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@feathersjs/hooks

Deno CI

@feathersjs/hooks brings middleware-like functionality to any async JavaScript or TypeScript function. It allows creation of composable and reusable workflows to handle functionality like

  • Logging
  • Profiling
  • Validation
  • Caching / Debouncing
  • Permissions
  • Data pre- and post-processing
  • etc.

This functionality can be added without having to change the original function. The pattern also keeps everything cleanly separated and testable.

import { hooks } from '@feathersjs/hooks';

// We're going to wrap `sayHi` with hook middleware.
class Hello {
  async sayHi (name) {
    return `Hi ${name}`
  }
}

// This logRuntime hook will be used as middleware
const logRuntime = async (context, next) => {
  const start = new Date().getTime();

  await next(); // In this example, `next` is `sayHi`.

  const duration = new Date().getTime() - start;
  console.log(`Function '${context.method}' returned '${context.result}' after ${duration}ms`);
}

// The `hooks` utility wraps `logRuntime` around `sayHi`.
hooks(Hello, {
  sayHi: [ logRuntime ]
});

// Calling `sayHi` will start by calling the `logRuntime` hook.
(async () => {
  const hi = new Hello();

  console.log(await hi.sayHi('Dave'));
})();

The Hooks middleware pattern was originally implemented directly in FeathersJS. Having been recognized as a powerful pattern with more broad-scale usefulness, it has been extracted from FeathersJS into this standalone utility.

See the release post for a quick overview.

Installation

Node

npm install @feathersjs/hooks --save
yarn add @feathersjs/hooks

Deno

@feathersjs/hooks releases are published to deno.land/x/hooks:

import { hooks } from 'https://deno.land/x/hooks/src/index.ts';

Browser

The @feathersjs/hooks npm package works in any modern browser and is compatible with any module loader like Webpack.

Documentation

Intro to Async Hooks

The fundamental building block of @feathersjs/hooks is the "Async Hook". An "Async Hook" is an async function that accepts two arguments:

  • A context object containing the arguments for the function call.
  • An asynchronous next function. Somewhere in the body of a hook function, there is a call to await next(), which calls the next hook OR the original function if all other hooks have run.

In its simplest form, an Async Hook looks like this:

const myAsyncHook = async (context, next) => {
  // Code before `await next()` runs before the main function
  await next();
  // Code after `await next()` runs after the main function.
}

Any Async Hook can be wrapped around another function, essentially becoming a middleware function. Calling await next() will either call the next middleware in the chain or the original function if all middleware have run. In the next section you'll learn how to wrap hooks around other functions.

The hooks Function

hooks(fn, middleware[]|manager) returns a new function that wraps fn with middleware

The hooks function wraps one or more Async Hooks around another function, setting up the hooks as middleware. The following examples all show the default functionality of passing an array of hooks as the second argument. Learn about additional functionality in the section about Middleware Managers

Example with a Function

The example below demonstrates the concept of wrapping the make_request function with the verify_auth hook function.

import { hooks } from '@feathersjs/hooks'

const make_request = () => { /* make a request to the database server */ }

const verify_auth = (context, next) => {
  /* Do auth verification before calling `await next()` */
  await next()
}

const request_with_middleware = hooks(make_request, [verify_auth])

In the above example, calling request_with_middleware will call the verify_auth function before calling make_request. The verify_auth function will have a context.arguments array containing the original arguments for the function call. A hook can modify the context object before calling await next(). (In this case, the next function IS the make_request function.) Alternatively, verify_auth could throw an error to prevent the request from ever getting to the make_request function. Check the hook context section to learn how to turn the context.arguments array into named parameters.

Important: A wrapped function will always return a Promise even if it was not originally async.

We've seen how to wrap a single function, but the hooks utility is more powerful. It can also wrap object methods and class methods. The following example shows how to use it with a class.

