Null and undefined in JavaScript

Null and undefined in javaScript seems the same but there is a significant difference between the two. In this article let us explore the similarities and differences between them.

Undefined

Undefined means no-value. if you have declared a variable but did not initialize/assign it with any value,By default it stays undefined, If you try to read a non-existing property in an object it gives undefined. you can also assign a variable as undefined.

   let i;
   console.log(i); //undefined
   i={};
   console.log(i.something); //undefined
   i= undefined;
   console.log(i); //undefined

Null

Null is non-existing value or you can also say no object value.A variable becomes null only when you assign it as null. so null doesn't come by default.

   let i;
   console.log(i);// undefined
   i = null;
   console.log(i);// null

Both null and undefined evaluates to falsy.

   let i; 
   if(i){
     console.log('truthy');
   }
   else{
     console.log('falsy');
    } //prints falsy
   i=null;
   if(i){
     console.log('truthy');
    }
   else{
     console.log('falsy');
    } // prints falsy

as we can see both null and undefined evaluates to falsy values.

But interestingly if we check the type of null it gives object even though
both null and undefined are said to be primitives. This is regarded as
mistake in javaScript implementation.

   console.log(typeof null); //object
   conosle.log(typeof undefined); //undefined

Why typeof null gives object is explained in this v8 article.

following the above image Brendan Eich the (creator of javascript) designed JavaScript to make typeof return 'object' for all values on the right-hand side. That’s why typeof null gives object despite the spec having a separate Null type.

one more thing is when we compare null and undefined with === it returns false but when we compare null == undefined it returns true because of type-coercion.

summarizing the post:

1. undefined means not-defined. a variable is undefined until you give it some value.
2. null means no-value. something becomes null only if it is made(assigned) null.
3. null and undefined both evaluate to falsy.
4. null and undefined both are primitive values.
5. typeof null gives object.

if you like my post please do like. you can also follow me here on dev.to or on twitter

Comments

[Jan Melicherik]

Simple and non confusing explanation, thank you

[aravind_reddy]

Thank you for your kind words Jan!