The difference between the two functions? (“function x” vs “var x = function”) [duplicate]

Answer 1

The first tends to be used for a few reasons:

1. The name "sum" shows up in the stacktrace which makes debugging easier in many browsers.
2. The name "sum" can be used inside the function body which makes it easier to use for recursive functions.
3. function declarations are "hoisted" in javascript, so in the first case, the function is guaranteed to be defined exactly once.

4. Semicolon insertion causes

   var f = function (x) { return 4; }
   
   (f)
   

   to assign 4 to f.

There are a few caveats to keep in mind though. Do not do

  var sum = function sum(x, y) { ... };

on IE 6 since it will result in two function objects being created. Especially confusing if you do

  var sum = function mySym(x, y) { ... };

According to the standard, function sum(x, y) { ... } cannot appear inside an if block or a loop body, so different interpreters will treat

  if (0) {
    function foo() { return 1; }
  } else {
    function foo() { return 2; }
  }
  return foo();

differently. In this case, you should do

  var foo;
  if (0) {
    foo = function () { return 1; }
  } ...