javascript – What is the difference between var name = function() and function name()?

Question:

I see in some codes different ways to define a function, but I don't know the difference if it even exists. What is the difference between these functions?

Function 1

function teste(valor){
    return (valor > 1) ? true : false;
}

Function 2

var teste = function(valor){
    return (valor > 1) ? true : false;
}

Answer:

Function Declaration: function name()

A function declaration defines a function variable without the need for variable assignment. They are independent constructs , and cannot be nested in non-function blocks (see comment below). It would be like a "kin" of variable declarations. Just as variables need to be declared with var , functions are declared with function .

function bar() {
   return 3;
}

bar() //3
bar  //function

Function Expression: var name = function()

A function expression defines a function as part of a larger syntactic expression, typically a variable assignment. Functions defined with function expressions can be named or anonymous. Cannot start with function , so parentheses are used in "auto invocation" in the example below:

//expressão anônima de função
var a = function() {
   return 3;
}

//expressão nominada de função
var a = function bar() {
   return 3;
}

//expressão "autoinvocada" de função
(function digaOi() {
    alert("Oi!");
})();

Scope visibility example:

<script>
  funcaoUm(); // Da erro, ainda nao foi definida
  var funcaoUm = function() { }
</script>

<script>
  var funcaoDois = function banana() {
    banana(); // Nao da erro, está no escopo
  }
  banana(); // Da erro, está fora do escopo (exceto no IE)
</script>

<script>
  funcaoTres(); // Nao da erro, é resolvida ao compilar
  function funcaoTres() { }
</script>

"Functions cannot be declared in non-code blocks"

Let's look at this code:

if (true) {
  function foo() {
    return 'primeiro';
  }
} else {
  function foo() {
    return 'segundo';
  }
}
foo();

Some browsers will return "first", some "second"!

According to the specification, function declarations can even appear inside function blocks, but not inside if , while , for and other building blocks.

In these cases, what should happen is a syntax error , but practically none of the implementations do this in practice, and what's worse, each one handles code like the example in a different way (with the exception of BESEB and DMDScripot, as per this one. source ).

Sources: Function Declarations vs. Function Expressions , SOen , kangax