Question

Consider following JavaScript code (tested in Firefox):

function f(a) {

  if (a == undefined) {
    alert('undefined');
  }

  if (a == null) {
    alert('null');
  }
}

f();

Both alerts are shown, suggesting that both statements are true.

Could you provide a reasonable explanation?

Answer 1

== is a “soft” equality operator.
It uses type coercion to compare two equivalent objects as equal.

All of the following are true:

42 == "42"
0 == false
0 == ""
[] == ""
{} == "[object Object]"
'/(?:)/' == new RegExp

Instead, you should use the === operator, which checks for strict equality.