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prettier

Prettier

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Prettier is an opinionated JavaScript formatter inspired by refmt with advanced support for language features from ES2017, JSX, and Flow. It removes all original styling and ensures that all outputted JavaScript conforms to a consistent style. (See this blog post)

If you are interested in the details, you can watch those two conference talks:

This goes way beyond ESLint and other projects built on it. Unlike ESLint, there aren't a million configuration options and rules. But more importantly: everything is fixable. This works because Prettier never "checks" anything; it takes JavaScript as input and delivers the formatted JavaScript as output.

In technical terms: Prettier parses your JavaScript into an AST (Abstract Syntax Tree) and pretty-prints the AST, completely ignoring any of the original formatting. Say hello to completely consistent syntax!

There's an extremely important piece missing from existing styling tools: the maximum line length. Sure, you can tell ESLint to warn you when you have a line that's too long, but that's an after-thought (ESLint never knows how to fix it). The maximum line length is a critical piece the formatter needs for laying out and wrapping code.

For example, take the following code:

foo(arg1, arg2, arg3, arg4);

That looks like the right way to format it. However, we've all run into this situation:

foo(reallyLongArg(), omgSoManyParameters(), IShouldRefactorThis(), isThereSeriouslyAnotherOne());

Suddenly our previous format for calling function breaks down because this is too long. What you would probably do is this instead:

foo(
  reallyLongArg(),
  omgSoManyParameters(),
  IShouldRefactorThis(),
  isThereSeriouslyAnotherOne()
);

This clearly shows that the maximum line length has a direct impact on the style of code we desire. The fact that current style tools ignore this means they can't really help with the situations that are actually the most troublesome. Individuals on teams will all format these differently according to their own rules and we lose the consistency we sought after.

Even if we disregard line widths, it's too easy to sneak in various styles of code in all other linters. The most strict linter I know happily lets all these styles happen:

foo({ num: 3 },
  1, 2)
 
foo(
  { num: 3 },
  1, 2)
 
foo(
  { num: 3 },
  1,
  2
)

Prettier bans all custom styling by parsing it away and re-printing the parsed AST with its own rules that take the maximum line width into account, wrapping code when necessary.

Usage

Install:

yarn add prettier --dev

You can install it globally if you like:

yarn global add prettier

We're defaulting to yarn but you can use npm if you like:

npm install [-g] prettier

CLI

Run Prettier through the CLI with this script. Run it without any arguments to see the options.

To format a file in-place, use --write. You may want to consider committing your code before doing that, just in case.

prettier [opts] [filename ...]

In practice, this may look something like:

prettier --single-quote --trailing-comma es5 --write "{app,__{tests,mocks}__}/**/*.js"

(Don't forget the quotes around the globs! The quotes make sure that Prettier expands the globs rather than your shell, for cross-platform usage.)

In the future we will have better support for formatting whole projects.

Pre-commit hook for changed files

lint-staged can re-format your files that are marked as "staged" via git add before you commit.

Install it along with husky:

yarn add lint-staged husky --dev

and add this config to your package.json:

{
  "scripts": {
    "precommit": "lint-staged"
  },
  "lint-staged": {
    "*.js": [
      "prettier --write",
      "git add"
    ]
  }
}

See https://github.com/okonet/lint-staged#configuration for more details about how you can configure lint-staged.

Alternately you can just save this script as .git/hooks/pre-commit and give it execute permission:

#!/bin/sh 
jsfiles=$(git diff --cached --name-only --diff-filter=ACM | grep '\.jsx\?$' | tr '\n' ' ')
[ -z "$jsfiles" ] && exit 0
 
diffs=$(node_modules/.bin/prettier -l $jsfiles)
[ -z "$diffs" ] && exit 0
 
echo "here"
echo >&2 "Javascript files must be formatted with prettier. Please run:"
echo >&2 "node_modules/.bin/prettier --write "$diffs""
 
exit 1

API

The API has two functions, exported as format and check. The options argument is optional, and all of the defaults are shown below:

const prettier = require("prettier");
 
prettier.format(source, {
  // Indent lines with tabs 
  useTabs: false,
 
  // Fit code within this line limit 
  printWidth: 80,
 
  // Number of spaces it should use per tab 
  tabWidth: 2,
 
  // If true, will use single instead of double quotes 
  singleQuote: false,
 
  // Controls the printing of trailing commas wherever possible. Valid options: 
  // "none" - No trailing commas 
  // "es5"  - Trailing commas where valid in ES5 (objects, arrays, etc) 
  // "all"  - Trailing commas wherever possible (function arguments) 
  // 
  // NOTE: Above is only available in 0.19.0 and above. Previously this was 
  // a boolean argument. 
  trailingComma: "none",
 
  // Controls the printing of spaces inside object literals 
  bracketSpacing: true,
 
  // If true, puts the `>` of a multi-line jsx element at the end of 
  // the last line instead of being alone on the next line 
  jsxBracketSameLine: false,
 
  // Which parser to use. Valid options are "flow" and "babylon" 
  parser: "babylon",
 
  // Whether to add a semicolon at the end of every line (semi: true), 
  // or only at the beginning of lines that may introduce ASI failures (semi: false) 
  semi: true
});

check checks to see if the file has been formatted with Prettier given those options and returns a Boolean. This is similar to the --list-different parameter in the CLI and is useful for running Prettier in CI scenarios.

Excluding code from formatting

A JavaScript comment of // prettier-ignore will exclude the next node in the abstract syntax tree from formatting.

