为什么该Javascript函数在被调用时返回“ Undefined”？

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.

Answer 1

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/

Answer 2

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));