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JavaScript Style Guide

Roughly based on node.js guide.

Run ESLint before committing

Always run grunt eslint before committing into repositories.

4 Spaces for indentation

Use 4 spaces for indenting your code and swear an oath to never mix tabs and spaces - a special kind of hell is awaiting you otherwise.

Newlines

Use UNIX-style newlines (\n), and a newline character as the last character of a file. Windows-style newlines (\r\n) are forbidden inside any repository.

No trailing whitespace

Clean up any trailing whitespace in your JS files before committing.

Use Semicolons

Yes everywhere.

100 characters per line

Limit your lines to 100 characters.

Use single quotes

Use single quotes, unless you are writing JSON or need to double escape.

Right:

var foo = 'bar';
var qux = "foo bar 'baz' qux";

Wrong:

var foo = "bar";

Opening braces go on the same line

Your opening braces go on the same line as the statement.

Right:

if (true) {
    console.log('winning');
}

Wrong:

if (true)
{
    console.log('losing');
}

Also, notice the use of whitespace before and after the condition statement.

Declare one variable per var statement

Declare one variable per var statement, it makes it easier to re-order the lines.

Right:

var keys   = ['foo', 'bar'];
var values = [23, 42];

var object = {};
while (keys.length) {
  var key = keys.pop();
  object[key] = values.pop();
}

Wrong:

var keys = ['foo', 'bar'],
    values = [23, 42],
    object = {},
    key;

while (keys.length) {
  key = keys.pop();
  object[key] = values.pop();
}

Use lowerCamelCase for variables, properties and function names

Variables, properties and function names should use lowerCamelCase. They should also be descriptive.

Right:

var adminUser = db.query('SELECT * FROM users ...');

Wrong:

var admin_user = db.query('SELECT * FROM users ...');

Use UpperCamelCase for class names

Class names should be capitalized using UpperCamelCase.

Right:

function BankAccount() {
}

Wrong:

function bank_Account() {
}

Use UPPERCASE for Constants

Constants should be declared as regular variables or static class properties, using all uppercase letters.

Right:

var SECOND = 1 * 1000;

function File() {
}
File.FULL_PERMISSIONS = 0777;

Wrong:

const SECOND = 1 * 1000;

function File() {
}
File.fullPermissions = 0777;

Variables

Object / Array creation

Use trailing commas and put short declarations on a single line. Only quote keys when your interpreter complains:

Right:

var a = ['hello', 'world'];
var b = {
  good: 'code',
  'is generally': 'pretty',
};

Wrong:

var a = [
  'hello', 'world'
];
var b = {"good": 'code'
        , is generally: 'pretty'
        };

Conditionals

Use the === operator

Use the triple equality operator as it will work just as expected.

Right:

var a = 0;
if (a !== '') {
  console.log('winning');
}

Wrong:

var a = 0;
if (a == '') {
  console.log('losing');
}

Use single-line ternary operator

The ternary operator should be used on a single line. It can be split into multiple lines in situations when it improves redability.

Right:

var foo = (a === b) ? 1 : 2;

Wrong:

var foo = (a === b)
    ? 1
    : 2;

Functions

Return early from functions

To avoid deep nesting of if-statements, always return a function's value as early as possible.

Right:

function isPercentage(val) {
  if (val < 0) {
    return false;
  }

  if (val > 100) {
    return false;
  }

  return true;
}

Wrong:

function isPercentage(val) {
  if (val >= 0) {
    if (val < 100) {
      return true;
    } else {
      return false;
    }
  } else {
    return false;
  }
}

Or for this particular example it may also be fine to shorten things even further:

function isPercentage(val) {
  var isInRange = (val >= 0 && val <= 100);
  return isInRange;
}

Name your closures

Feel free to give your closures a name. It shows that you care about them, and will produce better stack traces, heap and cpu profiles.

Right:

req.on('end', function onEnd() {
  console.log('winning');
});

Wrong:

req.on('end', function() {
  console.log('losing');
});

No nested closures

Use closures, but don't nest them. Otherwise your code will become a mess.

Right:

setTimeout(function() {
  client.connect(afterConnect);
}, 1000);

function afterConnect() {
  console.log('winning');
}

Wrong:

setTimeout(function() {
  client.connect(function() {
    console.log('losing');
  });
}, 1000);

Method chaining

One method per line should be used if you want to chain methods.

You should also indent these methods so it's easier to tell they are part of the same chain.

Right:

User.findOne({ name: 'foo' })
    .populate('bar')
    .exec(function(err, user) {
      return true;
    });

Wrong:

User
.findOne({ name: 'foo' })
.populate('bar')
.exec(function(err, user) {
    return true;
});

User.findOne({ name: 'foo' }).populate('bar')
.exec(function(err, user) {
    return true;
});

User.findOne({ name: 'foo' }).populate('bar')
  .exec(function(err, user) {
    return true;
  });

Comments

Use slashes for comments

Use slashes for both single line and multi line comments. Try to write comments that explain higher level mechanisms or clarify difficult segments of your code. Don't use comments to restate trivial things.

Right:

// 'ID_SOMETHING=VALUE' -> ['ID_SOMETHING=VALUE', 'SOMETHING', 'VALUE']
var matches = item.match(/ID_([^\n]+)=([^\n]+)/));

// This function has a nasty side effect where a failure to increment a
// redis counter used for statistics will cause an exception. This needs
// to be fixed in a later iteration.
function loadUser(id, cb) {
  // ...
}

var isSessionValid = (session.expires < Date.now());
if (isSessionValid) {
  // ...
}

Wrong:

// Execute a regex
var matches = item.match(/ID_([^\n]+)=([^\n]+)/);

// Usage: loadUser(5, function() { ... })
function loadUser(id, cb) {
  // ...
}

// Check if the session is valid
var isSessionValid = (session.expires < Date.now());
// If the session is valid
if (isSessionValid) {
  // ...
}

Miscellaneous

Object.freeze, Object.preventExtensions, Object.seal, with, eval

Crazy shit that you will probably never need. Stay away from it.

Requires At Top

Always put requires at top of file to clearly illustrate a file's dependencies. Besides giving an overview for others at a quick glance of dependencies and possible memory impact, it allows one to determine if they need a package.json file should they choose to use the file elsewhere.

Getters and setters

Do not use setters, they cause more problems for people who try to use your software than they can solve.

Feel free to use getters that are free from side effects, like providing a length property for a collection class.

Do not extend built-in prototypes

Do not extend the prototype of native JavaScript objects. Your future self will be forever grateful.

Right:

var a = [];
if (!a.length) {
  console.log('winning');
}

Wrong:

Array.prototype.empty = function() {
  return !this.length;
}

var a = [];
if (a.empty()) {
  console.log('losing');
}