JavaScript Algorithms and Data Structures
Basic JavaScript
Understanding 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.

Initialize the three variables a, b, and c with 5, 10, and "I am a" respectively so that they will not be undefined.
// Only change code below this line
var a;
var b;
var c;
// Only change code above this line

a = a + 1;
b = b + 5;
c = c + " String!";
for more:• https://www.clictune.com/ex7f