Fun Hooks

Installation

npm install fun-hooks

Motivation

Fun(ctional) Hooks is a generalized and simple hooking API for creating runtime extensible applications. This function-based approach operates on functions themselves rather than object methods to be fully compatible with purely functional code, but still maintains some convenience operations for dealing with object-oriented code.

The goals of this library are the following (in priority order):

1. Easy debugging
2. Performance
3. Simple but powerful API
4. Limited footprint

Compatibility for older environments

If you will be running fun-hooks in an environment that doesn't support the use of Proxy objects or you are using MooTools, Prototype.js, or some other old school js library that incorrectly patches Array and Function prototype methods, you should use the latest 0.9.x version of fun-hooks that includes the appropriate compatibility polyfills.

Usage

Hooks follow the same format whether they're sync or async and whether they're before or after hooks; however, it's important to remember that sync after hooks act on the return result and async after hooks act on the callback's arguments.

Configuration

• ready : number - (Optional, default: 0 (meaning no ready() call required)) See Ready.

import funHooks from "fun-hooks";    // es6 (using webpack or babel)
let funHooks = require("fun-hooks"); // or in node
let funHooks = window.funHooks;      // or directly in browser from somewhere like https://unpkg.com/fun-hooks@latest
let createHook = funHooks({
  ready: funHooks.ASYNC | funHooks.QUEUE
});

Sync (before, after)

function sum(a, b) {
  return a + b;
}

let hookedSum = createHook("sync", sum);

// sync `before` hooks accept the arguments to sum, and `next` passes the arguments to sum (or next `before` hook)
hookedSum.before(function(next, a, b) {
  a = a + 1;    // modify arguments or do some operation
  next(a, b);   // call next when you're done
});

// sync `after` hooks accepts the return result from sum, and `next` returns the result (or calls next `after` hook)
hookedSum.after(function(next, result) {
  next(result + 1);
});

let result = hookedSum(1, 1);
// hookedSum(1, 1) -> hookedSum.before(1, 1) -> sum(2, 1) -> hookedSum.after(3) -> 4

console.log(result); // 4

Note: You should always use sync if you are returning a value. This includes if you are returning a Promise.
Also, if you're hooking a function with sync your hooks should all call next synchronously (e.g. no ajax) so that your value can be returned. If you asynchronously call next in a sync hook then the return value will be undefined.

Async (before, after)

function increment(a, b, callback) {
  callback(a + 1, b + 1);
}

let hookedIncrement = createHook("async", increment);

// async `before` hooks accept the arguments to sum, and `next` passes the arguments to the next `before` hook or sum (same as sync)
hookedIncrement.before(function(next, a, b) {
  a = a + 1;
  next(a, b);
});

// async `after` hooks accept the arguments sum passed to callback, and `next` calls sum's actual callback (or next `after` hook)
hookedIncrement.after(function(next, a, b) {
  next(a, b + 1);
});

hookedIncrement(1, 1, function(a, b) {
  console.log(a, b); // 3, 3
})
// hookedIncrement(1, 1) -> hookedIncrement.before(1, 1) -> increment(2, 1) -> hookedIncrement.after(3, 2) -> callback(3, 3)

You'll notice no difference above in sync or async with the before hooks, but the after hooks are dealing with the callback's parameters in one case and the return result (which will always be a single value) in the other.

Removing hooks

Hooks can be removed using the remove method. You can either use a match object to remove a specific hook or pass nothing to remove all hooks.

function beforeHook() {
  console.log("called");
}

hookedSum.before(beforeHook);

hookedSum(1, 1); // "called"

hookedSum.getHooks({hook: beforeHook}).remove();

hookedSum(1, 1); // hook not called

hookedSum.before(beforeHook);
hookedSum.after(afterHook);
hookedSum.removeAll(); // remove all before and after hooks

Priority

You can attach as many before or after hooks as you'd like to a function. The order in which the hooks are ran is dependant on the order they're added or an optional priority argument set when creating the hook (which defaults to a priority of 10).

hookedSum.before(beforeHook1);
hookedSum.before(beforeHook2, 9);
hookedSum.before(beforeHook3, 11);
hookedSum.after(afterHook1, 8);
hookedSum.after(afterHook2);

hookedSum(1, 1); // hookedSum -> beforeHook3 -> beforeHook1 -> beforeHook2 -> sum -> afterHook2 -> afterHook1

Bailing

A hook can bail early to skip the other hooks or to skip the hooked function altogether (effectively stubbing it).

function bailHook(next, a, b) {
  next.bail(1, 1);
}

hookedIncrement.after(bailHook);
hookedIncrement.after(afterHook2);

// notice `afterHook2` is not called
hookedIncrement(1, 1, function callback(a, b) {}); // hookedIncrement -> increment -> bailHook -> callback(1, 1)

hookedIncrement.before(bailHook);

// notice not even the original `increment` function is called now
hookedIncrement(1, 1, function callback(a, b) {}); // hookedIncrement -> bailHook -> callback(1, 1)

If you want to bail completely (i.e. not even call the callback) then just don't call next.

