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

For now, here’s the best solution I could think of.

makeFunc = function (length, fn) {
    switch (length) {
    case 0 : return function () { return fn.apply(this, arguments); };
    case 1 : return function (a) { return fn.apply(this, arguments); };
    case 2 : return function (a,b) { return fn.apply(this, arguments); };
    case 3 : return function (a,b,c) { return fn.apply(this, arguments); };
    case 4 : return function (a,b,c,d) { return fn.apply(this, arguments); };
    case 5 : return function (a,b,c,d,e) { return fn.apply(this, arguments); };
    case 6 : return function (a,b,c,d,e,f) { return fn.apply(this, arguments); };
    case 7 : return function (a,b,c,d,e,f,g) { return fn.apply(this, arguments); };
    case 8 : return function (a,b,c,d,e,f,g,h) { return fn.apply(this, arguments); };
    case 9 : return function (a,b,c,d,e,f,g,h,i) { return fn.apply(this, arguments); };
    default : return function (a,b,c,d,e,f,g,h,i,j) { return fn.apply(this, arguments); };
    }
};

Example usage:

var realFn = function () {
    return "blah";
};

lengthSix = makeFunc(6, realFn);

lengthSix.length; // 6
lengthSix(); // "blah"

Personally, I always cringe whenever I use copy and paste when programming, so I’d be very happy to hear of any better options.

Update

I thought of a method which could work for any arbitrary size, unlike the example above which is limited by how many times you want to copy-and-paste. Essentially, it dynamically creates a function (using new Function) which will return a function of the right size which then just proxies through to whatever function you pass to it. Yeah that does hurt your head. Anyway, I thought I’d benchmark it against the above…

http://jsperf.com/functions-with-custom-length (you can see the ‘evil’ code there too).

The evil method is many hundreds of times slower than the hacky copypasta method, so there you go.