I am trying to write a helper method to find the intersection of two hashes/Objects like this...
Object::intersect = (obj)->
t = {}
t[k] = @[k] for k of obj
t
x = { a: 1, b: 2, c: 3 }
w = { a: true, b: 3 }
x.intersect(w) #=> { a: 1, b: 2, intersect: [Function] }
the results are as I expected/wanted EXCEPT for the addition of 'intersect' key/value.
What am I missing here?
Your property is enumerable, that's why it appears in both objects when you iterate.
A solution would be this :
Object.defineProperty Object.prototype, 'intersect', value:(obj)->
t = {}
t[k] = @[k] for k of obj
t
Demonstration (compiled as JS)
But modifying objects you don't own, especially Object is terrible practice and can lead to many unexpected problems for other coders. Read Maintainable JavaScript: Don’t modify objects you don’t own.
Here it would be as easy and so much cleaner to simply define a function taking two objects as arguments...
intersect = (a,b)-> t={};(t[k] = a[k] for k of b);t I never would have raised the question. sigh. Many thanks!
defineProperty. developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…prototype= intersect is both enumerable and writeable (using the defineProperty options).Adding the 'ownProperty' check removes this intersect property
Object::intersect= (obj) ->
t = {}
t[k] = @[k] for own k of obj
t
coffee> x = {a:1, b:2, c:3}; w = {a:true, b:3}
{ a: true, b: 3 }
coffee> x.intersect(w)
{ a: 1, b: 2 }
If w has keys that are not in x, they will appear in the result with an undefined value.
A more robust function could be constructed from underscore
_ = require 'underscore'
_.pick(x, _.keys(w))
w = { a: true, b: 3, d: 4}, thend: undefinedappears in the result.