Node.js Style Guide

This is a guide for writing consistent and aesthetically pleasing node.js code. It is inspired by what is popular within the community, and flavored with some personal opinions.

This guide was created by Felix Geisendörfer and is licensed under the CC BY-SA 3.0 license. You are encouraged to fork this repository and make adjustments according to your preferences.

• 2012-12-02: adjusted and clariefied according to personal preferences by Kai Koch

4 Spaces for indention

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. While it is common in the node.js community to use only 2 spaces as ident, my personal preference are 4 space. 4 spaces make your code more readable.

But if you code on a colaborative project, either use the standard set by the creator or if you start a new project arrange that all contributors use the same number of spaces.

No trailing whitespace

Just like you brush your teeth after every meal, you clean up any trailing whitespace in your JS files before committing. Otherwise the rotten smell of careless neglect will eventually drive away contributors and/or co-workers.

Use Semicolons

According to scientific research, the usage of semicolons is a core values of our community. Consider the points of the opposition, but be a traditionalist when it comes to abusing error correction mechanisms for cheap syntactic pleasures.

80 characters per line

Limit your lines to 80 characters. Yes, screens have gotten much bigger over the last few years, but your brain has not. Use the additional room for split screen, your editor supports that, right?

Use single quotes

Use single quotes, unless you are writing JSON.

Right:

var foo = 'bar';

Wrong:

var foo = "bar";

Opening braces go on the same line

Your opening braces go on the same line as the statement. In other programming languages this might be just personal preference, in JavaScript it is madatory!

Right:

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

Wrong:

if (true)
{
    console.log('losing');
}
// the following returns undefined instead of the object,
// because of the semicolon insertion the JavaScript interpreter does!
return
{
    foo: "bar"
};

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.

Exception variables that only get declared for later use.

JavaScript has no block scope, there for put "var" declaration at the begining of the function scoope, otherwise you might confuse programmers comming from other languages. See Crockford on this.

Right:

var i, k, key, len, tmp;
var keys   = ['foo', 'bar'];
var values = [23, 42];

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

Wrong:

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

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

var len = 4711;
for (var i=0; i < len; i++) {
    // ... stuff ...
}

Use lowerCamelCase for variables, properties and function names

Variables, properties and function names should use lowerCamelCase. They should also be descriptive. Single character variables and uncommon abbreviations should generally be avoided.

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.

Node.js / V8 actually supports mozilla's const extension, but unfortunately that cannot be applied to class members, nor is it part of any ECMA standard.

Right:

var SECOND = 1 * 1000;

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

Wrong:

const SECOND = 1 * 1000;

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

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'
        };

Use the === operator

Programming is not about remembering stupid rules. 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 multi-line ternary operator

The ternary operator should not be used on a single line. Split it up into multiple lines instead.

Right:

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

Wrong:

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

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');
}

Use descriptive conditions

Any non-trivial conditions should be assigned to a descriptive variable:

Right:

var isAuthorized = (user.isAdmin() || user.isModerator());
if (isAuthorized) {
    console.log('winning');
}

Wrong:

if (user.isAdmin() || user.isModerator()) {
    console.log('losing');
}

Write small functions

Keep your functions short. A good function fits on a slide that the people in the last row of a big room can comfortably read. So don't count on them having perfect vision and limit yourself to ~15 lines of code per function.

Return early from functions

To avoid deep nesting of if-statements, always return a functions 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);

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.

Exception: use JsDoc-style comments for describing variables, functions and classes, this will help generating automatic documentation, using compiling tools like Google's Closure Tools and code-hinds and code-completion of common IDEs like Netbeans or Eclipse

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.
 * @param {integer} id Id of the User
 * @param {function} cb Callback function
*/
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) {
  // ...
}

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

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

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.

Code quality tool

Assure constant code quality and catch some errors early with JSLint. E.g. use of reserved words, missing semicolons, missing function declarations or variables not being declared locally cluttering the global name space.

Used JSLint options

/*jslint vars: true, node: true, indent: 4, maxlen: 80 */