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javascriptcoffeescriptdestructuring

Can destructuring assignment be used to effect a projection in CoffeeScript?

I'm having some trouble understanding destructuring assignment in CoffeeScript. The documentation contains a couple of examples which together seem to imply that renaming objects during assignment can be used to project (i.e. map, translate, transform) a source object.

I am trying to project a = [ { Id: 1, Name: 'Foo' }, { Id: 2, Name: 'Bar' } ] into b = [ { x: 1 }, { x: 2 } ]. I've tried the following without success; I've clearly misunderstood something. Can anyone explain whether this is possible?

My Poor Attempts That Don't Return [ { x: 1 }, { x: 2 } ]

a = [ { Id: 1, Name: 'Foo' }, { Id: 2, Name: 'Bar' } ]

# Huh? This returns 1.
x = [ { Id } ] = a

# Boo! This returns [ { Id: 1, Name: 'Foo' }, { Id: 2, Name: 'Bar' } ] 
y = [ { x: Id } ] = a

# Boo! This returns [ { Id: 1, Name: 'Foo' }, { Id: 2, Name: 'Bar' } ]
z = [ { Id: x } ] = a

CoffeeScript's Parallel Assignment Example

theBait   = 1000
theSwitch = 0

[theBait, theSwitch] = [theSwitch, theBait]

I understand this example as implying that variables can be renamed which in this case is used to perform a swap.

CoffeeScript's Arbitrary Nesting Example

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

I understand this example as defining a selection of properties from an arbitrary object which includes assigning the elements of an array to variables.

Update: Using thejh's Solution to Flatten an Array of Nested Objects

a = [ 
  { Id: 0, Name: { First: 'George', Last: 'Clinton' } },
  { Id: 1, Name: { First: 'Bill', Last: 'Bush' } },
]

# The old way I was doing it.
old_way = _.map a, x ->
    { Id: id, Name: { First: first, Last: last } } = x
    { id, first, last }

# Using thejh's solution...
new_way = ({id, first, last} for {Id: id, Name: {First: first, Last: last}} in a)

console.log new_way
Solution

  • b = ({x} for {Id: x} in a) works:

    coffee> a = [ { Id: 1, Name: 'Foo' }, { Id: 2, Name: 'Bar' } ]
[ { Id: 1, Name: 'Foo' },
  { Id: 2, Name: 'Bar' } ]
coffee> b = ({x} for {Id: x} in a)
[ { x: 1 }, { x: 2 } ]
coffee>