Why does “undefined equals false” return false?

Question

When I compare undefined and null against Boolean false, the statement returns false:

undefined == false;
null == false;

It return false. W

Answer 1

From the incomparable MDN, sponsored by the company of JavaScript's creator.

JavaScript provides three different value-comparison operations:

• strict equality (or "triple equals" or "identity") using ===,
• loose equality ("double equals") using ==,
• and Object.is (new in ECMAScript > 6).

The choice of which operation to use depends on what sort of comparison you are looking to perform.

Briefly, double equals will perform a type conversion when comparing two things; triple equals will do the same comparison without type conversion (by simply always returning false if the types differ); and Object.is will behave the same way as triple equals, but with special handling for NaN and -0 and +0 so that the last two are not said to be the same, while Object.is(NaN, NaN) will be true. (Comparing NaN with NaN ordinarily—i.e., using either double equals or triple equals—evaluates to false, because IEEE 754 says so.) Do note that the distinction between these all have to do with their handling of primitives; none of them compares whether the parameters are conceptually similar in structure. For any non-primitive objects x and y which have the same structure but are distinct objects themselves, all of the above forms will evaluate to false.

For a visual overview of the whole picture of equality in JavaScript: https://dorey.github.io/JavaScript-Equality-Table/

The truth is, this seemingly "bad" aspect of JavaScript is a source of power when you understand how it works.