, b) {})

// after replacing with an arrow became:


  ) => {},

This is bad for obvious reasons - because it is purely junk lines.
From extensive testing, I found that the issue ONLY occurs if there is a docblock.

My hypothesis is that when prettier lays out things like function parameters it looks to see if there is a comment on the line. If there is then it puts the param on the next line.
Though - I don't have a true root cause for this as I haven't properly dug into prettier's source code and it's a pain to do remote nodejs debugging from a devserver and I cannot be bothered checking out locally.

I tried a few fixes, but from my testing this fix (setting detached nodes to have range `[1, 1]`) works for cases of brand new detached nodes, and cloned detached nodes.

Reviewed By: pieterv

Differential Revision: D37499940

fbshipit-source-id: 6398186696692c047a6e4cb65a3901d57ef8745b

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Jun 10, 2022

Hermes JS Engine

MIT license npm version PRs Welcome Hermes logo - large H with wings

Hermes is a JavaScript engine optimized for fast start-up of React Native apps. It features ahead-of-time static optimization and compact bytecode.

If you're only interested in using pre-built Hermes in a new or existing React Native app, you do not need to follow this guide or have direct access to the Hermes source. Instead, just follow these instructions to enable Hermes.

Noted that each Hermes release is aimed at a specific RN version. The rule of thumb is to always follow Hermes releases strictly. Version mismatch can result in instant crash of your apps in the worst case scenario.

If you want to know how to build and hack on Hermes directly, and/or integrate Hermes built from source into a React Native app then read on.

The instructions here very briefly cover steps to build the Hermes CLI. They assume you have typical native development tools setup for your OS, and support for cmake and Ninja. For more details of required dependencies, building Hermes with different options, etc. follow these links instead:

To build a local debug version of the Hermes CLI tools the following steps should get you started on macOS/Linux:

mkdir hermes_workingdir
cd hermes_workingdir
git clone
cmake -S hermes -B build -G Ninja
cmake --build ./build

Or if you're using Windows, the following should get you going in a Git Bash shell:

mkdir hermes_workingdir
cd hermes_workingdir
git -c core.autocrlf=false clone
cmake -S hermes -B build -G 'Visual Studio 16 2019' -A x64
cmake --build ./build

You will now be in a directory with the output of building Hermes into CLI tools. From here you can run a piece of JavaScript as follows:

echo "'use strict'; function hello() { print('Hello World'); } hello();" | ./bin/hermes


The main purpose of this repository is to continue to evolve Hermes, making it faster and more efficient. We are grateful to the community for contributing bugfixes and improvements. Read below to learn how you can take part in improving Hermes.

Code of Conduct

Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please read the full text so that you can understand what actions will and will not be tolerated.

Contributing Guide

Read our contributing guide to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to Hermes.


Hermes is MIT licensed.