Question

The noisy function defined earlier, which wraps its argument in another function, has a rather serious deficit.

function noisy(f) {
  return function(arg) {
    console.log("calling with", arg);
    var val = f(arg);
    console.log("called with", arg, "- got", val);
    return val;
  };
}

If f takes more than one parameter, it gets only the first one. We could add a bunch of arguments to the inner function ( arg1 , arg2 , and so on) and pass them all to f , but it is not clear how many would be enough. This solution would also deprive f of the information in arguments.length . Since we’d always pass the same number of arguments, it wouldn’t know how many arguments were originally given.

For these kinds of situations, JavaScript functions have an apply method. You pass it an array (or array-like object) of arguments, and it will call the function with those arguments.

function transparentWrapping(f) {
  return function() {
    return f.apply(null, arguments);
  };
}

That’s a useless function, but it shows the pattern we are interested in—the function it returns passes all of the given arguments, and only those arguments, to f . It does this by passing its own arguments object to apply . The first argument to apply , for which we are passing null here, can be used to simulate a method call. We will come back to that in the next chapter .

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.