在CoffeeScript中使用indexOf

Question

I'm using the following code in CoffeeScript:

if elem in my_array
  do_something()

Which compiles to this javascript:

if (__indexOf.call(my_array, elem) < 0) {
  my_array.push(elem);
}

I can see it's using the function __indexOf which is defined at the top of the script.

My question is regarding this use case: I want to remove an element from an array, and I want to support IE8. I can do that easily with indexOf and splice in browsers who support indexOf on an array object. However, in IE8 this doesn't work:

if (attr_index = my_array.indexOf(elem)) > -1
  my_array.splice(attr_index, 1)

I tried using the __indexOf function defined by CoffeScript but I get a reserved word error in the compiler.

if (attr_index = __indexOf.call(my_array, elem) > -1
  my_array.splice(attr_index, 1)

So how can I use CoffeScript or is there a more unobtrusive method for calling indexOf? It seems weird to define the same function twice, just because CoffeeScript won't let me use theirs...

Answer 1

No, CoffeeScript precludes you from using its helpers directly, since that would break down the distinction between the language and the implementation. To support IE8, I would add a shim like

Array::indexOf or= (item) ->
  for x, i in this
    return i if x is item
  return -1

or use a library like Underscore.js for array manipulation.

Answer 2

CoffeeScript adds the following to the top of file scope:

var __indexOf = [].indexOf || function(item) {
  for (var i = 0, l = this.length; i < l; i++) {
    if (i in this && this[i] === item) return i;
  }
  return -1;
};

If we attempted to utilise this by doing:

indexOf = __indexOf

This would produce a compiler error: RESERVED WORD "__INDEXOF"

The solution is to subvert the compiler using backticks:

indexOf = `__indexOf`

Then use it with

indexOf.call([1,2,3,4], 3) //2

Or we could reduce the duplicate code from @Trevor Burnham's answer:

Array::indexOf or= `__indexOf`

However, you need to be sure CoffeeScript will be adding this definition by making use of the in operator as boolean expression (and with a dynamic length array on the right hand side). At the end of the day, it might just be easier for some just to redefine it :)