Example with a Class

The following example updates a class's sayHi method to log information about a function call. This syntax also works on plain objects.

const { hooks } = require('@feathersjs/hooks');

// This class has a `sayHi` instance method we're going to wrap with hooks.
// This would also work with an object containing a `sayHi` method.
class Hello {
  async sayHi (name) {
    return `Hi ${name}`
  }
}

// This logRuntime hook will be used as middleware
const logRuntime = async (context, next) => {
  // Code before `await next()` runs before the original function
  const start = new Date().getTime();

  await next();

  // Code after `await next()` runs after the original function.
  const end = new Date().getTime();
  console.log(`Function '${context.method || '[no name]'}' returned '${context.result}' after ${end - start}ms`);
}

// Enhance class (or object) methods using an object of method names as the 2nd argument
hooks(Hello, { sayHi: [ logRuntime ] });

// You can now use the wrapped instance methods inside any async function.
(async () => {
  const hi = new Hello();

  console.log(await hi.sayHi('Dave'));
})();

TypeScript with the @hooks Decorator

With TypeScript, you can use hooks the same was as shown in the above JavaScript example, or you can use decorators. Using decorators requires the experimentalDecorators option in tsconfig.json to be enabled.

"experimentalDecorators": true, /* Enables experimental support for ES7 decorators. */

Now hooks can be registered using the @hooks decorator:

import { hooks, HookContext, NextFunction } from '@feathersjs/hooks';

const logRuntime = async (context: HookContext, next: NextFunction) => {
  const start = new Date().getTime();

  await next();

  const end = new Date().getTime();
  console.log(`Function '${context.method || '[no name]'}' returned '${context.result}' after ${end - start}ms`);
}

class Hello {
  @hooks([ logRuntime ]) // the @hooks decorator
  async sayHi (name: string) {
    return `Hi ${name}`;
  }
}

(async () => {
  const hi = new Hello();

  console.log(await hi.sayHi('David'));
})();

The middleware Manager

You can use a middleware manager, instead of a plain array of hook functions, to enable additional functionality.

In all previous examples, when calling hooks with an array in the second argument ﹣ either directly like hooks(someFn, []) or as the value of an object key like hooks(someObj, { prop: [] }) ﹣ the array gets wrapped into an internal middleware Manager.

The middleware function creates a middleware Manager which has three important methods:

  • params()
  • props()
  • defaults()
const { hooks, middleware } = require('@feathersjs/hooks');

const sayHiWithHooks = hooks(sayHi, middleware([ hook1, hook2, hook3 ]));

(async () => {
  await sayHiWithHooks('David');
})();

params(...names)

Supplies names for original function arguments. Instead of appearing in params.arguments, the arguments will be named in the order provided.

const sayHiWithHooks = hooks(sayHi,
  middleware([ hook1, hook2, hook3 ]).params('name', 'age')
);

props(properties)

Initializes properties on the context

const sayHiWithHooks = hooks(sayHi,
  middleware([ hook1, hook2, hook3 ]).params('name').props({ customProperty: true })
);

Note: .props must not contain any of the field names defined in .params.

defaults(callback)

Calls a callback(self, arguments, context) that returns default values which will be set if the property on the hook context is undefined. Applies to both, params and other properties.

const sayHi = async name => `Hello ${name}`;

const sayHiWithHooks = hooks(sayHi,
  middleware([])
    .params('name')
    .defaults((self, args, context) => {
      return {
        name: 'Unknown human'
      }
    })
);

Global Hooks

Sometimes you want to run a set of hooks on all of the methods in a class or object.

Global Hooks on an Object

Hooks can also be registered at the object level which will run before any specific hooks on a hook enabled function:

const { hooks } = require('@feathersjs/hooks');

const o = {
  async sayHi (name) {
    return `Hi ${name}!`;
  }
  async sayHello (name) {
    return `Hello ${name}!`;
  }
}

// This hook will run first for every hook-enabled method on the object
hooks(o, [
  async (context, next) => {
    console.log('Top level hook');
    await next();
  }
]);
// The global hooks only run if you enable hooks on the method:
hooks(o, {
  sayHello: middleware([ logRuntime ]).params('name', 'quote'),
  sayHi: []
});

hooks(o, {
  sayHi: [ logRuntime ]
});

Global Hooks on a Class

Similar to object hooks, class hooks modify the class (or class prototype). Just like for objects it is possible to register hooks that are global to the class or object. Registering hooks also works with inheritance.

