Why does typeof(null) return "object", but you can't assign properties to it?
javascript
Solution
This page has a nice description of the history here surrounding why `typeof(null)` gives "object": JS Data Types - Null
Here is the relevant portion (although I would suggest you read the whole post):
Why does `typeof null` return `"object"`? `// What's happening here?` `typeof null === "object"; // true`
The answer might disappoint some, but the truth is simply because the table above says to do so.
The reasoning behind this is that `null`, in contrast with `undefined`, was (and still is) often used where objects appear. In other words, `null` is often used to signify an empty reference to an object. When Brendan Eich created JavaScript, he followed the same paradigm, and it made sense (arguably) to return `"object"`. In fact, the ECMAScript specification defines `null` as the primitive value that represents the intentional absence of any object value (ECMA-262, 11.4.11).
To draw a parallel here, consider `typeof(NaN) === "number"`. So why does JavaScript give "number" as the type of `NaN` (not a number)? It is because `NaN` is used where numbers appear, it is a value that represents the intentional absence of a number value. Similar reasoning applies to `null`, the only difference being that `null` is implemented as a primitive and `NaN` is actually implemented as a `Number` object (so `NaN.foo = 42` would actually work).
Problem
Bear with me here, I'm going to try something stupid. When I evaluate `typeof(null)` in the console, I get `"object"`, so logically, I should be able to assign properties to it, but `null.foo = 42` gives `TypeError: Cannot set property 'foo' of null`. Is Javascript just picky when it comes to which global objects are mutable?