为什么我的函数在遇到返回后执行剩余的代码（返回两个循环内部）

Question

I don't understand why this function will run the remaining code after the return statement is supposed to be called:

function func(i) {
while (i >= 0) {
if (i < 0) {    
    return;
    }    
    document.write("inside loop, i is  " + i);
    document.write("</br>");

i--;
} //end of while 
document.write("end of function, i is " + i);
document.write("</br>");
}
func(2);

Output:

inside loop, i is 2; 
inside loop, i is 1; 
inside loop, i is 0; 
end of function, i is -1; 

I was expecting:

inside loop, i is 2; 
inside loop, i is 1; 
inside loop, i is 0; 

1. Why is the line "end of function, i is -1" being written if the return should be called before it?

2. If I replace while(i >= 0) with while(true) the function gives the expected output (without the last document.write). Why is that?

3. If I replace the code after the while loop with:

    return document.write("end of function, i is " + i); document.write("end 2 of function, i is " + i); 

the last code line (end 2 of function, i is ) is not executed. Why does the code keep executing after the first return and not after the second?

4. If I take the return out of the if statement the function stops when return is called:

    while (i >= 0) { document.write("inside loop, i is " + i); document.write("</br>"); i--; return; } 

output is:

inside loop, i is 2

Why doesn't the last document.write ("end of function, i is ") execute in this case?

Answer 1

Your while condition:

while (i >= 0) {

will never be true at the same time that the return test is true:

if (i < 0) {    
  return;
}    

Since i doesn't get changed between the top of the while loop and the i < 0 test, the if will never be fulfilled. On the last iteration, i will be decremented from 0 to -1 just before the end of the while block, and since an i of -1 fails the while test, no further iterations will run (and the if check and return will not be encountered).

If you had the while loop continue indefinitely, until the if statement was fulfiled, the function would indeed return:

 function func(i) { while (true) { if (i < 0) { return; } document.write("inside loop, i is " + i); document.write("</br>"); i--; } //end of while document.write("end of function, i is " + i); document.write("</br>"); } func(2); 

As for

3. If after the while loop I replace the code with:

    return document.write("end of function, i is " + i); document.write("end 2 of function, i is " + i); 

and

4. If I take the return out of the if statement the function stops when return is called:

    while (i >= 0) { ... return; 

In both of these cases, the return statement is actually being encountered, so the function terminates there as expected. (In your original code, the interpreter never runs across the return , because the if is never fulfilled)

Answer 2

Use break; instead or better still use return false; like so

function func(i) {
while (true) {
if (i < 0) {    
    break;
    }    
    document.write("inside loop, i is  " + i);
    document.write("</br>");

i--;
} //end of while 
document.write("end of function, i is " + i);
document.write("</br>");
}
func(2);

OR

function func(i) {
while (true) {
if (i < 0) {    
    return false;
    }    
    document.write("inside loop, i is  " + i);
    document.write("</br>");

i--;
} //end of while 
document.write("end of function, i is " + i);
document.write("</br>");
}
func(2);

I don't know about the second (return false) but the first should do the trick. I hope it helps.