Closure Compilation

What is type safety?

Strongly-typed languages like C++ and Java have the notion of variable types.

This is typically baked into how you declare variables:

const int32 kUniversalAnswer = 42;  // type = 32-bit integer

or as templates for containers or generics:

std::vector<int64> fibonacci_numbers;  // a vector of 64-bit integers

When differently-typed variables interact with each other, the compiler can warn you if there's no sane default action to take.

Typing can also be manually annotated via mechanisms like dynamic_cast and static_cast or older C-style casts (i.e. (Type)).

Using stongly-typed languages provide some level of protection against accidentally using variables in the wrong context.

JavaScript is weakly-typed and doesn't offer this safety by default. This makes writing JavaScript more error prone, and various type errors have resulted in real bugs seen by many users.

Chrome's solution to typechecking JavaScript

Enter Closure Compiler, a tool for analyzing JavaScript and checking for syntax errors, variable references, and other common JavaScript pitfalls.

To get the fullest type safety possible, it's often required to annotate your JavaScript explicitly with Closure-flavored @jsdoc tags

/**
 * @param {string} version A software version number (i.e. "50.0.2661.94").
 * @return {!Array<number>} Numbers corresponding to |version| (i.e. [50, 0, 2661, 94]).
 */
function versionSplit(version) {
  return version.split('.').map(Number);
}

See also: the design doc.

Typechecking Your Javascript

Given an example file structure of:

• lib/does_the_hard_stuff.js
• ui/makes_things_pretty.js

lib/does_the_hard_stuff.js:

var wit = 100;

// ... later on, sneakily ...

wit += ' IQ';  // '100 IQ'

ui/makes_things_pretty.js:

/** @type {number} */ var mensa = wit + 50;

alert(mensa);  // '100 IQ50' instead of 150

Closure compiler can notify us if we're using strings and numbers in dangerous ways.

To do this, we can create:

• ui/BUILD.gn

With these contents:

# Copyright 2018 The Chromium Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

import("//third_party/closure_compiler/compile_js.gni")

js_type_check("closure_compile") {
  deps = [
    ":make_things_pretty",
  ]
}

js_library("make_things_pretty") {
  deps = [
    "../lib:does_the_hard_stuff",
  ]

  externs_list = [
    "$externs_path/extern_name_goes_here.js"
  ]
}

Running Closure compiler locally

You can locally test that your code compiles on Linux or Mac. This requires Java and a Chrome checkout (i.e. python, depot_tools). Note: on Ubuntu, you can probably just run sudo apt-get install openjdk-7-jre.

First, add the following to your GN args:

enable_js_type_check = true

Then you should be able to run:

ninja -C out/Default webui_closure_compile

and should see output like this:

ninja: Entering directory `out/Default/'
[0/1] ACTION Compiling ui/makes_things_pretty.js

To compile only a specific folder, add an argument after the script name:

ninja -C out/Default ui:closure_compile

In our example code, this error should appear:

(ERROR) Error in: ui/makes_things_pretty.js
## /my/home/chromium/src/ui/makes_things_pretty.js:1: ERROR - initializing variable
## found   : string
## required: number
## /** @type {number} */ var mensa = wit + 50;
##                                   ^

Hooray! We can catch type errors in JavaScript!

Preferred BUILD.gn structure

• Make all individual JS file targets a js_library.
• The top level target should be called “closure_compile”.
• If you have subfolders that need compiling, make “closure_compile” a group(), and any files in the current directory a js_type_check() called “_resources”.
• Otherwise, just make “closure_compile” a js_type_check with all your js_library targets as deps
• Leave all closure targets below other kinds of targets becaure they’re less ‘important’

See also: Closure Compilation with GN.

Trying your change

Closure compilation runs in the compile step of Linux, Android and ChromeOS builds.

From the command line, you try your change with:

git cl try -b linux-rel

Integrating with the continuous build

To compile your code on every commit, add your file to the 'webui_closure_compile' target in src/BUILD.gn:

  group("webui_closure_compile") {
    data_deps = [
      # Other projects
      "my/project:closure_compile",
    ]
  }

Externs

Externs files define APIs external to your JavaScript. They provide the compiler with the type information needed to check usage of these APIs in your JavaScript, much like forward declarations do in C++.

Third-party libraries like Polymer often provide externs. Chrome must also provide externs for its extension APIs. Whenever an extension API's idl or json schema is updated in Chrome, the corresponding externs file must be regenerated:

./tools/json_schema_compiler/compiler.py -g externs \
  extensions/common/api/your_api_here.idl \
  > third_party/closure_compiler/externs/your_api_here.js