function definition mode function A () {} and A=function () {}

In general, the result of the call is the same, but there is a difference.

The first way:

functiona(){    alert(‘old‘);  }var b=a;functiona(){    b();    alert(‘new‘);    }a();//浏览器就会出现内存溢出的情况

The second way:

sequentially

function  a () { Code class= "JS spaces" >     alert ( ' old ' } var  b=a; var  a= function () {      b ();      alert ( ' new ' } a (); //browser will alert out ' old ' and ' new '

The difference between the two ways can be clearly distinguished here. The order of the definitions is different.
The first, in fact, does not redefine a function and executes itself in it.
In the second way, a = function () code A that is not executed in the function is already redefined. So the redefinition here is valid.

Supplement 1:

function   a () {       alert ( ' old ' } var  b=a; function  a () {       b ();      alert ( ' new ' }

Compile time: First A is defined as alert ("old") and then is defined as B (); Alert ("new");
Runtime: b = function A () {B (); Alert ("New");}, at this point B and a, call B directly in the function body, whether from a or B call results are the same, resulting in a stack overflow

On the other hand

functiona(){    alert(‘old‘);  }var b=a;vara=function(){    b();    alert(‘new‘);}

Compile time: A is defined as alert ("old")
Run Time:

b=functiona(){alert("old")}; a=function(){b();alert("new")};

At this point the function body of B does not include any of the AB, a only call B ... No stack overflow is generated ...

Supplement 2:

In general, you will use the first way to avoid code pollution, but if you need to retain the original function, you have to use the second way of writing, anyway, both methods are consistent.

In fact, the first form of writing is later, and this is optimized.

function definition mode function A () {} and A=function () {}