double-splat

In python I can pass a dict whose keys match parameters' names with the ** (double-splat) operator: def foo(a, b): print (a - b) args = {'b': 7, 'a': 10} foo(**args) # prints 3 How to do the same in ES6? This doesn't work: function foo(a, b) { console.log(a - b) } args = {b: 7, a: 10} foo(...args) NB: I'm looking for a solution that wouldn't involve changing the signature of foo because I want it to be used either way (with and without destructuring). So the following should work: foo(<magic>args); foo(123, 456); Bonus question: why is the error message "undefined is not a function"? What

What is happening below seems a little strange to me. def f(a, b) puts "#{a} :: #{b}" end f(*[1, 2], **{}) # prints "1 :: 2" hash = {} f(*[1, 2], **hash) ArgumentError: wrong number of arguments (3 for 2) f(*[1, 2], **Hash.new) ArgumentError: wrong number of arguments (3 for 2) Is this a compiler optimization feature? That is a Ruby's bug that has been reported several times (for example here by me) but has not been fixed. I guess that since the keyword argument feature has been introduced, the double splat syntax has become murky, and that is the indirect cause of this bug. I heard that Matz

I noticed what I find to be a very surprising behavior with the ** (double-splat) operator in Ruby 2.1.1. When key-value pairs are used before a **hash , the hash remains unmodified; however, when key-value pairs are only used after the **hash , the hash is permanently modified. h = { b: 2 } { a: 1, **h } # => { a: 1, b: 2 } h # => { b: 2 } { a: 1, **h, c: 3 } # => { a: 1, b: 2, c: 3 } h # => { b: 2 } { **h, c: 3 } # => { b: 2, c: 3 } h # => { b: 2, c: 3 } For comparison, consider the behavior of the single- * operator on arrays: a = [2] [1, *a] # => [1, 2] a # => [2] [1, *a, 3] # => [1, 2, 3]