Question

I am a little confused by why the below code returns NaN for output when the individual “i” values return as numbers. How do I produce a cumulative sum of all the “i” variables?

<script> 
var i;

var output;
for (i=0;i<20;i+=5){
console.log(i);
output+=i;
console.log(output);

} 
document.write (output);
console.log(output);
</script>

Answer 1

You need to initialize output to 0, otherwise you are adding numbers to undefined

<script> 
    var i,  
        output= 0;

    for (i = 0; i < 20; i += 5) {
        console.log(i);
        output+=i;
        console.log(output);  
    } 

    console.log(output);
</script>