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.
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