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This document attempts to explain the basic styles and patterns that are used in the Jetpack codebase. While existing code may not always comply to this style, new code should try to conform to these standards so that it is as easy to maintain as existing code. Of course every rule has an exception, but it's important to know the rules nonetheless!

General

2-column indentation. 80-character limit for all lines.

Use double quotation symbol " for strings as they are a standard in rest of Mozilla code base. Although some files may be authored with single quotes ', stay consistent with the style use by a file when making changes to it and use '. Do not mix styles in the same file:

// Good!
const { Panel } = require("panel");
const { Widget } = require("widget");

// Acceptable
const { Panel } = require('panel');
const { Widget } = require('widget');

// Bad
const { Panel } = require('panel');
const { Widget } = require("widget");

Use dots at the end of lines, not at the front:

// Good
const browsers = Cc["@mozilla.org/appshell/window-mediator;1"].
                  getService(Ci.nsIWindowMediator).
                  getEnumerator("navigator:browser");

// Bad
const browsers = Cc["@mozilla.org/appshell/window-mediator;1"]
                  .getService(Ci.nsIWindowMediator)
                  .getEnumerator("navigator:browser");

Avoid use of error prone undefined (which can be redefined) and prefer void(0) instead.

// Good
if (x === void(0)) {
  // ...
}

// Bad
if (x === undefined) {
  // ...
}

Declarations

Variables

Forget about var and use const instead. Use let only when variable will be (re)assigned different value later in the code. It's only ok to use var if code is going to be used in other runtimes, but such files should not use let or const.

// Good
const count = observers.length;
let index = 0;
while (index < count) {
  const observer = observers[index];
  observer(...args);
  index = index + 1;
}

// Bad
// Reader expects count to change!
let count = observers.length;
let index = 0;
while (index < count) {
  const observer = observers[index];
  observer(...args);
  index = index + 1;
}

// Bad
const count = observers.length;
let index = 0;
while (index < count) {
  // Reader expect observer to change with in the block!
  let observer = observers[index];
  observer(...args);
  index = index + 1;
}

// Bad
const count = observers.length;
// Do not use `var` if `let` or `var` used in file.
var index = 0;
while (index < count) {
  observers[index](...args);
  index = index + 1;
}

// Acceptable if file only uses var
var count = observers.length;
// Do not use `var` if `let` or `var` used in file.
var index = 0;
while (index < count) {
  observers[index](...args);
  index = index + 1;
}

Constants

For constants values use const declaration and ALL_CAPITAL_SNAKE_CASE naming convention.

// Good
const NORMAL_FILE_TYPE = 0;
const DIRECTORY_TYPE = 1;

// Bad
const normalFileType = 0;
const DirectoryType = 1;
let NORMAL_FILE_TYPE = 0;
var DIRECTORY_TYPE = 1;

Functions

Use arrow functions unless you are defining a "class" or a method.

// Good
const find = (collection, predicate, fallback) => {
  for (let item of collection)
    if (predicate(item)) return item;

  return fallback;
};

// Good
const Curse = function() {
  // ...
}
Curse.prototype.cry = function() {
  // ...
}

// Bad
function find(collection, predicate, fallback) {
  for (let item of collection)
    if (predicate(item)) return item;

  return fallback;
};

const find = function(collection, predicate, fallback) {
  for (let item of collection)
    if (predicate(item)) return item;

  return fallback;
};

Curse.prototype.cry = () => {
  // ...
}

Prefer shorter syntax with implicit return. Never use Mozilla's specific short functions syntax though.

// Good
const add = (x, y) => x + y

// Bad
function add(x, y) {
  return x + y;
}

// No No No!!
function add(x, y) x + y;

Use short arrow syntax even if single expression does not fits on one / same line. Do not add {} or return keyword unless function contains multiple statements.

// Good
const getInnerId = window =>
  window.
  QueryInterface(Ci.nsIInterfaceRequestor).
  getInterface(Ci.nsIDOMWindowUtils).
  currentInnerWindowID;

// Bad
const getInnerId = window => {
  return window.
           QueryInterface(Ci.nsIInterfaceRequestor).
           getInterface(Ci.nsIDOMWindowUtils).
           currentInnerWindowID;
}

// Bad
function getInnerId(window) {
  return window.
          QueryInterface(Ci.nsIInterfaceRequestor).
          getInterface(Ci.nsIDOMWindowUtils).
          currentInnerWindowID;
};

Identify arguments that function ignores via _ name:

// Good
const constant = x => _ => x
const five = constant(5);
five(); // => 5

Use rest parameters syntax for capturing arguments in the array:

// Good
const multiply = (multiplier, ...operands) =>
  operands.map(operand => multiplier * operand)

// Bad
const multiply = (multiplier) =>
  Array.slice(arguments, 1).map(operand => multiplier * operand)

// Bad
const multiply = (multiplier) =>
  Array.prototype.slice.call(arguments, 1).map(operand => multiplier * operand)

Use spread operator for applying arguments to a function. Forget that .apply exists!

