JavaScript

Identifiers

  // GOOD

  var User = function (name, age) {
      this.name = name;
      this.age = age;
  };
  var user = new User('bob', 32);

  // BAD

  var user = function (name, age) {
      this.name = name;
      this.age = age;
  };
  var u = new user('bob', 32);

RULE: Identifiers bound to a variable or function must start with a minuscule.
RULE: Identifiers bound to a variable or function must be CamelCased or lowercase, depending on their length.

  // GOOD

  var someKindOfProcess = function () {};
  var tmpvar = 42;

  // BAD

  var some_kind_of_process = function () {};
  var somekindofprocess = function () {};

RULE: Variables must always be declared, prior to use.
RULE: Variable declarations should appear at the top of functions, and not inside other blocks. The exception is for loops.

Variable declarations are moved up to the top of the function scope anyway, so that's where they belong.

  // GOOD

  function (a, b) {
      var k;

      if (a == b) {
          k = true;
      }
      ...
  }

  for (var i = 0; i < l; i ++) { ... }

  // BAD

  function (a, b) {
      if (a == b) {
          var k = true;
      }
      ...
  }

RULE: Control-flow statements, such as if, while and for must have a space between the keyword and the left parenthesis.

They aren't functions, and thus better distinguished like this.

  // GOOD

  if (a) {
      return true;
  }

  // BAD

  if(a) {
      return true;
  }

RULE: Anonymous functions must have a space between the function keyword and the left parenthesis.

To emphasise the lack of identifier and differentiate them with named functions.

  // GOOD

  function (a, b) {}

  // BAD

  function(a, b) {}

RULE: Named functions must not have a space between the function name and the left parenthesis.
RULE: Function calls should not have a space between the function name and the left parenthesis.

  // GOOD

  function add(a, b) {}

  // BAD

  function add (a, b) {}

RULE: Semicolons ; must be added at the end of every statement, except when the next character is a closing bracket }. In that case, they may be omitted.

  // GOOD

  var f = function add(a, b) {
      if (a == b) { return a * 2 }   // No `;` here.
      return a + b;
  };

  // BAD

  var f = function add (a, b) {
      return a + b
  }

RULE: Braces should be used in all circumstances. They may be omitted around simple statements.

  // GOOD

  if (x) { return true }

  // BAD

  if (x)
    while (1)
      i ++;
  else
    ...

  // OK

  if (x) return true;

  // OK

  if (x)
      return true;

Vertical screen space is precious.

  // GOOD

  if (x) {
      return true;
  }

  // BAD

  if (x)
  {
      return true;
  }

  // BAD

  if (x) {
      return true; }