Decorators and forwarding, call/apply

Domains: Javascript

JavaScript gives exceptional flexibility when dealing with functions. They can be passed around, used as objects, and now we'll see how to forward calls between them and decorate them.

Transparent caching

Let's say we have a function slow(x) which is CPU-heavy, but its results are stable. In other words, for the same x it always returns the same result.

If the function is called often, we may want to cache (remember) the results for different x to avoid spending extra-time on recalculations.

But instead of adding that functionality into slow() we'll create a wrapper. As we'll see, there are many benefits of doing so.

Here's the code, and explanations follow:

function slow(x) {
  // there can be a heavy CPU-intensive job here
  alert(`Called with ${x}`);
  return x;
}

function cachingDecorator(func) {
  let cache = new Map();

  return function(x) {
    if (cache.has(x)) { // if the result is in the map
      return cache.get(x); // return it
    }

    let result = func(x); // otherwise call func

    cache.set(x, result); // and cache (remember) the result
    return result;
  };
}

slow = cachingDecorator(slow);

alert( slow(1) ); // slow(1) is cached
alert( "Again: " + slow(1) ); // the same

alert( slow(2) ); // slow(2) is cached
alert( "Again: " + slow(2) ); // the same as the previous line

In the code above cachingDecorator is a decorator: a special function that takes another function and alters its behavior.

The idea is that we can call cachingDecorator for any function, and it will return the caching wrapper. That's great, because we can have many functions that could use such a feature, and all we need to do is to apply cachingDecorator to them.

By separating caching from the main function code we also keep the main code simpler.

Now let's get into details of how it works.

The result of cachingDecorator(func) is a "wrapper": function(x) that "wraps" the call of func(x) into caching logic:

As we can see, the wrapper returns the result of func(x) "as is". From an outside code, the wrapped slow function still does the same. It just got a caching aspect added to its behavior.

To summarize, there are several benefits of using a separate cachingDecorator instead of altering the code of slow itself:

The cachingDecorator is reusable. We can apply it to another function.
The caching logic is separate, it did not increase the complexity of slow itself (if there were any).
We can combine multiple decorators if needed (other decorators will follow).

Using "func.call" for the context

The caching decorator mentioned above is not suited to work with object methods.

For instance, in the code below worker.slow() stops working after the decoration:

// we'll make worker.slow caching
let worker = {
  someMethod() {
    return 1;
  },

  slow(x) {
    // actually, there can be a scary CPU-heavy task here  
    alert("Called with " + x);
    return x * this.someMethod(); // (*)
  }
};

// same code as before
function cachingDecorator(func) {
  let cache = new Map();
  return function(x) {
    if (cache.has(x)) {
      return cache.get(x);
    }
*!*
    let result = func(x); // (**)
*/!*
    cache.set(x, result);
    return result;
  };
}

alert( worker.slow(1) ); // the original method works

worker.slow = cachingDecorator(worker.slow); // now make it caching

*!*
alert( worker.slow(2) ); // Whoops! Error: Cannot read property 'someMethod' of undefined
*/!*

The error occurs in the line (*) that tries to access this.someMethod and fails. Can you see why?

The reason is that the wrapper calls the original function as func(x) in the line (**). And, when called like that, the function gets this = undefined.

We would observe a similar symptom if we tried to run:

let func = worker.slow;
func(2);

So, the wrapper passes the call to the original method, but without the context this. Hence the error.

Let's fix it.

There's a special built-in function method func.call(context, ...args) that allows to call a function explicitly setting this.

The syntax is:

func.call(context, arg1, arg2, ...)

It runs func providing the first argument as this, and the next as the arguments.

To put it simply, these two calls do almost the same:

func(1, 2, 3);
func.call(obj, 1, 2, 3)

They both call func with arguments 1, 2 and 3. The only difference is that func.call also sets this to obj.

