docs: Document that we use Prettier.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

README.md

@@ -11,6 +11,7 @@ largest and fastest growing open source group chat project.
 [![CircleCI branch](https://img.shields.io/circleci/project/github/zulip/zulip/master.svg)](https://circleci.com/gh/zulip/zulip/tree/master)
 [![Coverage Status](https://img.shields.io/codecov/c/github/zulip/zulip/master.svg)](https://codecov.io/gh/zulip/zulip/branch/master)
 [![Mypy coverage](https://img.shields.io/badge/mypy-100%25-green.svg)][mypy-coverage]
+[![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg)](https://github.com/prettier/prettier)
 [![GitHub release](https://img.shields.io/github/release/zulip/zulip.svg)](https://github.com/zulip/zulip/releases/latest)
 [![docs](https://readthedocs.org/projects/zulip/badge/?version=latest)](https://zulip.readthedocs.io/en/latest/)
 [![Zulip chat](https://img.shields.io/badge/zulip-join_chat-brightgreen.svg)](https://chat.zulip.org)

docs/contributing/code-style.md

@@ -251,19 +251,11 @@ code a lot uglier, in which case it's fine to go up to 120 or so.
 
 ### JavaScript and TypeScript
 
-When calling a function with an anonymous function as an argument, use
-this style:
-
-    my_function('foo', data => {
-        var x = ...;
-        // ...
-    });
-
-The inner function body is indented one level from the outer function
-call. The closing brace for the inner function and the closing
-parenthesis for the outer call are together on the same line. This style
-isn't necessarily appropriate for calls with multiple anonymous
-functions or other arguments following them.
+Our JavaScript and TypeScript code is formatted with
+[Prettier](https://prettier.io/).  You can ask Prettier to reformat
+all code via our [linter tool](../testing/linters.md) with `tools/lint
+--only=prettier --fix`.  You can also [integrate it with your
+editor](https://prettier.io/docs/en/editors.html).
 
 Combine adjacent on-ready functions, if they are logically related.
 

docs/testing/linters.md

@@ -19,6 +19,7 @@ below will direct you to the official documentation for these projects.
 
 - [eslint](https://eslint.org)
 - [mypy](http://mypy-lang.org/)
+- [Prettier](https://prettier.io/)
 - [puppet](https://puppet.com/) (puppet provides its own mechanism for
   validating manifests)
 - [pyflakes](https://pypi.python.org/pypi/pyflakes)
@@ -100,6 +101,7 @@ following checks:
 
 - Check Python code with pyflakes.
 - Check JavaScript and TypeScript code with eslint.
+- Check JavaScript, TypeScript, and YAML formatting with Prettier.
 - Check Python code for custom Zulip rules.
 - Check non-Python code for custom Zulip rules.
 - Check puppet manifests with the puppet validator.
@@ -184,6 +186,7 @@ that we exempt may be deemed not worthwhile to fix.
 
 We check our JavaScript code in a few different ways:
 - We run eslint.
+- We check code formatting with Prettier.
 - We perform custom Zulip regex checks on the code.
 
 #### Puppet manifests