Get Hooks

You can get all the hook entries attached to a hooked function using hookedFn.getHooks(). An optional argument can be passed for matching only specific kinds of hooks: e.g. hookedFn.getHooks({type: "before"}) or hookedFn.getHooks({hook: myBeforeHook}) to get a specific hook entry.

Side-effect (or pass-through) only hooks

If you want to have a hook that just performs some side-effect before or after the hooked function but does not modify arguments, just call next and pass-through the arguments without modifying them. It's important that next is still called with the original arguments so that the hook-chain can continue.

hookedIncrement.before(function sideEffect(next, ...args) {
  console.log("I'm a side-effect!");
  next.apply(this, args);
})

Naming

Hooks can be given a name and then accessed using the .get method. This can be useful for defining the extensible API for your application. _Note: You can also just expose references to the hooked functions themselves, this is just a convenience. Also, when using named hooks, you can reference the hook by name using .get and add before and after hooks before the hook itself has actually been created!

Note: For Typescript users, the .get function requires a type parameter defining the type of hook to expect. Type helpers are exported as SyncHook<T> and AsyncHook<T> where T is the hooked function signature. If you want Typescript to infer proper types then you should just expose references to the hooked function themselves.

// some-applicaiton
import hookFactory from "fun-hooks"; 
let hook = hookFactory(); // default configuration

function getItem(id, cb) {
  fetchItem(id).then(cb);
}

function getPrice(item) {
  return item.price;
}

// works, even though the "item" hook isn't defined until below!
hook.get("item").after(function(next, id) {
  console.log("accessing item: " + id);
  next(id);
});

hook("async", getItem, "item"); // naming this hook `item`
hook("sync", getPrice, "price"); // naming this hook `price`

export const getHook = hook.get;

// extending application
import { getHook } from "some-application";

getHook("item").before(function modifyId(next, id) {
  let newId = getUpdatedId(id); // `id` naming scheme changed... luckily we have this hook available!
  next(newId);
});

getHook("price").after(function currencyConversion(next, price) {
  let newPrice = convert(price, "USD");
  next(newPrice);
});

Objects

While functions are the base unit of extension in this library, there is a convenience provided to apply hooks to object methods if an object is passed to the hook creator. Note: this will be bound correctly in the hooked function as well as in the before and after hooks (i.e. this refers to the object instance inside hooks).

class Thing {
  constructor() {
    this.value = 1;
  }
  setValue(value) {
    this.value = value;
  }
  getValue() {
    return this.value;
  }
}
hook(Thing.prototype, ["setValue", "getValue"]);

Thing.prototype.getValue.after(function(next) {
  next(this.value + 2);
});

let myThing = new Thing();
myThing.setValue(1);

console.log(myThing.getValue()); // 3

Note: hook will also walk the prototype chain and find getValue if it were an inherited method.

If ["setValue", "getValue"] were omitted then hook would hook the results of Object.getOwnPropertyNames(Thing.prototype) excluding constructor and any methods marked private with a preceding underscore (e.g. _privateMethod() {}). Also, if the list of methods to hook is omitted, hook will no longer walk the prototype chain to avoid creating accidental hooks.

Hooked methods are all assumed to be sync unless otherwise specified.

hook(Thing.prototype, ["setValue", "sync:getValue" /* same as "getValue" */, "async:loadData"]);

If a third argument, name, is provided, then the object's hooked methods will be made accessible to the .get method described above using <Object Name>.<Method Name> in Naming.

hook(Thing.prototype, ["setValue", "getValue"], "thing");

// grab the collection of hooks
hook.get("thing"); // {thing: {setValue, getValue}}
// or grab an individual hook
hook.get("thing.setValue");

Ready

Fun hooks allows you to specify whether hooked functions should either throw an error or queue (for async hooks only) when they are called before being "ready". To utilize this feature, use the ready configuration option when setting up the hooking library. The ready API is turned off by default.

e.g.

