On the console, when you enter
blahblah and hit enter, you expectedly get the Unrecognized ReferenceError.
but...
When you enter this.blahblah instead, you get undefined ( meaning no errors )
Why is that?
When did blahblah find a space for itself in the memory to have that special value (undefined) assigned to it? - It looks like, when I do this.blahblah, it has the effect of executingvar blahblah; on the fly.
Any comments?
In the case of this.blahblah you are referring to the property of the this object (the window in your case).
In JavaScript, when you try to get a non existing property of an object it will return an undefined type.
In the first case however, calling simply blahblah, you are trying to call an object with the reference called blahblah. If that does not exist, JavaScript will throw a ReferenceError instead.
JavaScript has many ways of telling you something is not there.
this.blahblah is just undefined.
blahblah is not "calling" any objectYou can do the same when you write to console
obj
Uncaught ReferenceError: obj is not defined(…)
and
var obj = {a: 1}
obj.b
undefined
with this - you are refering to global object
The first one is undeclared, the second one undefined. Since this refers to the window in the browser, the interpreter tries to find a property blahblah of it and fails, so returns undefined.
You can verify that no variables are automatically created by trying to find the property of an undefined variable, such as bluh.blahblah. You'll get an undeclared error again because bluh won't be found in any scope.
Accessing a member of an object that doesn't exist simply returns undefined on reading because the language was "designed" that way.
If instead you assign a value to it then you're creating the member.
When using an identifier that is not a local the language accesses the global window object, but first checks if the member actually exists and gives an error otherwise. This is weird but just accept it... it's this way because it's this way. There's no solid logic reason.
If you think this is questionable then be prepared for much much worse. Logic is not a really important part of the Javascript language. You should remember that Javascript wasn't indeed given any serious thought at design time (basically it was created overnight) and, as a language, is close to hideous.
The internet big bang however solidified it in its current sad state and it's not something that will be "fixed" in incompatible ways as changing it would simply break the whole world wide web. Normally "standards" are defined after a lot of thinking and experimentation but this didn't happen for Javascript and what was standardized it's that overnight hack.
Just enjoy the really fantastic runtime environment (HTML5 browsers, node js) and pay a lot of attention when writing your code.
PS: Don't even use or look the specs about what == does in Javascript. It's just nonsense.
Accessing a member of an object that doesn't exist simply returns undefined on reading because the language was "designed" that way.[1]==1, 1=="1 " but [1]!="1 "... i.e. equality is not transitive... and this without using any "strange" value like undefined, null, NaN).== as opposed to using ===. If we ,for comparison purposes, were to stick to using === (or the !== for that matter ) ALL THE TIME , then the results would be much less surprising. It would be predictable to say the least. Most of us use == instead of === though. That's a big mistake. I understand your frustration but no language is perfect anyway and certainly JS is no exception. But it does support functions being first class citizens and as well as closures.. And the node.js is no joke. That's my opinion.== is the most evident (i.e. the "default" comparison operator is basically unusable and modern IDEs even warn you on every single use as a potential mistake). Another pearl is with that is totally broken by "design". Javascript is however powerful and the runtime environments and the implementations are amazing (thanks to the billions of dollars that were thrown at it). As a formal language however it sucks big time for many many reasons. Today is one of my preferred tools.