JavaScript checking for null vs. undefined and difference between == and ===

Question

1. How do I check a variable if it\'s null or undefined and what is the difference between the null and undefined?<

Answer 1

Ad 1. null is not an identifier for a property of the global object, like undefined can be

let x;      // undefined
let y=null; // null
let z=3;    // has value
// 'w'      // is undeclared

if(!x) console.log('x is null or undefined');
if(!y) console.log('y is null or undefined');
if(!z) console.log('z is null or undefined');

try { if(w) 0 } catch(e) { console.log('w is undeclared') }
// typeof not throw exception for undelared variabels
if(typeof w === 'undefined') console.log('w is undefined');

Ad 2. The === check values and types. The == dont require same types and made implicit conversion before comparison (using .valueOf() and .toString()). Here you have all (src):

if

== (its negation !=)

=== (its negation !==)