For example:

matrix(
  1, 0, 0,
  0, 1, 0,
  0, 0, 1
)
 
// prettier-ignore 
matrix(
  1, 0, 0,
  0, 1, 0,
  0, 0, 1
)

will be transformed to:

matrix(1, 0, 0, 0, 1, 0, 0, 0, 1);
 
// prettier-ignore 
matrix(
  1, 0, 0,
  0, 1, 0,
  0, 0, 1
)

Editor Integration

Atom

Atom users can simply install the prettier-atom package and use Ctrl+Alt+F to format a file (or format on save if enabled).

Emacs

Emacs users should see this directory for on-demand formatting.

Vim

For Vim users, there are two main approaches: one that leans on sbdchd/neoformat, which has the advantage of leaving the cursor in the same position despite changes, or a vanilla approach which can only approximate the cursor location, but might be good enough for your needs.

Vanilla approach

Vim users can add the following to their .vimrc:

autocmd FileType javascript set formatprg=prettier\ --stdin

If you use the vim-jsx plugin without requiring the .jsx file extension (See https://github.com/mxw/vim-jsx#usage), the FileType needs to include javascript.jsx:

autocmd FileType javascript.jsx,javascript setlocal formatprg=prettier\ --stdin

This makes Prettier power the gq command for automatic formatting without any plugins. You can also add the following to your .vimrc to run Prettier when .js files are saved:

autocmd BufWritePre *.js :normal gggqG

If you want to restore cursor position after formatting, try this (although it's not guaranteed that it will be restored to the same place in the code since it may have moved):

autocmd BufWritePre *.js exe "normal! gggqG\<C-o>\<C-o>"

Neoformat approach

Add sbdchd/neoformat to your list based on the tool you use:

Plug 'sbdchd/neoformat'

Then make Neoformat run on save:

autocmd BufWritePre *.js Neoformat

Other autocmd events

You can also make Vim format your code more frequently, by setting an autocmd for other events. Here are a couple of useful ones:

  • TextChanged: after a change was made to the text in Normal mode
  • InsertLeave: when leaving Insert mode

For example, you can format on both of the above events together with BufWritePre like this:

autocmd BufWritePre,TextChanged,InsertLeave *.js Neoformat

See :help autocmd-events in Vim for details.

Customizing Prettier in Vim

If your project requires settings other than the default Prettier settings, you can pass arguments to do so in your .vimrc or vim project, you can do so:

autocmd FileType javascript set formatprg=prettier\ --stdin\ --parser\ flow\ --single-quote\ --trailing-comma\ es5

Each command needs to be escaped with \. If you are using Neoformat and you want it to recognize your formatprg settings you can also do that by adding the following to your .vimrc:

" Use formatprg when available
let g:neoformat_try_formatprg = 1

Visual Studio Code

Can be installed using the extension sidebar. Search for Prettier - JavaScript formatter.

Can also be installed using ext install prettier-vscode.

Check its repository for configuration and shortcuts

Visual Studio

Install the JavaScript Prettier extension.

Sublime Text

Sublime Text support is available through Package Control and the JsPrettier plug-in.

JetBrains

JetBrains users can configure prettier as an External Tool. See this blog post or this directory with examples.

More editors are coming soon.

Language Support

Prettier attempts to support all JavaScript language features, including non-standardized ones. By default it uses the Babylon parser with all language features enabled, but you can also use the Flow parser with the parser API or --parser CLI option.

All of JSX and Flow syntax is supported. In fact, the test suite in tests is the entire Flow test suite and they all pass.

Related Projects

Technical Details

This printer is a fork of recast's printer with its algorithm replaced by the one described by Wadler in "A prettier printer". There still may be leftover code from recast that needs to be cleaned up.

The basic idea is that the printer takes an AST and returns an intermediate representation of the output, and the printer uses that to generate a string. The advantage is that the printer can "measure" the IR and see if the output is going to fit on a line, and break if not.

This means that most of the logic of printing an AST involves generating an abstract representation of the output involving certain commands. For example, concat(["(", line, arg, line ")"]) would represent a concatentation of opening parens, an argument, and closing parens. But if that doesn't fit on one line, the printer can break where line is specified.

More (rough) details can be found in commands.md. Better docs will come soon.

Badge

Show the world you're using Prettierstyled with prettier

[![styled with prettier](https://img.shields.io/badge/styled_with-prettier-ff69b4.svg)](https://github.com/prettier/prettier)

Contributing

To get up and running, install the dependencies and run the tests:

yarn
yarn test

Here's what you need to know about the tests:

  • The tests uses Jest snapshots.
  • You can make changes and run jest -u to update the snapshots. Then run git diff to take a look at what changed. Always update the snapshots when opening a PR.
  • You can run AST_COMPARE=1 jest for a more robust test run. That formats each file, re-parses it, and compares the new AST with the original one and makes sure they are semantically equivalent.
  • Each test folder has a jsfmt.spec.js that runs the tests. Normally you can just put run_spec(__dirname); there. You can also pass options and additional parsers, like this: run_spec(__dirname, { trailingComma: "es5" }, ["babylon"]);
  • tests/flow/ contains the Flow test suite, and is not supposed to be edited by hand. To update it, clone the Flow repo next to the Prettier repo and run: node scripts/sync-flow-tests.js ../flow/tests/.
  • If you would like to debug prettier locally, you can either debug it in node or the browser. The easiest way to debug it in the browser is to run the interactive docs REPL locally. The easiest way to debug it in node, is to create a local test file and run it in an editor like VS Code.

If you can, take look at commands.md and check out Wadler's paper to understand how Prettier works.