On May 31, 2014, at 8:59 PM, Nicholas C. Zakas <[email protected]> wrote:
> I've been playing around with using destructuring as function arguments and > have come across some odd behaviors that I'm not sure are intentional (or > perhaps, not to spec). Argument binding initialization takes place as part of http://people.mozilla.org/~jorendorff/es6-draft.html#sec-functiondeclarationinstantiation The actual initialization takes place in steps 24-25, essentially treating the entire parameter list as an array-like restructuring Note that there has been significant tweaks to this section in each of the 3 latest spec. drafts. You favorite implementation may not match the current spec. text. > For context, consider the following function: > > ``` > function setCookie(name, value, { secure, path, domain, expires }) { > console.log(secure); > // ... > } > > // called as > setCookie("type", "js", { > secure: true > }); > ``` > > What happens here: > * `secure === true` > * `path === undefined` > * `domain === undefined` > * `expires === undefined` > > I'd say all of that behavior is as expected. However, if I omit the third > argument completely, things get a bit strange: > > ``` > setCookie("type", "js"); // throws error at console.log > ``` > correct, this should throw a TypeError exception because desctructuring patterns (in any context) require an object value to restructure. Since you did not pass a third argument there is no object to restructure into {secure, path, domain, expires} > What happens here is that none of `secure`, `path`, `domain`, `expires` are > defined. I can use `typeof` on them to protect against this, but then I end > up with some lousy looking code: not really, the thrown exception while processing the arguments should terminate the function and you should never start executing the function body > > ``` > function setCookie(name, value, { secure, path, domain, expires }) { > > if (typeof secure !== "undefined") { > // use it > } > > if (typeof path !== "undefined") { > // use it > } > > if (typeof domain !== "undefined") { > // use it > } > > if (typeof expires !== "undefined") { > // use it > } > > // ... > } > ``` > > My first thought was that this behavior made sense, since no destructuring > can happen on undefined. However, the workaround for dealing with that > behavior seems a bit heavy-handed. > > I thought perhaps I could assign a default value, and that would solve the > problem: > > ``` > function setCookie(name, value, { secure, path, domain, expires } = {}) { > console.log(secure); > // … yes the above is exactly what you are expected to do. Or you could include some defaults in the default object: {secure, path, domain, expires } = {secure: false} > } > ``` > > Unfortunately, that resulted in a syntax error in Firefox. Traceur seems to > have no problem with it. that sounds like a Firefox bug > > So I really have two questions: > > 1. Who is right about assigning a default value to a destructured parameter, > Firefox or Traceur? Traceur > 2. Is the behavior of not having any bindings for destructured parameter > properties correct? And if so, is it desirable? no, it is supposed to throw as discussed above. Allen > > Thanks. > > -- > ___________________________ > Nicholas C. Zakas > http://www.nczonline.net > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss >
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