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'undefined' And 'null'

Out of the six primitive data types, two data types, 'undefined' and 'null', can only have one value, which is 'undefined' and 'null' themselves respectively. 'undefined' is a result of attempts to access variables which have not been initialized with a value. 'null' is a default placeholder for an object.

'undefined' and 'null' both evaluate to false in a Boolean expression, which explains why (a==b) is true. NaN (Not a Number) actually has the Number type.
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<!DOCTYPE html><html><body><script>

<!DOCTYPE html><html><body><script>
   var a;
   console.log(typeof a);  // undefined
   console.log(typeof b);  // undefined
      var b = document.getElementById("x");
   console.log(b);     // null
      console.log(a==b);  // true
   console.log(a===b); // false
      var c = null;
   var d = undefined;
   console.log(c+d);   // NaN
   console.log(typeof (c+d));   // number
      var e = new Object();
   console.log(e);     // Object {}
   console.log(e.a);   // undefined
   if (typeof yourvar == 'undefined') console.log("variable does not exist");</script></body></html>

</script></body><html>
Some declarations are hoisted in JavaScript. This means that a variable can be used above its declaration within the scope. <br>Here v1 is initialized with 'undefined' initially. v2 and v3 stay inaccessible until they are declared. The period between entering scope and being declared is called the temporal dead zone.
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<!DOCTYPE html><html><body><script>

<!DOCTYPE html><html><body><script>
   console.log(typeof v1);  // undefined   //console.log(typeof v2); // ReferenceError   //console.log(typeof v3); // ReferenceError
   var v1=10;
   let v2=20;
   const v3=30;</script></body></html>

</script></body><html>