Question

This is how JsFromHell defines a function to find sum of a numeric array (http://jsfromhell.com/array/sum)

sum = function(o){
    for(var s = 0, i = o.length; i; s += o[--i]);
    return s;
};
//sum([1, 2, 3, 4, 5, 6, 7, 8, 9])

Can someone explain what’s happening within second part of the for loop? What’s the meaning of “i;”? It appears like its same as i >= 0. But that returns a NaN.

Answer 1

JavaScript has various ways of coercing non-Boolean values to true or false. One of them has to do with numbers: zero is false, any other number is true.

For strings, an empty string is false, others are true. The null value is coerced to false, as is the somewhat zen-like “undefined” non-value.

You could write that code:

for (var s = 0, i = o.length; i > 0; s += o[--i]);

and it might be even more efficient. (Or it might not be; it’s the kind of micro-optimization that only library maintainers should worry about, since next week the browser vendors may rev their interpreters and flip the situation on its head.)

Finally, if you’re getting a NaN, it means that you don’t really have an array of numbers. If there’s a single thing in the array that can’t cleanly be converted to a numeric value in the third part of the “for” loop, you’ll get a NaN result. edit — oh wait, I see; you tried i >= 0 and not i > 0. That means the loop will try to access o[-1] which is undefined. That’ll give you a NaN when you try to convert it to a number.