Oh man, you've got me itching to get into the intricacies of JavaScript…
One fun example of the difference: when doing arithmetic operations, null is indeed converted to 0, but undefined is converted to NaN. This has to do with null being an assigned value that represents empty, whereas undefined is not actually a value but a response indicating that there was no value assigned in the first place.
response indicating that there was no value assigned in the first place.
You can explicitly assign undefined to a variable though.
Another fun fact about JavaScript is that undefined never used to be a keyword. If you did var foo = undefined, foo would indeed have a value of undefined, but it was only because there was no variable called undefined in scope!
You could do varundefined = 42 then var foo = undefined would actually set foo to 42! window.undefined = 42 would break all sorts of things.
Thankfully this was fixed with ES5 in 2009, although it took a few years for browsers to make the change.
Oh man, you've got me itching to get into the intricacies of JavaScript…
One fun example of the difference: when doing arithmetic operations,
nullis indeed converted to 0, butundefinedis converted to NaN. This has to do withnullbeing an assigned value that represents empty, whereasundefinedis not actually a value but a response indicating that there was no value assigned in the first place.You can explicitly assign undefined to a variable though.
Another fun fact about JavaScript is that
undefinednever used to be a keyword. If you didvar foo = undefined,foowould indeed have a value ofundefined, but it was only because there was no variable calledundefinedin scope!You could do
var undefined = 42thenvar foo = undefinedwould actually setfooto42!window.undefined = 42would break all sorts of things.Thankfully this was fixed with ES5 in 2009, although it took a few years for browsers to make the change.
javascript moment