Explore the order in which the three parts of the For loop are executed separated by semicolons

What causes this question is the result of running a js program:

 
var i = 0; 
function a(){ 
for(i=0;i<20;i++){ 
} 
} 
function b(){ 
for(i=0;i<3;i++){ 
a(); 
} 
return i; 
} 
var Result = b(); 

Result = 21;

And what we can see from this program is that I is going to be 20 when a returns and that's fine.
And when b returns, it's worth discussing whether I is 20 or 21.
The essence of the problem is that the judgment was made first. 3. Once again, i++ was conducted first, and then i< was judged; 3.

As you can see from the results of the execution, i++ was executed first.

 
function a(){ 
for(i=0;i<20;i++){ 
//No var I
//The I here is the global variable that everyone can access
} 
} 
function b(){ 
for(i=0;i<3;i++){ 
//alert(i);// Same thing here i Also a global variable, returns 0 And only return once  
a();//This function returns I =20
//When I =20 goes through I ++, I =21, then it does not conform to i<Condition of 3, exit directly. So return I =21 which is normal!
} 
return i; 
} 
var Result = b(); 

Here we complete the execution order of the for loop:
Take the following program as an example

 
for(int i=0;i<10;i++) 
{ 
} 

I =0; I< 10; Then execute the first round of the loop body
Then execute: i++,i< 10; Then execute a second round of the loop body
Until the last I ++ after I > =10, and the loop ends.

namely

Statement 1 is executed before the loop (block of code) begins

Statement 2 defines the conditions for running the loop (block of code)

Statement 3 is executed after the loop (block of code) has been executed