Javascript:Operations on Uninitialized Variables

When JavaScript variables are declared, they have an initial value of undefined.

If you do a mathematical operation on an undefined variable your result will be

NaN which means "Not a Number".

If you concatenate a string with an undefined variable, you will get a literal

string of "undefined".

consider following code snippet

Code:

x++

console.log(x);

var x = 21;

output:

NaN

Here at first line x is undefined any after mathematical operation it becomes

NaN.

consider another code snippet

Code:

x = x + 'my data'

console.log(x);

var x = "original data";

Output:

undefinedmy data

Here again x at first line is undefined when we concat it,its to toString() is

called first then latter part concatenated.