Question

Leave a comment if I’m wrong: In my memory, I thought the difference between the two style of function creation:

function myFunc(){};

and

myFunc = function(){};

is the first evaluate right away, and the later waits for the function call.

so I got this code in this article about global variable is attach to window object:

function setGloabalVariable(){
  window.variable = '1';
}

after loading the page, why the variable still undefined?

Answer 1

The difference between these two types of declarations is that the first is a named function statement, the second is an anonymous function expression. Neither of them is automatically executed at their point of declaration (you may be thinking of an IIFE).

// Create a symbol named "my_named_function"
// that points at the named function "my_named_function"
function my_named_function() {
}

// Create a symbol named "my_anonymous_func"
// that points to the *unnamed* function <anonymous function>
var my_anonymous_func = function() {
}

Both my_named_function and my_anonymous_func are unexecuted. Both may be executed by calling them:

my_named_function();
my_anonymous_func();

An IIFE (Immediately Executed Function Expression) works a little differently:

(/* define a function */ function() {

})(/* and *immediately* execute it */);

I recommend reading through kangax’s excellent article on function expressions and statements for more information on this subject.