As an example, in the code below we call sayHi in the context of different objects: sayHi.call(user) runs sayHi providing this=user, and the next line sets this=admin:

function sayHi() {
  alert(this.name);
}

let user = { name: "John" };
let admin = { name: "Admin" };

// use call to pass different objects as "this"
sayHi.call( user ); // this = John
sayHi.call( admin ); // this = Admin

And here we use call to call say with the given context and phrase:

function say(phrase) {
  alert(this.name + ': ' + phrase);
}

let user = { name: "John" };

// user becomes this, and "Hello" becomes the first argument
say.call( user, "Hello" ); // John: Hello

In our case, we can use call in the wrapper to pass the context to the original function:

let worker = {
  someMethod() {
    return 1;
  },

  slow(x) {
    alert("Called with " + x);
    return x * this.someMethod(); // (*)
  }
};

function cachingDecorator(func) {
  let cache = new Map();
  return function(x) {
    if (cache.has(x)) {
      return cache.get(x);
    }
*!*
    let result = func.call(this, x); // "this" is passed correctly now
*/!*
    cache.set(x, result);
    return result;
  };
}

worker.slow = cachingDecorator(worker.slow); // now make it caching

alert( worker.slow(2) ); // works
alert( worker.slow(2) ); // works, doesn't call the original (cached)

Now everything is fine.

To make it all clear, let's see more deeply how this is passed along:

After the decoration worker.slow is now the wrapper function (x) { ... }.
So when worker.slow(2) is executed, the wrapper gets 2 as an argument and this=worker (it's the object before dot).
Inside the wrapper, assuming the result is not yet cached, func.call(this, x) passes the current this (=worker) and the current argument (=2) to the original method.

Going multi-argument with "func.apply"

Now let's make cachingDecorator even more universal. Till now it was working only with single-argument functions.

Now how to cache the multi-argument worker.slow method?

let worker = {
  slow(min, max) {
    return min + max; // scary CPU-hogger is assumed
  }
};

// should remember same-argument calls
worker.slow = cachingDecorator(worker.slow);

We have two tasks to solve here.

First is how to use both arguments min and max for the key in cache map. Previously, for a single argument x we could just cache.set(x, result) to save the result and cache.get(x) to retrieve it. But now we need to remember the result for a combination of arguments (min,max). The native Map takes single value only as the key.

There are many solutions possible:

Implement a new (or use a third-party) map-like data structure that is more versatile and allows multi-keys.
Use nested maps: cache.set(min) will be a Map that stores the pair (max, result). So we can get result as cache.get(min).get(max).
Join two values into one. In our particular case we can just use a string "min,max" as the Map key. For flexibility, we can allow to provide a hashing function for the decorator, that knows how to make one value from many.

For many practical applications, the 3rd variant is good enough, so we'll stick to it.

The second task to solve is how to pass many arguments to func. Currently, the wrapper function(x) assumes a single argument, and func.call(this, x) passes it.

Here we can use another built-in method func.apply.

The syntax is:

func.apply(context, args)

It runs the func setting this=context and using an array-like object args as the list of arguments.

For instance, these two calls are almost the same:

func(1, 2, 3);
func.apply(context, [1, 2, 3])

Both run func giving it arguments 1,2,3. But apply also sets this=context.

For instance, here say is called with this=user and messageData as a list of arguments:

function say(time, phrase) {
  alert(`[${time}] ${this.name}: ${phrase}`);
}

let user = { name: "John" };

let messageData = ['10:00', 'Hello']; // become time and phrase

*!*
// user becomes this, messageData is passed as a list of arguments (time, phrase)
say.apply(user, messageData); // [10:00] John: Hello (this=user)
*/!*

The only syntax difference between call and apply is that call expects a list of arguments, while apply takes an array-like object with them.

We already know the spread operator ... from the chapter info:rest-parameters-spread-operator that can pass an array (or any iterable) as a list of arguments. So if we use it with call, we can achieve almost the same as apply.

These two calls are almost equivalent:

let args = [1, 2, 3];

*!*
func.call(context, ...args); // pass an array as list with spread operator
func.apply(context, args);   // is same as using apply
*/!*

If we look more closely, there's a minor difference between such uses of call and apply.

The spread operator ... allows to pass iterable args as the list to call.
The apply accepts only array-like args.

