Language

US English should be used.

preferred

var authorized = true;

not preferred

var authorised = true;

Indentation

Use 2 spaces per indentation level. No tabs.

preferred

function sumArgs() {
  for (var i = 0, sum = 0; i < arguments.length; i++) {
    sum += arguments[i];
  }

  return sum;
}

not preferred

function sumArgs() {
    for (var i = 0, sum = 0; i < arguments.length; i++) {
        sum += arguments[i];
    }

    return sum;
}

Semicolons

Use semicolons.

Do not place semicolons after any function unless it is a declaration (like var y = function(){};).

Don't omit the semicolon after return.

preferred

var x = 12;
var y = function() {};

for (var i = 0, l = 5; i < l; i++) {
  // something
}

function handler() {
  // something
  return;
}

not preferred

var x = 12
var y = function() {}

for (var i = 0, l = 5; i < l; i++) {
  // something
};

function handler() {
  // something
  return
};

;

Naming

All names should be limited to using alpha a-z and numeric 0-9 characters. All names should start with an alpha character. One exception is when dealing with an outside dependency, like SQL, where a column name may contain underscores.

Names should be camel cased. Any constructor should start with a capital letter. Any other type of variable should start with a lowercase character.

Constants do not exist in Node, since the idea of a frozen variable is loose. Global variables should be named like normal variables.

preferred

var defaultValue3 = 30;

function Parent() {
  this.myValue1 = 10;
  this.myValue2 = 20;
  this.myValue3 = defaultValue3;
}
var child = new Parent();

not preferred

var DEFAULTVALUE3 = 30;

function parent() {
  this.MyValue1 = 10;
  this.myvalue2 = 20;
  this.my_value_3 = 30;
}
var Child = new Parent();

Function Declarations

If possible, do not define a function, rather than assigned a function to a variable.

preferred

function doSomething() {}

not preferred

var doSomething = function() {}

Object Declarations

Objects should always be broken across lines.

preferred

var obj = {
  key1: 'val1',
  key2: 'val2',
  key3: {
    key4: 'val4',
    key5: 'val5'
  }
};

not preferred

  var obj = { key1: 'val1', key2: 'val2', key3: { key4: 'val4', key5: 'val5' } };

Golden Path

When coding with conditionals, the left hand margin of the code should be the "golden" or "happy" path. That is, don't nest if statements if you don't need to. Multiple return statements are OK.

preferred

function someMethod(err, callback) {
  if (err) {
    return callback(err);
  }

  // more logic

  callback();
}

not preferred

function someMethod(err, callback) {
  if (!err) {
    // more logic

    callback();
  }
}

not preferred

function someMethod(err, callback) {
  if (err) {
    return callback(err);
  } else {
    // more logic
    callback();
  }
}

Callback Hell

If you have tiers of callbacks, then use the async module, or something similar, to make it more readable.

not preferred

module.exports = function(callback) {
  methodOne(function(err, resOne) {
    if (err) {
      return callback(err);
    }

    methodTwo(function(err, resTwo) {
      if (err) {
        return callback(err);
      }

      methodThree(function(err, resThree) {
        callback(err);
      });
    });
  });
};

preferred

var async = require('async');

module.exports = function(callback) {
  async.series([
    methodOne,
    methodTwo,
    methodThree
  ], callback);
};

Whitespace

Every statement (if, for, while, do, switch, try, catch, finally) should have a space before the following { bracket or ( parenthese.

if (x) {
  // logic
}

Simple arrays do not need spacing. But if there are references like the one below, then it should have spaces included.

var arr = [1, 2, 3];

var arr2 = [ anotherObj[key] ];

Vertical white-space is needed for readability. Don't shy away from using space to make things readable.

If Statements

All if statements should be encapsulated by {} brackets.

No if statements should be a single line.

preferred

if (x === y) {
  a = 10;
} else if (x === z) {
  a = 20;
} else {
  a = 30;
}

not preferred

if (x === y) a = 10;
else if (x === z) a = 20;
else a = 30;

not preferred

if (x === y)
  a = 10;
else if (x === z)
  a = 20;
else
  a = 30;

Try Catch

Try catch statements should follow the same rules as an if else. See the above documentation.

No function calls should be made from within a try (since any error within would bubble up to the catch)

preferred

try {
  regionId = user.payment.region.id; // will fail if any part of dot notation is undefined
  passed = true; // will only be reached if no err
} catch(err) {
  passed = false;
}

if (!passed) {
  processUnavailableRegion(user);
} else {
  processAvailableRegion(regionId);
}

not preferred

try {
  regionId = user.payment.region.id; // will fail if any part of dot notation is undefined
  processAvailableRegion(regionId); // any error in this call will bubble to the catch
} catch(err) {
  processUnavailableRegion(user);
}

Use Shorthand Constructors

Do not use new Array() or new Object(). Instead, use [] and {}. This is actually more efficient, and saves a few bytes.

Evil keywords

Do not use eval. It, in general, is not very safe.

Do not use with. It leads to some confusing situations with scope.