How to programmatically set the length of a function

Question

The length property of functions tells how long the 'expected' argument list is:

console.log((function () {}).length);  /* 0 */
console.log((function (a) {}).length); /* 1 */
console.log((function (a, b) {}).length); /* 2 etc. */

However, it is a readonly method:

f = function (a) {};
alert(f.length); // 1
f.length = 3;
alert(f.length); // 1

Is there a way to programmatically set that length? The closest I've come so far is to use the Function constructor:

f = new Function("a,b,c", "/* function body here */");
f.length; // 3

However, using Function is essentially the same as eval and we all know how bad that is. What other options do I have here?

Answer 1

It turns out the length property on functions is configurable, which means you can use .defineProperty to change the value of length on a function. Example:

function hi() {}
hi.length === 0; // Expected

Object.defineProperty(hi, "length", { value: 5 })
hi.length === 5; // Intriguing

This works in the latest version of Chrome and Firefox, but it does not work in Safari (v9.1.1).

Answer 2

According to the ECMA Script standard, revision 5.1 on page 103, the .length parameter on a Function object is not writable so it is set when the function is declared and not changable (if implemented per spec).

Thus, the only way to create a function with a particular .length upon demand is to either have a bunch of functions lying around of various length (as nickf suggests), create a Function object (as you've already mentioned) or use eval() with a dynamically created string. I don't know what problem yu're actually trying to solve, but I personally find nothing wrong with using eval() if you know the source of the code you're using it with and have a way of either checking it or knowing what it will or won't have in it. In this case, programmatically generating a certain number of parameters in a string before calling eval() on it poses no security risk I'm aware of.

If this is all your own code, you can just create a new property on the function .dynLength that is mutable and set it to whatever you want and have your code use that.