letta

Promisify sync, async or generator function, using relike. Kind of promisify, but lower level. Full compatibility with co4 and passing 100% of its tests.

You might also be interested in relike, relike-all, relike-value and letta-value.

Table Of Contents

• Highlights
• Notes
  • Sync and async functions
  • Absolutely silent - never crash
• Install
• Usage
• API
  • letta
  • .promisify
  • .Promise
• Examples
  • Callback functions
  • Generator functions
  • JSON.stringify
  • Synchronous functions
  • Exceptions and rejections
  • Returning errors
  • Passing function as last argument
• Related
• Contributing

(TOC generated by verb using markdown-toc)

Highlights

A few features and main points.

• promisify sync, async and generator functions
• lower lever than "promisify" - giving function to 1st argument, the next arguments are passed to it
• thin wrapper around relike package to add support for generators, using co
• full compatibility with co@4 and passing 100% of its tests
• correct detecting async (callback-style) functions
• correct handling of optional arguments, just fn.length not works - believe
• great handling of errors, uncaught exceptions, rejections and optional arguments
• never crash, all is silent, listening on unhandledRejection and uncaughtException
• always stay to the standards and specs
• always use native Promise, you can't trick that
• only using Bluebird, if not other Promise constructor provided through .Promise property
• Bluebird or the custom constructor is used only on enviroments that don't have support for native Promise
• works on any nodejs version - from v0.10.x to latest v6+ Node.js
• accept and works with javascript internal functions like JSON.stringify and JSON.parse

Notes

Sync and async functions

Note that it treats functions as asynchronous, based on is-async-function.

Why you should be aware of that? Because if you give async function which don't have last argument called with some of the common-callback-names it will treat that function as synchronous and things may not work as expected.

It's not a problem for most of the cases and for node's native packages, because that's a convention.

Absolutely silent - never crash

Using letta you should be absolutely careful. Because it makes your application absolutely silent. Which means if you have some ReferenceError or something like it, after the execution of letta it will be muted. And the only way to handle it is through .catch from the returned promise.

Let's visualise it. In the following examples we'll use relike first and then letta, and you can see the differences.

var relike = require('relike')
var promise = relike(function () {
  return 123
})

promise.then(console.log, err => {
  console.error(err.stack)
  // => errors only happened in function wrapped by relike
})

foo
// => throws ReferenceError directly, immediately
// and your application will crash

But the things, using letta are little bit different, because we have listeners on unhandledRejection and on uncaughtException events. The same example from above, using letta

var letta = require('letta')
var promise = letta(function () {
  return 123
})

promise.then(console.log, err => {
  console.error(err.stack)
  // => ReferenceError: foo is not defined
})

foo
// => never throws directly, never crash
// this error should be handled from the promise

So, if you don't want this behavior, you should use relike. But if you want generators support, you should do some little wrapper for relike.

Install

npm i letta --save

Usage

For more use-cases see the tests, examples or all the passing co@4 tests

const fs = require('fs')
const letta = require('letta')

const promise = letta(function * () {
  let result = yield Promise.resolve(123)
  return result
})

promise.then(value => {
  console.log(value) // => 123
}, err => {
  console.error(err.stack)
})

If you want to convert generator function to regular function that returns a Promise use letta.promisify

Example

const letta = require('letta')
const fn = letta.promisify(function * (number) {
  return yield Promise.resolve(number)
})

fn(456).then(number => {
  console.log(number) // => 456
}, err => {
  console.error(err.stack)
})

If you want to promisify any type of function, again, just use the .promisify method, like you do with bluebird.promisify.

const fs = require('fs')
const letta = require('letta')
const readFile = letta.promisify(fs.readFile)

readFile('package.json', 'utf8')
  .then(JSON.parse)
  .then(value => {
    console.log(value.name) // => 'letta'
  })
  .catch(SyntaxError, err => {
    console.error('File had syntax error', err)
  })
  // Catch any other error
  .catch(err => {
    console.error(err.stack)
  })

API

letta

Control flow for now and then.

