Why is output of variable NaN

Question

I wrote simple code like this

var value = function(a){return a*3;}
var number = value(a);

when i type value(5); i will get number 15 thats OK, but after typing number; ill get NaN. I just want to write in variable the result of function.

Sorry, it's kinda unclear. So it works as expected. Where do you want to write the number? Any sample use case? — Praveen Kumar Purushothaman, Dec 16 '17 at 13:15
Is it because your passing `a` as an argument `var number = value(a);` and `a` doesn't hold a number? — Claudio, Dec 16 '17 at 13:16
For a code like this: `var value = function(a) { return a * 3; } var number = value(5); console.log(number);` I get `15` Not sure what you mean by your question. — Praveen Kumar Purushothaman, Dec 16 '17 at 13:16
It looks like `a` wasn't set to any value when you defined `number = value(a)`. So `value(a)` returned NaN and so that's what `number` was set to. — lurker, Dec 16 '17 at 13:16
The `a` is local to the scope of the `value` function. You cannot use it outside the function. — Praveen Kumar Purushothaman, Dec 16 '17 at 13:17
yes you are right it works like this .....`var number = value (5);` `console.log('result is ' + number);` in my value its alwas need to be only number which i want work with. — Perezo123, Dec 16 '17 at 13:48
What you *should* be getting, if you don't have something like `var a;` earlier in your code, is a `ReferenceError: a is not defined` exception. — Ilmari Karonen, Dec 16 '17 at 15:22

score 0 · Accepted Answer · answered Dec 16 '17 at 15:27

JavaScript is not a computer algebra system, and it cannot store an algebraic expression like x * 3 in a variable. Instead, when you do:

var y = x * 3;

the variable y will be set to 3 times the value that x had when that statement was executed. If no value has been assigned to x before, then its default value will be undefined. And according to JavaScript's arithmetic rules undefined is automatically converted to NaN when used in an arithmetic expression, and NaN * 3 equals NaN.

(Actually, what you should be getting, if you try to execute the code above without declaring the variable x first, is a ReferenceError: x is not defined exception. But if you add a var x; statement before that code, to declare the variable x but leave its value undefined, then you'll indeed get NaN as the value of y.)

The fact that you've wrapped the multiplication inside a function makes no difference here. The following code:

var f = function (a) { return a * 3 };
var y = f(x);

does exactly the same thing as:

var y = x * 3;

(except, of course, for the fact that the former version also defines the function f.)

Why is output of variable NaN

1 Answers1