Very simply I am seeing the following syntax using coffeescript in node, that I never saw using coffeescript in the browser.
{foo} = app.locals.foo
I headed over to js2coffee to see what this does, but it appears to be identical in js output to
foo = app.locals.foo
So what is going on here? What does the {} syntax mean in this context? Why would you use it?
From the fine manual:
Destructuring Assignment
To make extracting values from complex arrays and objects more convenient, CoffeeScript implements ECMAScript Harmony's proposed destructuring assignment syntax. When you assign an array or object literal to a value, CoffeeScript breaks up and matches both sides against each other, assigning the values on the right to the variables on the left.
[...]
Destructuring assignment can be used with any depth of array and object nesting, to help pull out deeply nested properties.
The relevant example is this one:
futurists =
sculptor: "Umberto Boccioni"
painter: "Vladimir Burliuk"
poet:
name: "F.T. Marinetti"
address: [
"Via Roma 42R"
"Bellagio, Italy 22021"
]
{poet: {name, address: [street, city]}} = futurists
That is short hand for this:
name = futurists.poet.name
street = futurists.poet.address[0]
city = futurists.poet.address[1]
and you can see it in action over here.
Basically, the object form of destructured assignment allows you to unpack objects in a somewhat natural way. Perhaps a simpler example would help:
o =
a: 'b'
c: 'd'
e: 'f'
{a, e} = o
That's shorthand for:
a = o.a
e = o.e
And another demo.
You can think of the left hand side of a destructured assignment as a pattern that is used to unpack the right hand side.