Params

• <fn> {Function}: Regular function (including arrow function) or generator function.
• [...args] {Mixed}: Any number of any type of arguments, they are passed to fn.
• returns {Promise}: Always native Promise if supported on enviroment.

Example

const letta = require('letta')

letta((foo, bar, baz) => {
  console.log(foo, bar, baz) // => 'foo bar baz'
  return foo
}, 'foo', 'bar', 'baz')
.then(console.log) // => 'foo'

.promisify

Returns a function that will wrap the given fn. Instead of taking a callback, the returned function will return a promise whose fate is decided by the callback behavior of the given fn node function. The node function should conform to node.js convention of accepting a callback as last argument and calling that callback with error as the first argument and success value on the second argument. – Bluebird Docs on .promisify

Params

• <fn> {Function}: Regular function (including arrow function) or generator function.
• [Promize] {Function}: Promise constructor to be used on enviroment where no support for native.
• returns {Function}: Promisified function, which always return a Promise when called.

Example

const fs = require('fs')
const letta = require('letta')
const readFile = letta.promisify(fs.readFile)

readFile('package.json', 'utf8')
  .then(JSON.parse)
  .then(value => {
    console.log(value.name) // => 'letta'
  })
  .catch(SyntaxError, err => {
    console.error('File had syntax error', err)
  })
  // Catch any other error
  .catch(err => {
    console.error(err.stack)
  })

// or promisify generator function
const promise = letta(function * () {
  let result = yield Promise.resolve(123)
  return result
})

promise.then(value => {
  console.log(value) // => 123
}, err => {
  console.error(err.stack)
})

.Promise

While letta always trying to use native Promise if available in the enviroment, you can give a Promise constructor to be used on enviroment where there's no support - for example, old broswers or node's 0.10 version. By default, letta will use and include bluebird on old enviroments, as it is the fastest implementation of Promises. So, you are able to give Promise constructor, but it won't be used in modern enviroments - it always will use native Promise, you can't trick that. You can't give custom promise implementation to be used in any enviroment.

Example

var fs = require('fs')
var letta = require('letta')

letta.Promise = require('q') // using `Q` promise on node 0.10
var readFile = letta.promisify(fs.readFile)

readFile('package.json', 'utf8')
  .then(console.log, err => {
    console.error(err.stack)
  })

One way to pass a custom Promise constructor is as shown above. But the other way is passing it to .Promise of the promisified function, like that

var fs = require('fs')
var letta = require('letta')
var statFile = letta.promisify(fs.stat)

statFile.Promise = require('when') // using `when` promise on node 0.10
statFile('package.json').then(console.log, console.error)

One more thing, is that you can access the used Promise and can detect what promise is used. It is easy, just as promise.Promise and you'll get it. Or look for promise.___bluebirdPromise and promise.___customPromise properties. .___bluebirdPromise (yea, with three underscores in front) will be true if enviroment is old and you didn't provide promise constructor to .Promise. So, when you give constructor .__customPromise will be true and .___bluebirdPromise will be false.

var fs = require('fs')
var letta = require('letta')

var promise = letta(fs.readFile, 'package.json', 'utf8')
promise.then(JSON.parse).then(function (val) {
  console.log(val.name) // => 'letta'
}, console.error)

console.log(promise.Promise) // => used Promise constructor
console.log(promise.___bluebirdPromise) // => `true` on old env, falsey otherwise
console.log(promise.___customPromise) // => `true` when pass `.Promise`, falsey otherwise

Examples

Few working examples with what can be passed and how letta acts.

• Callback functions
• Generator functions
• JSON.stringify
• Synchronous functions
• Exceptions and rejections
• Returning errors
• Passing function as last argument

Callback functions

Can accept asynchronous (callback) functions as well.

Example

const fs = require('fs')
const letta = require('letta')

letta(fs.readFile, 'package.json', 'utf8')
  .then(JSON.parse)
  .then(data => {
    console.log(data.name) // => 'letta'
  }, err => {
    console.error(err.stack)
  })

// callback `fs.stat` function
letta(fs.stat, 'package.json')
  .then(stats => {
    console.log(stats.isFile()) // => true
  }, err => {
    console.error(err.stack)
  })

Generator functions

Accept generator functions same as co and acts like co@4.

