Question

I would like to make a generic function wrapper that (for example) prints the called function and its arguments.

Doing so is easy through the arguments quasi-array and simple calls. For example:

function wrap(target, method) {
    return function() {
        console.log(Array.prototype.slice.call(arguments).join(', '));
        return method.apply(target, arguments);
    }
}

However, this way of doing of course completely loses the arity of the called function (if you didn’t know, one can obtain the arity (number of arguments) of a JavaScript function through its length property).

Is there any way to dynamically create a wrapper function that would copy the arguments of the wrapped function to itself?

---

I’ve thought about creating a new Function object, but I don’t see any way to statically extract the arguments list, since the arguments property is deprecated.

Answer 1

Here’s a solution using Function:

// could also generate arg0, arg1, arg2, ... or use the same name for each arg
var argNames = 'abcdefghijklmnopqrstuvwxyz';
var makeArgs = function(n) { return [].slice.call(argNames, 0, n).join(','); };

function wrap(target, method) {
    // We can't have a closure, so we shove all our data in one object
    var data = {
        method: method,
        target: target
    }

    // Build our function with the generated arg list, using `this.`
    // to access "closures"
    f = new Function(makeArgs(method.length),
        "console.log(Array.prototype.slice.call(arguments).join(', '));" +
        "return this.method.apply(this.target, arguments);"
    );
    // and bind `this` to refer to `data` within the function
    return f.bind(data);
}

EDIT:

Here’s a more abstract solution, which fixes the closure problem:

function giveArity(f, n) {
    return new Function(makeArgs(n),
        "return this.apply(null, arguments);"
    ).bind(f);
}

And a better one, that preserves context when invoked:

function giveArity(f, n) {
    return eval('(function('+makeArgs(n)+') { return f.apply(this, arguments); })')
}

Used as:

function wrap(target, method) {
    return giveArity(function() {
        console.log(Array.prototype.slice.call(arguments).join(', '));
        return method.apply(target, arguments);
    }, method.length)
}