(JavaScript) Why does a continue statement inside 'if' in a while loop crash the browser?

Question

I want to write a program in JavaScript to print all the numbers from 1 to 20 except 5 and 10. When I use a for loop like this:

for (x = 1; x <= 20; x++) {
    if (x == 5 || x == 10) {
        continue;
    }
    document.write(x + ' ');
}

It works properly and prints 1 2 3 4 6 7 8 9 11 12 13 14 15 16 17 18 19 20 .

However, when I try to use a while loop like this:

var x = 1;

while (x <= 20) {
    if (x == 5 || x == 10) {
        continue;
    }
    document.write(x + ' ');
    x++;
}

It makes the webpage unresponsive and I get a prompt asking me to close it. What is the problem here?

Answer 1

The problem is the following, inside the for-loop the continue will jump back to the update expression x++ but in the while loop it will jump back to the running condition while(x <= 20)

Referencing the mdn docs .

In a while loop, it jumps back to the condition. In a for loop, it jumps to the update expression.

Because you are not updating the counter inside your condition

while (x <= 20) {
    if (x == 5 || x == 10) {
        continue;            
    }

x will remain 5 and gets never updated because with the continue inside the while-loop it jumps back to the running condition . This will end up in an endless loop .

To solve it you can increment the counter before the continue statement inside the while-loop

while (x <= 20) {
    if (x == 5 || x == 10) {
        x++
        continue;

 // for (x = 1; x <= 20; x++) { // if (x == 5 || x == 10) { // continue; // } // document.write(x + ' '); //} var x = 1; while (x <= 20) { if (x == 5 || x == 10) { x++ continue; } document.write(x + ' '); x++; }

Answer 2

In the first case the loop declaration for (x = 1; x <= 20; x++) handles incrementing x between iterations, so every time the loop body executes, you get a different value of x .

In the second case, x++ is only executed if x is neither 5 nor 10 . So, when x reaches 5 it's the loop will keep executing with x = 5 forever, since nothing is changing it.