Why does this Javascript function return "Undefined" when called?

Question

I've written the following simple JavaScript code. But whenever I run it, it displays "Undefined" as a result.

Following is the code:

function naturalSum(num1,num2) {
    var sumValue = 0;
    for (i=0;i<1000;i++)
    {
        if (i%num1 === 0 || i%num2 === 0) {
            sumValue += i ;    
        }
    }
}
document.write(naturalSum(3,5));

I want to know why this is happening. When I'm not enclosing the code within the function, the code's working fine. Kindly help me on this.

have you not posted the return call, or is that your problem — MDEV
– MDEV, Commented Mar 3, 2014 at 14:33

Community · Accepted Answer · 2017-05-23 11:49:45Z

3

JavaScript functions return undefined by default, unless you explicitly return something.

So, if you want your function to return sumValue, you should add return sumValue; as the function’s last line:

function naturalSum(num1,num2) {
    var sumValue = 0;
    for (i=0;i<1000;i++)
    {
        if (i%num1 === 0 || i%num2 === 0) {
            sumValue += i ;
        }
    }
    return sumValue;
}

@AmitJoki created a JSFiddle to demonstrate:

http://jsfiddle.net/dgC46/

edited May 23, 2017 at 11:49

CommunityBot

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answered Mar 3, 2014 at 14:33

Paul D. Waite

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1 Comment

Amit Joki Over a year ago

hi, I just created a fiddle, when your answer sprang up. can you include it please ? The url is jsfiddle.net/dgC46

Dennington-bear · Accepted Answer · 2014-03-03 14:37:30Z

1

function naturalSum(num1,num2) 
{
    var sumValue = 0;
    for (i=0;i<1000;i++)
    {
        if (i%num1 === 0 || i%num2 === 0) 
        {
            sumValue += i ;
        }
    }
    //You forgot to return the value
    return sumValue;
}
document.write(naturalSum(3,5));

answered Mar 3, 2014 at 14:37

Dennington-bear

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4 Comments

saadi123 Over a year ago

Yup, problem solved. However, I have one more question. I used the return keyword with like this: function naturalSum(num1,num2) { var sumValue = 0; for (i=0;i<1000;i++) { if (i%num1 === 0 || i%num2 === 0) { return sumValue += i ; } } } document.write(naturalSum(3,5)); But then the editor was displaying an error that the return stating that the function does not always return a value. I want to ask that why is this happening?

Paul D. Waite Over a year ago

@saadi123: I suspect it’s because you put your return statement within an if statement, so it’ll only get called when the if’s condition is true.

Paul D. Waite Over a year ago

@Dennington-bear: this from Pedant’s Corner: a return statement outside the if but inside the for would make the function return the first time the for loop runs. It would never increment, because the return statement would have made the function return before it got a chance.

saadi123 Over a year ago

@PaulD.Waite Thanks for the guidance.

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