- There's both undefined and null
- There's three ways to declare variable
- typeof vs instanceof
- for in vs for of
- the way `this` works
- semantics of functions called with the wrong number of args
There's far more that I can't immediately recall.
A lot of the fixes are just removing the old broken way of doing things.
I'd still probably change 'for x of ...' to implicitly be 'const x' and require let if you are doing shenanigans with changing the values.
I am surprisingly ok with null and undefined. Null means not a thing, undefined means there is not even the notion of the thing.
I'm pretty sure there are more than three ways to declare a variable.
x=5
let x=5
var x=5
window.x=5
window["x"]=5 //arguably the same as above.
Object.defineProperty(window,"x",{value:5})
Object.assign(window,{x:5})
A variable reference can lookup properties on the global object, or other objects with ‘with’
If only "undefined" was consistently the same as lack of variable / collection entry, it would be understandable. But as things are, {x: 1, y: undefined} is not the same as {x: 1} in many ways - indeed, so much so that TS distinguishes between these two cases in its type system.
To make matters even worse, there's a difference between setting an object property to undefined as opposed to omitting it. So one could say there are two different "undefined"s.
FWIW Python has three (default, nonlocal, and global).
And many languages have two.