Callback decorators

This package has been replaced by async-deco

This is a collection of function decorators designed to work with functions using a callback or returning a promise. In case of callback, it must follow the node convention: the callback should be the last argument and its arguments should be, an error instance and the output of the function. Most of them are designed to make an asynchronous function call more robust and reliable. They can be combined together using the "compose" function (included).

Callback and promises

Every decorator is available in two different flavours:

• callback based:

var logDecorator = require('callback-decorators/callback/log');

This should be applied to functions with the node callback convention:

logDecorator(logger)(function (a, b, c, next) {
  ...
  next(undefined, result); // or next(error);
});

• and promise based:

var logDecorator = require('callback-decorators/promise/log');

This should be used for function returning promises:

logDecorator(logger)(function (a, b, c) {
  return new Promise(function (resolve, reject) {
    ...
    resolve(result); // or reject(error);    
  });
});

Requiring the library

You can either:

var memoizeDecorator = require('callback-decorators/callback/memoize');

or

var memoizeDecorator = require('callback-decorators').callback.memoize;

or

var callbackDecorators = require('callback-decorators');
var memoizeDecorator = callbackDecorators.callback.memoize;

I strongly advice to use the first method, especially when using browserify. It allows to import only the functions you are actually using.

About the logger

You can pass a logger to the decorators. It is a function with this signature:

function (type, obj)

• Type is the type of event to log
• obj contains useful informations, depending on the type

Decorators

The examples are related to the callback version. Just import the promise version in case of decorating promise based functions.

Memoize

It allows to remember the last results

var memoizeDecorator = require('callback-decorators/callback/memoize');

var simpleMemoize = memoizeDecorator(getKey, logger);
simpleMemoize(function (..., cb) { .... });

It takes 2 arguments:

• an optional getKey function: when it runs against the original arguments it returns the key used for the caching
• a logger function (logs "cachehit")

Cache

It is a more sophisticated version of the memoize decorator. It can be used to for caching in a db/file etc (You may have to write your own cache object). memoize-cache is an in-memory reference implementation (https://github.com/sithmel/memoize-cache).

var cacheDecorator = require('callback-decorators/callback/cache');

var cached = cacheDecorator(cache, logger);
cached(function (..., cb) { .... });

It takes 2 arguments:

• a cache object. The interface should be compatible with memoize-cache (https://github.com/sithmel/memoize-cache)
• a logger function (logs "cachehit")

Fallback

If a function fails, calls another one

var fallbackDecorator = require('callback-decorators/callback/fallback');

var fallback = fallbackDecorator(function (err, a, b, c, func) {
  func(undefined, 'giving up');
}, Error, logger);
fallback(function (..., cb) { .... });

It takes 3 arguments:

• fallback function. It takes the err, and the original arguments.
• error instance for deciding to fallback, or a function taking error and result (if it returns true it'll trigger the fallback)
• logger function (logs "fallback")

Fallback cache

If a function fails, it tries to use a previous cached result

var fallbackCacheDecorator = require('callback-decorators/callback/fallback-cache');

var fallback = fallbackCacheDecorator(cache, Error, logger);
fallback(function (..., cb) { .... });

It takes 3 arguments:

• a cache object. The interface should be compatible with memoize-cache (https://github.com/sithmel/memoize-cache)
• error instance for deciding to fallback, or a function taking error and result (if it returns true it'll trigger the fallback)
• logger function (logs "fallback-cache")

Log

Logs when a function start, end and fail

ar logDecorator = require('callback-decorators/callback/log');

var addLogs = logDecorator(logger);
addLogs(function (..., cb) { .... });

Timeout

If a function takes to much, returns a timeout exception

var timeoutDecorator = require('callback-decorators/callback/timeout');

var timeout20 = timeoutDecorator(20, logger);
timeout20(function (..., cb) { .... });

This will wait 20 ms before returning a TimeoutError. It takes 2 arguments:

• time in ms
• a logger function (logs "timeout")

Retry

If a function fails it retry it again

var retryDecorator = require('callback-decorators/callback/retry');

var retryTenTimes = retryDecorator(10, 0, Error, logger);
retryTenTimes(function (..., cb) { .... });

You can initialise the decorator with 3 arguments:

• number of retries
• interval for trying again (number of a function based on the number of times)
• error instance for deciding to retry, or function taking error and result (if it returns true it'll trigger the retry)
• logger function (logs "retry")

Utilities

Callbackify

Convert a synchronous/promise based function to a plain callback.

var callbackify = require('callback-decorators/utils/callbackify');

var func = callbackify(function (a, b){
  return a + b;
});
func(4, 6, function (err, result){
  ... // result === 10 here
})

Compose

It can combine more than one decorators. You can pass either an array or using multiple arguments. "undefined" functions are ignored.

var compose = require('callback-decorators/utils/compose');

var decorator = compose(
  retryDecorator(10, Error, logger),
  timeoutDecorator(20, logger));

decorator(function (..., cb) { .... });

Timeout after 20 ms and then retry 10 times before giving up. You should consider the last function is the one happen first!