Example

const fs = require('fs')
const letta = require('letta')

letta(function * (filepath) {
  return yield letta(fs.readFile, filepath, 'utf8')
}, 'package.json')
  .then(JSON.parse)
  .then(data => {
    console.log(data.name) // => 'letta'
  }, err => {
    console.error(err.stack)
  })

JSON.stringify

Specific use-case which shows correct handling of optional arguments.

const letta = require('letta')

letta(JSON.stringify, { foo: 'bar' })
  .then(data => {
    console.log(data) // => {"foo":"bar"}
  }, console.error)

// result with identation
letta(JSON.stringify, {foo: 'bar'}, null, 2)
  .then(data => {
    console.log(data)
    // =>
    // {
    //   "foo": "bar"
    // }
  }, console.error)

Synchronous functions

Again, showing correct handling of optinal arguments using native fs module.

const fs = require('fs')
const letta = require('letta')

// sync function
letta(fs.statSync, 'package.json')
  .then(stats => {
    console.log(stats.isFile()) // => true
  })
  .catch(err => console.error(err.stack))

// correct handling of optional arguments
letta(fs.readFileSync, 'package.json')
  .then(buf => {
    console.log(Buffer.isBuffer(buf)) // => true
  })
  .catch(err => {
    console.error(err.stack)
  })

Exceptions and rejections

Handles uncaughtException and unhandledRejection by default.

Example

const fs = require('fs')
const letta = require('letta')

letta(fs.readFile, 'foobar.json')
  .then(console.log, err => {
    console.error(err.code) // => 'ENOENT'
  })

// handles ReferenceError,
// SyntaxError and etc
const promise = letta(function () {
  foo
  return true
})

promise.catch(err => {
  console.error(err) // => 'ReferenceError: foo is not defined'
})

Returning errors

You should notice that if some function returns instance of Error it will acts as usual - receive it in .then not in .catch. Review the examples/errors.js example.

Example

const letta = require('letta')

const promise = letta(function () {
  return new Error('foo err bar')
})

promise.then(errorAsResultValue => {
  console.log(errorAsResultValue instanceof Error) // => true
  console.log(errorAsResultValue.message) // => 'foo err bar'
})

Passing function as last argument

You can also pass normal (non-callback) function as last argument without problem. It won't be assumed as callback, until you name it or have argument with some of common-callback-names.

Example

const assert = require('assert')
const letta = require('letta')

function regular (str, num, obj, fn) {
  assert.strictEqual(str, 'foo')
  assert.strictEqual(num, 123)
  assert.deepEqual(obj, { a: 'b' })
  assert.strictEqual(typeof str, 'string')
  assert.strictEqual(typeof num, 'number')
  assert.strictEqual(typeof obj, 'object')
  assert.strictEqual(typeof fn, 'function')
  return obj
}

letta(regular, 'foo', 123, {a: 'b'}, function someFn () {})
  .then(result => {
    console.log(result) // => { a: 'b' }
  })

Related

• callback2stream: Transform sync, async or generator function to Stream. Correctly handle errors. homepage
• letta-value: Extends letta to accept and handles more than functions only. Handles all… more | homepage
• mukla: Simple and fast test runner with basic reporter and clean stacktraces. Support… more | homepage
• promise2stream: Transform ES2015 Promise to Stream - specifically, Transform Stream using… more | homepage
• relike-all: Promisify all functions in an object, using relike. | homepage
• relike-value: Create promise from sync, async, string, number, array and so on. Handle… more | homepage
• relike: Simple promisify async or sync function with sane defaults. Lower level than… more | homepage
• value2stream: Transform any value to stream. Create a stream from any value -… more | homepage

Contributing

Pull requests and stars are always welcome. For bugs and feature requests, please create an issue. But before doing anything, please read the CONTRIBUTING.md guidelines.

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