Note: Object or class level global hooks will only run if the method itself has been enabled for hooks. This can be done by registering hooks with an empty array.

JavaScript Example

const { hooks } = require('@feathersjs/hooks');

class HelloSayer {
  async sayHello (name) {
    return `Hello ${name}`;
  }
}

class HappyHelloSayer extends HelloSayer {
  async sayHello (name) {
    const baseHello = await super.sayHello(name);
    return baseHello + '!!!!! :)';
  }
}

// Add global hooks to the class using its prototype
hooks(HelloSayer.prototype, [
  async (context, next) => {
    console.log('Hook on HelloSayer');
    await next();
  }
]);

hooks(HappyHelloSayer.prototype, [
  async (context, next) => {
    console.log('Hook on HappyHelloSayer');
    await next();
  }
]);

// Enabling hooks on sayHello also allows the global hooks to run.
hooks(HelloSayer, {
  sayHello: [async (context, next) => {
    console.log('Hook on HelloSayer.sayHello');
    await next();
  }]
});

(async () => {
  const happy = new HappyHelloSayer();

  console.log(await happy.sayHello('David'));
})();

TypeScript Example

Using decorators in TypeScript also respects inheritance:

import { hooks, HookContext, NextFunction } from '@feathersjs/hooks';

@hooks([
  async (context: HookContext, next: NextFunction) => {
    console.log('Hook on HelloSayer');
    await next();
  }
])
class HelloSayer {
  @hooks(middleware([
    async (context: HookContext, next: NextFunction) => {
      console.log('Hook on HelloSayer.sayHello');
      await next();
    }
  ]).params('name'))
  async sayHello (name: string) {
    return `Hello ${name}`;
  }

  async otherMethod () {
    return 'This will not run any hooks';
  }
}

@hooks([
  async (context: HookContext, next: NextFunction) => {
    console.log('Hook on HappyHelloSayer');
    await next();
  }
])
class HappyHelloSayer extends HelloSayer {
  async sayHello (name: string) {
    const message = await super.sayHello(name);
    return `${message}!!!!! :)`;
  }
}

(async () => {
  const happy = new HappyHelloSayer();

  console.log(await happy.sayHello('David'));
})();

Note: Decorators only work on classes and class methods, not on functions. Standalone (arrow) functions require the JavaScript function style hook registration.

Hook Context

The hook context in a middleware function is an object that contains information about the function call.

Context properties

The default properties available are:

  • context.arguments - The arguments of the function as an array
  • context.method - The name of the function (if it belongs to an object or class)
  • context.self - The this context of the function being called (may not always be available e.g. for top level arrow functions)
  • context.result - The result of the method call
  • context[name] - Value of a named parameter when using named arguments

Arguments

By default, the function call arguments will be available as an array in context.arguments. The values can be modified to change what is passed to the original function call:

const { hooks } = require('@feathersjs/hooks');

const sayHello = async (firstName, lastName) => {
  return `Hello ${firstName} ${lastName}!`;
};

const wrappedSayHello = hooks(sayHello, [
  async (context, next) => {
    // Replace the `lastName`
    context.arguments[1] = 'X';
    await next();
  }
]);

(async () => {
  console.log(await wrappedSayHello('David', 'L')); // Hello David X
})();

Using named parameters

It is also possible to turn the arguments into named parameters. In the above example we probably want to have context.firstName and context.lastName available. To do this, the context option can be initialized like this:

const { hooks, middleware } = require('@feathersjs/hooks');

const sayHello = async (firstName, lastName) => {
  return `Hello ${firstName} ${lastName}!`;
};

const wrappedSayHello = hooks(sayHello, middleware([
  async (context, next) => {
    // Now we can modify `context.lastName` instead
    context.lastName = 'X';
    await next();
  }
]).params('firstName', 'lastName'));

(async () => {
  console.log(await wrappedSayHello('David', 'L')); // Hello David X
})();

Note: When using named parameters, context.arguments is read only to preserve the order of named params.

