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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