JS函数定义顺序

Question

Very quick question as I am having some kind of strange bugs and I can't find any documentation on this. Does order in which functions are defined in a file matter?

For example:

function a() {
  b(); //defined below the current function
}

function b() {
  //do something
}

Is it considered proper or do I have to mind the order?

Answer 1

Due to variable hoisting, "var statements and function declarations will be moved to the top of their enclosing scope" [1].

This can cause gotchas at times, but as long as the file containing the function is loaded, the order shouldn't matter.

[1] http://bonsaiden.github.io/JavaScript-Garden/#function.scopes , http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html

Answer 2

在JavaScript中，只要被调用函数存在，创建函数的顺序就无关紧要。

Answer 3

The problem you're running into is variable hoisting. Read more about that here

It is considered improper to do that according to Crockford's JSLint. However, it shouldn't matter as long as you are defining the functions in that manner. For example, your code will work, but something like

function a(){
   b();
}
function b(){
   //do something
}

will work, but

function a(){
    b();
}
var b=function(){//do something};

will not work.

So basically, if everything is loaded before you call it, you should be fine. Consider wrapping it in a $("window").load()

Answer 4

No, it does not matter, where function is. It can be at top of file, at bottom of file even in another file. what matter is existence.

Once JavaScript is loaded it will treated as single file or script; that is the reason you may not have function with same signature even on two different script file using on same page.

Fiddle with both cases

a();
c();

function a() {
    b(); //defined below the current function
}

function b() {
    alert("called b");
}

function c() {
    b();//defined above the current function
}