Mutable `length` for functions?

[Claude Pache]

An alternative to a writable 'length' property, is to make it configurable and nonwritable. It would prevent the issue of accidental assignments to the 'length' property that used be ignored in non-strict mode (they'll still be ignored), but it would allow to modify its value using Object.defineProperty.

—Claude

Le 25 févr. 2013 à 07:27, Nathan Wall <nathan.wall at live.com> a écrit :

> Would it be possible to make / what are the thoughts on making `length` mutable on functions?
> 
> Writing to `length` could be a useful functionality for library code. For instance, implementing something like `bind` correctly requires the arity of the bound function to be the same as the original function minus the number of preloaded arguments.
> 
> Simplified example:
> 
>     var slice = Function.prototype.call.bind(Array.prototype.slice);
>     function bind(f, thisArg, ...args) {
>         function bound() {
>             return f.apply(thisArg, args.concat(slice(arguments)));
>         }
>         var L = f.length - args.length;
>         bound.length = L > 0 ? L : 0;
>         return bound;
>     }
> 
> Of course, `bind` is already on Function.prototype, so an ES6 library has no need to implement it. I'm only using bind as an example to establish precedent that writable `length` could be useful in implementing function wrappers.  Consider as a more necessary example implementing an `uncurryThis` function.
> 
>     var uncurryThis = Function.prototype.bind.bind(Function.prototype.call);
>     function foo(a, b, c) { /* ... */ }
>     var uFoo = uncurryThis(foo);
>     console.log(
>         foo.length, // => 3
>         uFoo.length // => 1
>     );
> 
> This is problematic because we lose information about the arity of the `uFoo` function which actually takes 4 arguments now: A `this` argument, `a`, `b`, and `c`.  A simple solution would be to write an uncurryThis which corrects the arity:
> 
>     var bind = Function.prototype.call.bind(Function.prototype.bind),
>         callMethod = Function.prototype.call;
>     function uncurryThis(fn) {
>         var F = bind(callMethod, fn);
>         // Correct arity.
>         F.length = fn.length + 1;
>         return F;
>     }
>     function foo(a, b, c) { /* ... */ }
>     var uFoo = uncurryThis(foo);
>     console.log(
>         foo.length, // => 3
>         uFoo.length // => 4
>     );
> 
> Currently I have resorted to an `eval` based solution for creating wrapper functions which are defined with the correct number of arguments when a certain arity is desired. Obviously this is not ideal.  Writable `length` would help a lot.
> 
> Thoughts?
> 
> Nathan 		 	   		  
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