Difference between Undeclared, Undefined and NULL in JavaScript

Undeclared

A variable used in the code which is not already declared with any keyword like var, let and const is called undeclared variable.

When we use it in the code gives an error.If we use typeof operator to get the value of it then return 'undefined' value.

    
        console.log(r); // output as an error - Uncaught ReferenceError: r is not defined
    

    
        console.log(typeof d); // undefined
    

Undefined

The undefined type has only one value, which is the special value undefined.

When a variable is declared using var or let but not initialized, It is assigned the value of undefined.

    
        var a; 
        let b; 
        console.log(a); // undefined 
        console.log(b); // undefined
    

Null

The Null type is the second data type that has only one value: the special value null. Logically, a null value is an empty object pointer, which is why typeof returns “object” when it ’s passed a null value

    
        var x = null; 
        console.log(typeof x); //object 
    

In summary:

• Undeclared refers to a variable that hasn't been declared at all.
• Undefined refers to a variable that has been declared but hasn't been given a value.
• Null is a value that can be assigned to a variable to represent the absence of any object value.

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