module-async-mapper

Discover and specify module methods' continuation schemes (callbacks vs promises vs generator functions, etc)

mapper.map(module[, hints, options])

Traverses the module and returns a JSONPath style map of the given module's methods, along with a heuristic best guess at their continuation scheme, accepting hints to inform guesses.

var mapper = require('module-async-mapper');
var fs = require('fs');
var map = mapper.map(fs);

// {
//   '$.readFile': 'standard',
//   '$.readFileSync': 'sync',
//   '$.writeFile': 'standard',
//   '$.writeFileSync': 'sync',
//   '$.exists': 'simple',
//   '$.existsSync': 'sync',
//   ...
// }

Hints are in the form of an object with JSONPath keys pointing to continuation scheme identifiers, optionally preceded by a + to force that option to take effect.

And another example: The mkdirp module exports a function that accepts a standard node callback, but that happens to be difficult to ascertain programmatically so we provide a hint.

var mapper = require('module-async-map');
var mkdirp = require('mkdirp');
var map = mapper.map(mkdirp, { '$': '+standard' });

// {
//  '$': 'standard',
//  '$.sync': 'sync'
// }

If module is a string, it will be resolved as require would do, and any hints in .asyncmap.json in the module's source root will be applied with hints merged on top, as well as any local hints from this package.

Takes the following options:

• traverseAll - Traverse the whole object rather than just the parts that match keys in hints; defaults to true
• loadModuleHints - Load any hints from .asyncmap files in module source root; defaults to true
• loadLocalHints - Load any hints from hints in this here module; defaults to true

mapper.scheme(fn)

Given a function returns a continuation scheme identifier based on a heuristic best guess. For example, detect a plain old synchronous function:

var scheme = mapper.scheme(function(a, b) { return a + b });
// "sync"

Or detect a function that accepts a standard callback:

var scheme = mapper.scheme(function(a, b, callback) { return callback(null, a + b) });
// "standard"

Continuation Schemes

JavaScript has a few options/conventions for continuing execution with the result of some asynchronous action.

Standard Node Callback

standard

The most common and scheme is the standard node callback -- a function that accepts an error as the first parameter and a return value as the second.

fs.readFile('/etc/passwd', function(err, contents) {
  console.log(contents);
});

Simple Callback

simple

An optimistic callback function, accepting just the return value as the first paramater, with no intrinsic mechanism for reporting error.

fs.exists('/etc/passwd', function(exists) {
  console.log(exists ? "File exists" : "File does not exist");
});

Promise

promise

A promise is an instance of a Promise from ES6 promises / bluebird / etc.

File.read('/etc/passwd')
  .then(function(contents) { console.log(contents) });

Generator Function

generator

A generator function may perform asynchronous work and assign results of that work via yielding within its body. Requires a framework like suspend as an intermediary.

suspend(function*() {
  var contents = yield read('/etc/passwd');
  console.log(contents);
})();

Synchronous Return

sync

A regular old function that directly returns the result of its work, potentially blocking the event loop for any I/O operations.

var contents = fs.readFileSync('/etc/passwd');
console.log(contents);

License

MIT