On Sun, Jan 1, 2012 at 5:12 AM, David Bruant <[email protected]> wrote:
> Le 01/01/2012 06:12, John J Barton a écrit : > > On Sat, Dec 31, 2011 at 7:53 PM, Axel Rauschmayer <[email protected]>wrote: > > >> >> 1. Checking whether a variable has been declared. >> Problematic: verbose and conflated with checking for a declared >> variable having the value `undefined`. >> Better: a dedicated operator or predicate for performing this check. >> > > Sorry, I don't think anyone checks if a is variable declared. You just > look at the source code. > > I have read code where people checked variable existence in global code. > Typeof prevents a ReferenceError if the variable is not declared. > Having been used to do it in global code (where you can't always "read the > code"), or just reading it somewhere, people reproduce within a function. > Maybe I am confused. Here is what I was imagining: var w = 1; // w is declared and defined. typeof gives 'number' var x; // x is declared, but is undefined. typeof gives 'undefined' y = 4; // y is not declared, is a global. typeof gives 'number' // z is not declared typeof gives 'undefined. So how can I check to see that y and z are not declared? In: var b; if (typeof x !== 'undefined') b = x I don't get a ReferenceError. What I am I missing? > > I agree that there are a lot of bad practices in here, but some people do > do it. > Moreover, the concept of static scoping and "just look at the code to see > if a variable is declared" is not that obvious for newcomers to the > language or newcomers to programming. > I guess newcomers won't understand typeof either ;-). jjb > > > David >
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