Sorry, ignore the line noise I just remembered that we decided to treat undefined as missing.
I do recall actually making the argument w.r.t defaults, that treating undefined as “not provided” would cause us difficulty if anyone ever wanted to distinguish undefined from not provided. —Oliver On Nov 10, 2013, at 12:52 PM, Oliver Hunt <[email protected]> wrote: > I don’t fully understand this — what is the reason for needing to know actual > argument count? The historical reason was that if i have a var arg function > with a few required parameters i don’t have any nice way of iterating the > optional args. > > Default and rest parameters seem to solve this use case. > > —Oliver > > On Nov 10, 2013, at 10:12 AM, Allen Wirfs-Brock <[email protected]> wrote: > >> One of the the few remaining uses of a function's 'arguments' binding is to >> determine the actual number of passed arguments. This is necessary in some >> overloading scenarios where a function has different behavior when an >> argument is completely absent then it has when undefined (or any other >> default value) is explicitly passed in that parameter position. That >> situation occurs in a number of DOM APIs and even a few ES library functions. >> >> For example(see https://bugs.ecmascript.org/show_bug.cgi?id=1877 ), >> Array.prototype.splice returns different results for: >> [1,2,3].splice() >> and >> [1,2,3].splice(undefined) >> >> The natural ES6 declaration for a splice function is: >> >> function splice(start, deleteCount, ...items) {... >> >> but if you write it this way then within the body you have to have a test >> like: >> >> if (arguments.length == 0) {... >> >> to implement the correct web-compatable behavior. >> >> Or, alternatively you could declare the functions as: >> >> function splice(...actualArgs) { >> let [start, stop, ...item] = actualArgs; >> ... >> if (actualArgs.length == 0) {... >> >> So, to implement a Web-compaable version of splice you either have to use >> 'arguments' to determine the actual number of passed objects or you need to >> declare it with a bogus parameter pattern and use explicit or implicit >> destructuring to parse out the positional parameters. >> >> One way around this dilemma would be to provide a syntactic affordance for >> determing the actual argument count. For example, one possibility would be >> to allow the last item of any formal parameter list to be an item of the >> syntactic form: >> >> ActualArgumentCount : '#' BindingIdentifier >> >> So, the declaration for splice could then be: >> >> function splice(start, deleteCount, ...items, #argCount) { >> ... >> if (argCount == 0) {... >> >> Thoughts? >> >> Allen >> >> >> _______________________________________________ >> es-discuss mailing list >> [email protected] >> https://mail.mozilla.org/listinfo/es-discuss > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss
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