why is second function parameter in comment block? just curious

Question

I found this code and I dont understand purpose of comment block in parameters:

if (!Array.prototype.some)
{
  Array.prototype.some = function(fun /*, thisp*/)
  {
    var len = this.length;
    if (typeof fun != "function")
      throw new TypeError();

    var thisp = arguments[1];
    for (var i = 0; i < len; i++)
    {
      if (i in this && fun.call(thisp, this[i], i, this))
        return true;
    }

    return false;
  };
}

In my opinion it could be normal second parameter and this row could be deleted then:

var thisp = arguments[1];

Dude, it's just a comment! it has no meaning, it's just there! who knows what he meant to do with that parameter? — gdoron, Apr 29 '12 at 20:01
It seems like this question might be better directed to the author of the code in question... [EDITED to add: oh look, others got there first and said the same. I'll leave this here as a reminder to myself to refresh before commenting :-).] — Gareth McCaughan, Apr 29 '12 at 20:03
@Gareth: I was under the impression that SO nowadays refreshes while you type ... — KooiInc, Apr 29 '12 at 20:09

score 1 · Answer 1 · answered Apr 29 '12 at 20:03

1

If Array.prototype.some is defined, the original method had a second parameter which is thisp, but since it is not defined, the author defines his/her own method that mimics the functionality.

The author expects that the caller passes the second parameter which is thisp. HTH.

answered Apr 29 '12 at 20:03

g13n

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Yes, but he could save one row of code avoiding this. Unreasonable behavior for me. :) – Luckylooke Apr 29 '12 at 20:07
@Luckylooke see this . – g13n Apr 29 '12 at 20:10

score 1 · Accepted Answer · answered Apr 29 '12 at 20:12

The implementation of Array.prototype.some built in to Firefox has an arity of 1, i.e. it accepts one argument. In order to implement the second optional argument without changing the arity the replacement code accesses the second argument via arguments[1] instead.

I don't actually know what the EcmaScript specification has to say about the arity of Array.prototype.some.

score 0 · Answer 3 · edited May 23 '17 at 11:49

you're right, it should be a parameter, because 1. using the functions arguments within a function is expensive and 2. it's hardwired into the function, although it can stay undefined in the actual loop. The method shouldn't be in production code, I'd say. Testing some more, it's not clear what the second parameter actually does anyway. The MDN shim is a strange duck, but maybe I've missed something.

why is second function parameter in comment block? just curious

3 Answers3