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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.
<!DOCTYPE html>
<html>
<body>
<script>
  var a;
  console.log(typeof a);
  console.log(typeof b);

  var b = document.getElementById("x");
  console.log(b);
  console.log(a == b);
  console.log(a === b);

  var c = null;
  var d = undefined;
  console.log(c + d);
  console.log(typeof (c + d));

  var e = new Object();
  console.log(e);
  console.log(e.a);

  if (typeof yourvar == 'undefined') console.log("variable does not exist");
</script>
</body>
</html>

undefined undefined null true false NaN number Object {} undefined variable does not exist
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.
<!DOCTYPE html>
<html>
<body>
<script>
  console.log(typeof v1);
  // console.log(typeof v2);
  // console.log(typeof v3);
  var v1 = 10;
  let v2 = 20;
  const v3 = 30;
</script>
</body>
</html>

undefined ReferenceError ReferenceError