Function.prototype.bind CoffeeScript polyfill

gistfile1.coffee

# https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind#Compatibility
Function::bind ?= (oThis, args...) ->
    fNOP = ->
    fNOP:: ?= @::

    fBound = =>
        context = if this instanceof fNOP and oThis then this else oThis
        @apply context, args.concat arguments

    fBound:: = new fNOP()
    fBound

Worth noting that the original polyfill assigns fNOP.prototype to this.prototype unconditionally, but when this.prototype is actually undefined, then you break instanceof. This is an edge-case that you can run into when you are running bind on a native method, such as Function.prototype.apply.bind.

So, a simple ?= fixes that issue. Shouldn't assign it a prototype that doesn't exist.

Whipped this up to better understand the issue. Here's a comparison of using the faulty MDN polyfill compared to native bind to show that the two are not equivalent.

Function::customBind = (oThis, args...) ->
    fNOP = ->
    fNOP:: = @::

    fBound = =>
        context = if this instanceof fNOP and oThis then this else oThis
        @apply context, args.concat arguments

    fBound:: = new fNOP()
    fBound

a = Function::apply.bind (class noop)
b = Function::apply.customBind (class noop)

# Test a()
try
  a()
  console.log 'a: OK'
catch error
  console.log "a: #{error.message}"

# Test b()
try
  b()
  console.log 'b: OK'
catch error
  console.log "b: #{error.message}"

Output

a: OK
b: Function has non-object prototype 'undefined' in instanceof check