// Good
const wrap = (f, g) => (...args) => g(f, ...args)
instance.method(...args)

// Bad
const wrap = (f, g) => (...args) => g.apply(g, [f].concat(args))
instance.method.apply(instance, args);

Only functions with Referential transparency should return non void values. If function causes any side effects on given arguments or on bindings in the outer scope it should return undefined. This makes it clear to a reader / user whether invoked function causes any side-effects or not without looking into implementation.

// Good

// Define does not returns value since it mutates `object`.
const define = (object, properties) => {
  let descriptor = {};
  Object.getOwnPropertyNames(properties).forEach(name => {
    descriptor[name] = Object.getOwnPropertyDescriptor(properties, name);
  });
  Object.defineProperties(object, properties);
}

// `extend` is referentially transparent, as it does not
// mutates given arguments nor things in the outer scope.
const extend = (target, properties) => {
  let result = Object.create(target);
  define(result, properties);
  return result
}

// Bad
// This is bad because it's no longer clear if `a = define(b, c)` has
// changed anything or not.
const define = (object, properties) => {
  let descriptor = {};
  Object.getOwnPropertyNames(properties).forEach(name => {
    descriptor[name] = Object.getOwnPropertyDescriptor(properties, name);
  });
  Object.defineProperties(object, properties);
  return object;
}

Do not declare function within blocks.

// Good
const readURIs = (uris, callback) => {
  let pending = uris.length;
  // Using `let` to signify that output is going
  // to be mutated.
  let output = [];
  uris.reduce((index, uri) => {
    // Defining functions with-in the functions is ok.
    readURI(uri, content => {
      output[index] = content;
      pending = pending - 1;

      if (!pending)
        callback(output)
    });
    return index + 1;
  }, 0);
}

// Good
const readURIs = (uris, callback) => {
  let pending = uri.length;
  let results = [];
  const makeFetchHandler = index =>
    content => {
      output[index] = content;
      pending = pending - 1;

      if (!pending)
        callback(output)
    }

  let index = 0;
  let count = uris.length;
  while (index < count) {
    readURI(uri, makeFetchHandler(index));
    index = index + 1;
  }
}

// Bad
const readURIs = (uris, callback) => {
  let pending = uris.length;
  // Using `let` to signify that output is going
  // to be mutated.
  let output = [];
  const count = uris.length;
  let index = 0;
  while (index < count) {
    let id = index;
    // Defining functions with-in the functions is ok.
    readURI(uri, content => {
      output[id] = content;
      pending = pending - 1;

      if (!pending)
        callback(output)
    });
    index = index + 1;
  }
}

If you ever find yourself in a need of using .call or .apply to pass in this pseudo-variable, you are doing it wrong. That method should have being a function in first place:

const Target = function() {
  // ...
}

const FancyTarget = function() {
}
FancyTarget.prototype = Object.create(Target.prototype);

// Good

// Good
Target.prototype.registerListener = function(listener) {
  registerListener(this, listener);
}

// Best

const registerListener = (target, listener) => {
  // ...
}

// See: https://addons.mozilla.org/en-US/developers/docs/sdk/latest/modules/sdk/lang/functional.html#method%28lambda%29
Target.prototype.registerListener = method(registerListener);
// ...
FancyTarget.prototype.registerListener = function(listener) {
  this.listenerCount = this.listenerCount + 1;
  registerListener(this, listener);
}

// Bad

FancyTarget.prototype.registerListener = function(listener) {
  this.listenerCount = this.listenerCount + 1;
  this.registerListener.call(this, listener);
}

Naming

  • Use camelCase for functions and variables.
  • Use capitalized CamelCase for classes / constructor functions.
  • Use all lowercase for file names in order to avoid confusion on case-sensitive platforms. Filenames should end in .js and should contain no punctuation except for - delimiters.

Conditionals

A branch follows its conditional on a new line and is indented:

if (foo)
  bar();

If all branches of a conditional are one-liner single statements, no braces needed:

if (foo)
  bar();
else if (baz)
  beep();
else
  qux();

A single-statement branch that requires multiple lines also requires braces. The opening brace follows the conditional on the same line:

if (foo) {
  if (bar)
    baz();
}

if (foo) {
  Cc['@mozilla.org/appshell/window-mediator;1'].
    getService(Ci.nsIWindowMediator).
    getEnumerator('navigator:browser').
    doSomethingOnce();
}

If any branch requires braces, use them for all:

if (foo) {
  bar();
}
else {
  doThis();
  andThat();
}

Do not cuddle else:

// Good
if (foo) {
  bar();
  baz();
}
else {
  qux();
  qix();
}

// Bad
if (foo) {
  bar();
  baz();
} else {
  qux();
  qix();
}

Use triple equal === instead of double == unless there is a reason not to. If in a given case == is preferred add a comment to explain why.

