function sumArray(numbers){
var sum;
for(var i in numbers){
sum += numbers[i];
}
return sum;
}
console.log(sumArray([1,2,3,4,5]));
Hi all,
The outcome is NaN. However, if I initialize sum with sum = 0, the outcome is 15. Why JS does not recognize the value type in the array and do the initialization for me? Why does it return NaN in the first case?
Thanks
When a variable is declared within current scope, it is initialized with undefined value.
var sum; // is initialized with undefined
In the for loop, the addition sum += numbers[i] is actually doing an undefined + 1 operation. Because both operands are not string types, they are converted to numbers:
undefined + 1NaN + 1NaNPlease check this article for more info about the addition operator (example 7).
Of course, to solve this problem just initialize it with 0:
var sum = 0;
Also I would sum the items simpler:
var sum = [1,2,3,4,5].reduce(function(sum, item) {
return sum + item;
});
when you create var sum; it's value is undefined [default]
so when you keep adding to undefined you get NaN [Not a Number]
but if you initialize as var sum=0;
then you are adding numbers to 0 so you get correct output
try
console.log(undefined+1); you get NaN but
if you do
console.log(0+1); then you get 1
This is because you have not initialized sum. So sum+ will try to add to undefined.
function sumArray(numbers){
var sum=0; // sum="" if you want output as 12345
for(var i in numbers){
sum += numbers[i];
}
return sum;
}
console.log(sumArray([1,2,3,4,5]));
The reason behind is very clear. When JavaScript finds any "undefined" or "unexpected" error then it returns NaN.
Where else in your second condition where you added sum=0 ,it already made a value of it then what ever you did it was addition to that value.
var sum = 0;