Introduction to the definition and difference between the two functions of javascript

Generally, the two call results are the same, but there are still differences.
Method 1:
Copy codeThe Code is as follows:
Function (){
Alert ('Old ');
}
Var B =;
Function (){
B ();
Alert ('new ');
}
A (); // memory overflow occurs in the browser.

Method 2:
Copy codeThe Code is as follows:
Function (){
Alert ('Old ');
}
Var B =;
Var a = function (){
B ();
Alert ('new ');
}
A (); // the browser will generate 'old' and 'new' in order'

Here we can clearly distinguish between the two methods. The order of definition is different.
First, the function a was not re-defined at the beginning and the function itself was executed in it.
In the second way, a = function () is not executed here. Code a in the function has been redefined. So the definition here is valid.
Supplement 1:
Copy codeThe Code is as follows:
Function (){
Alert ('Old ');
}
Var B =;
Function (){
B ();
Alert ('new ');
}

During Compilation: a is first defined as alert ("old") and then B (); alert ("new ");
Runtime: B = function a () {B (); alert ("new") ;}. in this case, B and a are the same, and B is called directly in the function body, both a call and B call results are the same, resulting in stack overflow.
On the other hand
Copy codeThe Code is as follows:
Function (){
Alert ('Old ');
}
Var B =;
Var a = function (){
B ();
Alert ('new ');
}

During Compilation: a is defined as alert ("old ")
Runtime: B = function a () {alert ("old")}; a = function () {B (); alert ("new ")}; at this time, the function body of B does not include any of AB. a only calls B... stack Overflow will not occur in any way...
Supplement 2:
Generally, the first method is used to avoid code pollution, but if you need to retain the original function, you need to use the second method. Both methods comply with w3c.
In fact, the first writing method was only available later, which was optimized.