Andreas Rossberg wrote:
On 25 June 2012 17:17, David Herman<[email protected]> wrote:
Implicit coercions suck, and IMO it would be better if destructuring didn't add
new ones to JS. In the current draft spec, I believe if you match an object
destructuring pattern against a non-object or an object that doesn't have one
or more of the properties, it quietly proceeds -- masking what is quite likely
an error:
var { foo, bar } = getFooAndBar();
If getFooAndBar() doesn't produce an object with foo and bar properties,
there's probably something going wrong! And most of the time when you do want
to be forgiving (say, in a function signature, or when reading a configuration
file), you have a specific default value you want to provide, which can be
specified with the default syntax:
var { foo = defaultFooValue, bar = defaultBarValue } = getFooAndBar();
I'd like to propose the following changes:
(a) Throw when matching null, undefined, booleans, or strings against an object
pattern. (I don't propose throwing when matching a string against an array
pattern, however, since strings behave structurally like read-only arrays.)
(b) Allow a shorthand "?" prefix for properties in an object destructuring
pattern. This is simply shorthand for defaulting to undefined. IOW, the following are all
equivalent:
var { ?foo } = getFoo();
var { foo = undefined } = getFoo();
var { foo: foo = undefined } = getFoo();
var { ?foo: foo } = getFoo();
Note, however, that (b) is not strictly necessary to get the fail-soft
behavior. You can always write the explicit default. But it makes it
considerably more convenient to get the fail-soft behavior when you want it --
only one character more expensive than fail-fast.
+10^20, although you don't say that "let {foo} = {}" should throw as
well -- but your (b) doesn't seem to be motivated without proposing
that.
Dave did write "If getFooAndBar() doesn't produce an object with foo and
bar properties, there's probably something going wrong!"
The test would be "in" not "own", to be clear, so if
Object.prototype.foo = 42 had been done previously, let {foo} = {} would
destructure 42 as the initial value of the foo let binding.
I'm ok with this even though it is not compatible with what we've
implemented and user-tested for almost six years, but I want (b) as part
of the deal. We have code that will use ?-prefixing.
/be
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