// Good
if (password === secret)
  authorize()
else
  deny()

// Bad
if (password == secret)
  authorize()
else
  deny()

Do not compare to booleans unless exactly true or false is expected (add comment if that's a case):

// Good
if (x)
  doThis()
else if (!y)
  doThat()
else
  doSomethingElse()

// Bad
if (x === true)
  doThis()
else if (y != false)
  doThat()
else
  doSomethingElse()

Loops

Conditional style also applies to loop style.

for (let i = 0; i < arr.len; i++)
  arr[i] = 0;

for (let i = 0; i < arr.len; i++) {
  if (i % 2)
    arr[i] = 0;
}

Prefer array methods to avoid loops, if you need loop for whatever reason prefer for of and if it's not a good fit then while, plain for loops is a last resort! If you have internal doubts read Learnable Programming essay.

// best
xs.reduce((sum, x) => sum + x)

// good
let sum = 0;
for (let x of xs)
  sum = sum + x

// ok
const count = xs.length;
let index = 0;
let sum = 0;
while (index < count)
  sum = sum + xs[index];

// bad

const count = xs.length;
let sum = 0;
for (let i = 0; i < count; i++)
  sum = sum + xs[i];

# Try - Catch

Do not cuddle **catch**:

```js
// Good
try {
  bar();
}
catch (err) {
  baz();
}

// Bad
try {
  bar();
} catch (err) {
  baz();
}

Good practices

Reuse functions where possible, creating closures on every call has worse performance and generates more garbage to be GC-ed.

// Good
const isOdd = x => x % 2
const sum = (x, y) => x + y
const foo = nums => nums.filter(isOdd).reduce(sum);

// Bad
const foo = nums => {
  return nums.filter(function(x) {
    return x % 2;
  }).reduce(function(a, b) {
    return a + b;
  });
}

Comments and Documentation

This applies to in-source docs only, not nice docs meant for end users.

All exported functions should be documented in JSDoc style. Their purpose is to help people looking at your code.

/**
 * This function registers given user.
 * @param {String} name
 *    The name of the user.
 * @param {String|Number} id
 *    Unique user ID.
 * @param {String[]} aliases
 *    Array of aliases user
 * @param {String} [accessLevel='user']
 *    Optional `accessLevel` for a user.
 */
const register = (name, id, aliases, accessLevel) => {
  // ...
}

/**
 * Registers user and returns associated ID.
 * @param {Object} options
 *    Information about the user.
 * @param {String} options.name
 *    The name of the user.
 * @param {String} [options.aliases]
 *    Optional array of aliases
 */
const registerUser = options => {
  // ...
}

Module internal utility functions don't need to be documented this way, but it's encouraged.

For all other comments use single line comments. If a comment is a full sentence, capitalize and punctuate it. If a comment is almost a full sentence, make it a full sentence. Full sentences are generally preferable to fragments, but fragments are sometimes more effective. Fragments should be very terse. Don't capitalize or punctuate fragments.

Quote identifiers with backticks:

// Returns string content under given `uri`. Throws
// exception if such `uri` does not exists.
const readURI = uri => {
}

Module Exports

Exported functions should be named and defined at the top level module scope. Assignment to exports should follow a definition as separate statement:

// Good
const doSomething = () => {
  // ....
}
exports.doSomething = doSomething;

// Bad
exports.doSomething = () => {
  // ...
}

// Bad
var doSomething = exports.doSomething = () => {
  // ...
}

Exported functions should be referenced via local name and not as an exported property. This is both faster and future proof, since in upcoming standard JS modules export will be a statement and exports will have to be referenced via local name:

// Good 
const doThis = () => {
  // ...
  runExportedF()
  // ...
}

// Bad
const doThis = () => {
  // ...
  exports.runExportedF()
  // ...
}

Same rules apply to non function exports as well:

// Good
var foo = {
  // ...
};
exports.foo = foo;

const bar = () => {
  // ...
  doSomething(foo);
  // ...
}

// Bad

exports.foo = {
  // ...
};
const bar = () => {
  // ...
  doSomething(exports.foo);
  // ...
}

Avoid

  • List Comprehensions. Prefer map, filter, reduce as they are short enough but a lot easier to read & understand.
const result = [fn() for (x in someArray)]; // bad

const result = someArray.map(fn);  // better

Further Reading

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