So, these calls complement each other. Where we expect an iterable, call works, where we expect an array-like, apply works.

And if args is both iterable and array-like, like a real array, then we technically could use any of them, but apply will probably be faster, because it's a single operation. Most JavaScript engines internally optimize is better than a pair call + spread.

One of the most important uses of apply is passing the call to another function, like this:

let wrapper = function() {
  return anotherFunction.apply(this, arguments);
};

That's called call forwarding. The wrapper passes everything it gets: the context this and arguments to anotherFunction and returns back its result.

When an external code calls such wrapper, it is indistinguishable from the call of the original function.

Now let's bake it all into the more powerful cachingDecorator:

let worker = {
  slow(min, max) {
    alert(`Called with ${min},${max}`);
    return min + max;
  }
};

function cachingDecorator(func, hash) {
  let cache = new Map();
  return function() {
*!*
    let key = hash(arguments); // (*)
*/!*
    if (cache.has(key)) {
      return cache.get(key);
    }

*!*
    let result = func.apply(this, arguments); // (**)
*/!*

    cache.set(key, result);
    return result;
  };
}

function hash(args) {
  return args[0] + ',' + args[1];
}

worker.slow = cachingDecorator(worker.slow, hash);

alert( worker.slow(3, 5) ); // works
alert( "Again " + worker.slow(3, 5) ); // same (cached)

Now the wrapper operates with any number of arguments.

There are two changes:

In the line (*) it calls hash to create a single key from arguments. Here we use a simple "joining" function that turns arguments (3, 5) into the key "3,5". More complex cases may require other hashing functions.
Then (**) uses func.apply to pass both the context and all arguments the wrapper got (no matter how many) to the original function.

Borrowing a method

Now let's make one more minor improvement in the hashing function:

function hash(args) {
  return args[0] + ',' + args[1];
}

As of now, it works only on two arguments. It would be better if it could glue any number of args.

The natural solution would be to use arr.join method:

function hash(args) {
  return args.join();
}

...Unfortunately, that won't work. Because we are calling hash(arguments) and arguments object is both iterable and array-like, but not a real array.

So calling join on it would fail, as we can see below:

function hash() {
*!*
  alert( arguments.join() ); // Error: arguments.join is not a function
*/!*
}

hash(1, 2);

Still, there's an easy way to use array join:

function hash() {
*!*
  alert( [].join.call(arguments) ); // 1,2
*/!*
}

hash(1, 2);

The trick is called method borrowing.

We take (borrow) a join method from a regular array [].join. And use [].join.call to run it in the context of arguments.

Why does it work?

That's because the internal algorithm of the native method arr.join(glue) is very simple.

Taken from the specification almost "as-is":

Let glue be the first argument or, if no arguments, then a comma ",".
Let result be an empty string.
Append this[0] to result.
Append glue and this[1].
Append glue and this[2].
...Do so until this.length items are glued.
Return result.

So, technically it takes this and joins this[0], this[1] ...etc together. It's intentionally written in a way that allows any array-like this (not a coincidence, many methods follow this practice). That's why it also works with this=arguments.

Summary

Decorator is a wrapper around a function that alters its behavior. The main job is still carried out by the function.

It is generally safe to replace a function or a method with a decorated one, except for one little thing. If the original function had properties on it, like func.calledCount or whatever, then the decorated one will not provide them. Because that is a wrapper. So one needs to be careful if one uses them. Some decorators provide their own properties.

Decorators can be seen as "features" or "aspects" that can be added to a function. We can add one or add many. And all this without changing its code!

To implement cachingDecorator, we studied methods:

func.call(context, arg1, arg2...) -- calls func with given context and arguments.
func.apply(context, args) -- calls func passing context as this and array-like args into a list of arguments.

The generic call forwarding is usually done with apply:

let wrapper = function() {
  return original.apply(this, arguments);
}

We also saw an example of method borrowing when we take a method from an object and call it in the context of another object. It is quite common to take array methods and apply them to arguments. The alternative is to use rest parameters object that is a real array.

There are many decorators there in the wild. Check how well you got them by solving the tasks of this chapter.