Default values

You can add default values using the manager's .defaults() method. See manager.defaults()

Note: Even if your original function contains a default value, it is important to specify it because the middleware runs before and the value will be undefined without a default value.

Modifying the result

In a hook function, context.result can be

  • Set before calling await next() to skip the original function call. Other hooks will still run.
  • Modified after calling await next() to modify what is being returned by the function.

See the cache example for how this can be used.

Calling the original function

The original function without any hooks is available as fn.original:

const { hooks } = require('@feathersjs/hooks');
const emphasize = async (context, next) => {
  await next();

  context.result += '!!!';
};
const sayHello = hooks(async name => `Hello ${name}`, [ emphasize ]);

const o = hooks({
  async sayHi(name) {
    return `Hi ${name}`;
  }
}, {
  sayHi: [ emphasize ]
});

(async () => {
  console.log(await sayHello.original('Dave')); // Hello Dave
  // Originals on object need to be called with an explicit `this` context
  console.log(await o.sayHi.original.call(o, 'David'))
})();

Customizing and returning the context

Once a function has been wrapped with hooks, the wrapped function will have a createContext method. This method can be used to create a custom context object. This custom context can then be passed as the last argument of a hook-enabled function call. In that case, the up-to-date context object - with all the information (like context.result) - will be returned:

const { hooks, HookContext } = require('@feathersjs/hooks');
const customContextData = async (context, next) => {
  console.log('Custom context message is', context.message);

  context.customProperty = 'Hi';

  await next();
}

const sayHello = hooks(async message => {
  return `Hello ${message}!`;
}, [ customContextData ]);

const customContext = sayHello.createContext({
  message: 'Hi from context'
});

(async () => {
  const finalContext = await sayHello('Dave', customContext);

  console.log(finalContext);
})();

Flow Control with Multiple Hooks

Async Hook Flow

Middleware functions (or hook functions) take a context and an asynchronous next function as their parameters. The context contains information about the function call (like the arguments, the result or this context) and the next function can be called to continue to the next hook or the original function.

A middleware function can do things before calling await next() and after all following middleware functions and the function call itself return. It can also try/catch the await next() call to handle and modify errors. This is the same control flow that the web framework KoaJS uses for handling HTTP requests and response.

Each hook function wraps around all other functions (like an onion). This means that the first registered middleware function will run first before await next() and as the very last after all following hooks.

Feathers hooks image

The following example uses hooks named one, two, and three to demonstrate how execution order works:

const { hooks } = require('@feathersjs/hooks');

const sayHello = async message => {
  console.log(`HELLO, ${message}!`)
};

const one = async (ctx, next) => {
  console.log('one   before');
  await next();
  console.log('one   after')
}

const two = async (ctx, next) => {
  console.log('two   before');
  await next();
  console.log('two   after')
}

const three = async (ctx, next) => {
  console.log('three before');
  await next();
  console.log('three after')
}

const sayHelloWithHooks = hooks(sayHello, [
  one,
  two,
  three
]);

(async () => {
  await sayHelloWithHooks('DAVID');
})();

Would print:

one   before
two   before
three before
HELLO, DAVID!
three after
two   after
one   after

This order also applies when using hooks on objects and classes and with inheritance.

Regular Hooks

You may have noticed that after-hook execution order is the reverse compared to before-hook execution order. This is due to how the hooks wrap around each other. If you prefer that the flow of the hooks matches the flow of the page, you can use Regular Hooks. Regular Hooks are similar to Async Hooks, but they do not receive a next function as the second argument. This means there is no async next() in the middle of the function body. This allows the code execution to match the natural reading flow on the page: top to bottom. Here's what a regular hook looks like:

// A Regular Hook is just an async function that receives the context object.
const regularHook = async (context) => {
  // All code goes here.
}

With @feathersjs/hooks, the collect utility enables the use of Regular Hooks.

Longtime FeathersJS developers will recognize Regular Hooks. They're the same type of hooks that have been around since the beginning.

