On Fri, Jan 7, 2011 at 10:46 AM, Mathias Bynens <math...@qiwi.be> wrote:
> > > Let’s assume you’re working in the global scope already and you want > to create a new global variable. What do you guys recommend? Should > the `var` keyword be used (first example) even though it’s not > necessary in this case? Are there any other implications I should be > aware of? What is considered to be the best practice here? > > > While answers above have already clarified the case (yes, a variable in JS is declared with using a `var` keyword), I add some notes below. First, if to nit-pick to technical part, strictly speaking, a variable is defined only with using `var` keyword (there is a special section in my old article: http://dmitrysoshnikov.com/ecmascript/chapter-2-variable-object/#about-variables ). However, if the technical part is not so essential, you may use an assignment to an undeclared identifier (your case with foo = 42) for creating a property of the global object. The only thing you need in ES3 for that is to watch attentively and manage carefully all your variables (to avoid collisions with other variables). In strict mode of ES5 though, you won't be able to use `foo = 42` case, since such assignments are banned and cause a `ReferenceError`. Info: http://dmitrysoshnikov.com/ecmascript/es5-chapter-2-strict-mode/#assignment-to-an-undeclared-identifier So if you need to define a global property in the some other context, use `global.foo = 42` (where `global` refers to the global object). Or via `Object.defineProperty(...)`. P.S.: The main thing is just in distinguishing local and non-local variables (there are no other ideological reasons). For example, from Python viewpoint, this JS's `var` is a syntactic noise. What for to repeat every time this `var` if I want to declare a variable in the local scope? Of course `one-var` pattern with using comma may help to manage the case and to decrease this noise. So this is all about which strategy to choose for distinguishing. Since Python declares vars without a special keyword, it needs vice-versa keyword then to define a non-local variable: # global var (which is # local in the global context) a = 10 def foo(): a = 20 # local print(a) # 20 print(a) # 10 def bar(): a = 30 # local def baz(): # if we need to use `a` # from the outer context we # we can't use just `a = 40` # since it will create a local `a` # so we use a special keyword nonlocal a = 40 b = 50 print(a + b) # 90 bar() # call the inner function print(a) # see that `a` of bar is changed Other languages also differently distinguish local and non-local vars. E.g. Lua uses a similar to JS notation -- assignment to an undeclared identifier creates always a global var. And is we need a local var, we use `local` keyword. a = 10 -- global function foo() b = 20 -- global local c = 30 -- local end CoffeScript also doesn't use any keyword for declaring a variable. But in Coffee it's not possible to create a local variable with the same name if already there is a global variable with the same name. a = 10 # global foo = () -> a = 20 # assignment to global `a` b = 30 # local Dmitry. -- To view archived discussions from the original JSMentors Mailman list: http://www.mail-archive.com/jsmentors@jsmentors.com/ To search via a non-Google archive, visit here: http://www.mail-archive.com/jsmentors@googlegroups.com/ To unsubscribe from this group, send email to jsmentors+unsubscr...@googlegroups.com
