How can I determine if a variable is 'undefined' or 'null'?

Question

How do I determine if variable is undefined or null?

My code is as follows:

var EmpN         


        

Answer 1

Let's look at this,

1.  

   let apple; // Only declare the variable as apple
   alert(apple); // undefined
   

   In the above, the variable is only declared as apple. In this case, if we call method alert it will display undefined.

2.  

      let apple = null; /* Declare the variable as apple and initialized but the value is null */
      alert(apple); // null
   

In the second one it displays null, because variable of apple value is null.

So you can check whether a value is undefined or null.

if(apple !== undefined || apple !== null) {
    // Can use variable without any error
}