The collect utility

The collect utility enables Regular Hooks functionality. It gathers hooks into before, after, and error hooks. Here's what it looks like.

import { hooks } from '@feathersjs/hooks'
import { discard } from 'feathers-hooks-common'

const make_request = () => { /* make a request to the database server */ }

const verify_auth = (context) => {
  /* Do auth verification, here */
}
const handle_error = (context) => {
  /* Do some error handling */
}

const request_with_middleware = hooks(
  make_request,
  middleware([
    collect({
      before: [verify_auth],
      after: [discard('password')],
      error: [handle_error]
    })
  ])
)

Or with a class:

import { hooks } from '@feathersjs/hooks'
import { discard } from 'feathers-hooks-common'

class DbAdapter {
  create() {
    /* create data in the db */
  }
}

const verify_auth = (context) => {
  /* Do auth verification, here */
}
const handle_error = (context) => {
  /* Do some error handling */
}

const request_with_middleware = hooks(
  DbAdapter,
  {
    create: middleware([
      collect({
        before: [verify_auth],
        after: [discard('password')],
        error: [handle_error]
      })
    ]),
  }
)

Best practises

  • Hooks can be registered at any time by calling hooks again but registration should be kept in one place for better visibility.

  • Decorators make the flow even more visible by putting it right next to the code the hooks are affecting.

  • The context will always be the same object in the hook flow. You can set any property on it.

  • If a parameter is an object, modifying that object will change the original parameter. This can cause subtle issues that are difficult to debug. Using the spread operator to add the new property and replacing the context property helps to avoid many of those problems:

    const updateQuery = async (context, next) => {
      // NOT: context.query.newProperty = 'something';
    
      // Instead
      context.query = {
        ...context.query,
        active: true
      }
    
      await next();
    }
    
    const findUser = hooks(async query => {
      return collection.find(query);
    }, middleware([ updateQuery ]).params('query'));

More Examples

Cache

The following example is a simple hook that caches the results of a function call and uses the cached value. It will clear the cache every 5 seconds. This is useful for any kind of expensive method call like an external HTTP request:

const { hooks } = require('@feathersjs/hooks');
const cache = () => {
  let cacheData = {};

  // Reset entire cache every 5 seconds
  setInterval(() => {
    cacheData = {};
  }, 5000);

  return async (context, next) => {
    const key = JSON.stringify(context);

    if (cacheData[key]) {
      // Setting context.result before `await next()`
      // will skip the (expensive function call) and
      // make it return the cached value
      context.result = cacheData[key];
    }

    await next();

    // Set the cached value to the result
    cacheData[key] = context.result;
  }
}

const getData = hooks(async url => {
  return axios.get(url);
}, [ cache() ]);

await getData('http://url-that-takes-long-to-respond');

Permissions

When passing e.g. a user object to a function call, hooks allow for a better separation of concerns by handling permissions in a hook:

const checkPermission = name => async (context, next) => {
  if (!context.user.permissions.includes(name)) {
    throw new Error(`User does not have ${name} permission`);
  }

  await next();
}

const deleteInvoice = hooks(async (id, user) => {
  return collection.delete(id);
}, middleware([ checkPermission('admin') ]).params('id', 'user'));

Cleaning up GraphQL resolvers

The above examples can both be useful for speeding up and locking down existing GraphQL resolvers:

const { hooks } = require('@feathersjs/hooks');

const checkPermission = name => async (ctx, next) => {
  const { context } = ctx;
  if (!context.user.permissions.includes(name)) {
    throw new Error(`User does not have ${name} permission`);
  }

  await next();
}

const resolvers = {
  Query: {
    human: hooks(async (obj, args, context, info) => {
      return context.db.loadHumanByID(args.id).then(
        userData => new Human(userData)
      )
    }, middleware([
      cache(),
      checkPermission('admin')
    ]).params('obj', 'args', 'context', 'info'))
  }
}

Contributing

For general contribution information refer to the Feathers contribution guideLINES.

@feathersjs/hooks modules are written in TypeScript using Deno as the runtime. With Deno 1.16 or later installed you can start contributing by cloning this repository or your own fork via:

git clone git@github.com:feathersjs/hooks.git
cd hooks/

And then run the tests via:

make test

License

Copyright (c) 2021

Licensed under the MIT license.