Why is there a potential infinite loop in my JavaScript Code?

Question

I wish to find the sum of all prime numbers between range defined from 1 to N. The code gives an infinite loop when I call the sumPrimes function with a value of 3. I have debugged the code and found out that it does that only for the number 3. It does not do so for any other numbers above 2.

JavaScript:

function sumPrimes(num) { 
    var sum=0;
    for (i = 2; i <= num; i++) {
        if (checkPrime(i)) {
            sum += i;
        }
    }

    return sum;
}

function checkPrime(num) {
    for (i = 2; i <= Math.sqrt(num); i++) {
        if (num % i === 0) {
            return false;
        }
    }

    return true;
}

There is no infinite loop on 3. However, it won't do anything for 3, as the square root of 3 is less than 2, so it will simply return true. Which is actually correct, as 3 is prime. — ManoDestra
– ManoDestra, Commented Jul 6, 2016 at 15:19
@GerardoFurtado it works for the number 2 though, I am confused as well — Pranav Ghate
– Pranav Ghate, Commented Jul 6, 2016 at 15:20
@ManoDestra, fixing the code in the question hides the actual problem. — zzzzBov
– zzzzBov, Commented Jul 6, 2016 at 15:23

A.Baudouin · Accepted Answer · 2016-07-06 15:36:29Z

4

Because you have to declare i with var : it will make it local to the function.

for (var i = 2; i <= num...

otherwise the two functions use the same global variable.

If you want to avoid this kind of bug, you should use strict mode. JavaScript Use Strict

You just have to put "use strict"; at the top of your .js file.

edited Jul 6, 2016 at 15:36

answered Jul 6, 2016 at 15:20

A.Baudouin

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3 Comments

Mike Robinson Over a year ago

And would you then please explain why that omission (which occurs in two places in the posted code) would produce "an obviously very-subtle bug?" It does not matter that the variable is used within the body of a function? Interesting ... (And yet-another reason why I long ago stopped writing source-code directly in JavaScript ...!)

Pranav Ghate Over a year ago

Doesnt matter if var is used or not

ManoDestra Over a year ago

It does matter. You need to add var prior to i in both functions above.

zzzzBov · Accepted Answer · 2016-07-06 15:17:58Z

1

You haven't declared a scope for i, which means that both loops will keep resetting the value of i in global scope, causing the loop to continue endlessly.

Add var i to the top of both functions and the problem will disappear.

answered Jul 6, 2016 at 15:17

zzzzBov

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