import funHooks from "fun-hooks"; 
let hook = funHooks({
  // ready accepts a bit mask to determine ready behavior for sync and async hooks
  // SYNC will cause sync hooks to throw if called before ready
  // ASYNC will cause async hooks to throw if called before ready
  // ASYNC + QUEUE will cause async hooks to queue (rather than throw) if called before ready (and execute immediately 
  // when `ready()` is called)
  ready: funHooks.SYNC | funHooks.ASYNC | funHooks.QUEUE
});

function sum(a + b) {
  return a + b;
}
let hookedSum = hook(sum);

hookedSum(1, 2); // throws "not ready" error

function addTen(a, cb) {
  cb(a + 10);
}

let hookedAddTen = hook(addTen);

hookedAddTen(6, function(result) {
  console.log(result);
}); // this will queue the call and `addOne` will not be executed

function addOneHook(next, a) {
  next(a + 1);
}
hookedAddTen.before(addOneHook);
hookedSum.before(addOneHook)

// all hooks are ready now, queued `addTen` and its hooks are now called and 17 is printed to screen.
// notice the that the `addOneHook` for `hookedAddTen` was used even though it was added after `hookedAddTen` was called
hook.ready(); 

hookedSum(1, 2); // prints 4 since addOne hook was installed

Additional Information

Debugging

One of the hardest parts of using libraries that allow for hooking, intercepting, and/or adding some sort of middleware comes about when attempting to debug the code. These libraries usually require a lot of scaffolding that manages the hooks, caches results, changes some internal state, and calls the appropriate next hook in the sequence; this creates a debugging nightmare when you step into your wrapped function or your hook's next call and are now sifting through some library's scaffolding code helplessly trying to find your way back to your own code base.

Fun Hooks solves this problem by doing as much work as possible when the hook is created (rather than when it's invoked) dynamically generating a wrapper function that already has the hooks (and the original function) chained in the proper callback order. This means that the next function passed into each hook immediately invokes the next hook and not some scaffolding code.

example:

wrappedFn() => hook1() -> hook2() -> wrappedFn() -> hook3() -> hook4()
rather than
wrappedFn() => hook1() -> scaffolding -> hook2() -> scaffolding -> wrappedFn() ... etc

(Depending on the type and nature of the hook, some might have a one-line wrapper function to do something like extract a return result from a sync function)

Also, when wrapping a function for extension, Fun Hooks returns a Proxy object rather than a wrapper function. This allows the library to skip scaffolding code altogether if there are no hooks to apply, meaning if you step into your wrappedFn() invocation but no hooks have been attached, you'll step directly into your wrappedFn code.

Finally, when you're stepping through your hooks or wrapped function code, the previously executed hooks will be still be present in the stack since they are all invoked by the previous hook as opposed to iteratively looped through in scaffolding code and invoked individually.

All of the above creates a pleasant hooking environment that is much easier to debug.

Performance

Since most of the scaffolding code involved with function invocation has been avoided, this makes the execution of a function and all its hooks more efficient at the sake of a little more overhead when creating the hook. Considering hooked functions (and their hooks) are usually invoked a lot more frequently than hooks are added/removed, I think this is a decent trade-off.

Proxy forwarding is also much more performant than a function wrapper* (in a browser; in Node.js that currently doesn't seem to be the case, but will probably change as Proxies are further optimized).

Footprint

Since this hooking library is written for Browsers first the footprint and API is kept as slim as possible.

Code readability

Currently parts of this library are not very readable. Since much of the code is dynamically generated (using new Function) and is in strings it can't be properly minified by a minifier and is therefore written into the code pre-minified. This hurts readability but is necessary to remain as small as possible. Readability could probably still be improved somewhat but is considered low-priority compared to the goals stated above. There are extensive tests to ensure the code is as bug-free as possible.

next as first argument

It is a common convention in Javascript to pass callbacks as the last argument to a function. However, fun-hooks breaks this convention for a two reasons.

1. Javascript provides Funciton.prototype.bind to set this as well as for providing partial function application. fun-hooks uses this feature extensively in order to stack the hooks in such a way that they directly call each other. However, bind only allows partially applying arguments starting from the left. If the next callback were the last argument, this would require each hook being wrapped in another function to perform right partial function application. (this is actually what is done for the hooked async function itself since it will have its callback on the right by convention)
2. Having the next callback on the left allows "pass-through" hooks to easily decouple themselves from worrying about function arity. (i.e. (next, ...args) => { doSomething(); next.apply(null, args) } is easy and still works if parameters are added to the function, whereas (a, b, next) => { doSomething(); next.call(null, a, b) } is coupled to the hooked function's interface and needs to be refactored if the interface changes)

CSP (Content Security Policy) considerations

Since this library uses new Function you may need to allow for 'unsafe-eval' if your website uses a CSP policy (Node.js users need not worry). Technically the way this library uses new Function is safe, but what can you do... If you use CSP and cannot apply 'unsafe-eval' then there is a "no-eval" version available at the cost of some overhead. You can use it with the following import:

import funHooks from 'fun-hooks/no-eval/index.js'

Development

npm run test 
# or with debugging
npm run test:debug

# lint
npm run lint