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.
Dave
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