Difference between var name; and var name = 0;

Question

I want to sum up myArray where i declared var sumUpArray = 0. It return the correct sum of myArray which is 6.

var myArray = [1, 2, 3];
var sumUpArray = 0;

for (i = 0; i < myArray.length; i++) {
  sumUpArray = sumUpArray + myArray[i];
}

console.log(sumUpArray);

But when I declared var sumUpArray; it return NaN.

var myArray = [1, 2, 3];
var sumUpArray;

for (i = 0; i < myArray.length; i++) {
  sumUpArray = sumUpArray + myArray[i];
}

console.log(sumUpArray);

What is the difference between the two declaration of variable?

Did you check what the value of sumUpArray is before the loop in the latter code? — Carcigenicate
– Carcigenicate, Commented Mar 15, 2018 at 16:54
When you use var sumUpArray, its value is undefined. So, if you add 0 to an undefined value, it will be NaN - Not a Number — Pedro Martins
– Pedro Martins, Commented Mar 15, 2018 at 16:54
By default the value of sumUpArray is undefined. When you sum undefined with a number, you are getting NaN. — GSSwain
– GSSwain, Commented Mar 15, 2018 at 16:54

Artur P. · Accepted Answer · 2018-03-15 16:59:17Z

1

When you declare variable

var sumUpArray;

is the same as

var sumUpArray = undefined;

So you try add integer to undefined results NaN

sumUpArray = sumUpArray + myArray[i];
sumUpArray = undefined + myArray[i]; // NaN

BTW: use let and const to declare variables.

answered Mar 15, 2018 at 16:59

Artur P.

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Comments

Chris Staikos · Accepted Answer · 2018-03-15 16:55:59Z

0

In the first example you declared the variable but didn't assign any value, so it starts out as undefined. I think you maybe expected it to be auto-assigned to 0, which it does not.

Then you tried to add some numbers to it, but undefined + {anyNumber} = NaN.

answered Mar 15, 2018 at 16:55

Chris Staikos

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Tiago Alves · Accepted Answer · 2018-03-15 16:54:26Z

0

Because when you use var sumUpArray, sumUpArray is undefined, not 0. undefined + any number will return NaN.

answered Mar 15, 2018 at 16:54

Tiago Alves

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Sagar Kharab · Accepted Answer · 2018-03-15 16:58:02Z

0

var name; is declared but not assigned or initialized or defined and hence is undefined however var name=0; is assigned a value '0'. typeof(name) will tell you that both are having different types.

answered Mar 15, 2018 at 16:58

Sagar Kharab

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