How to install ?

1. Run npm install promise-composer
2. Include the Promise Composer Object (PCO) with something like :

var PCO = require('promise-composer');

What does it do ?

Promise Composer is a small JS library that gives some utility tools and functions to help asynchronous processes to dynamically check/validate data.

I wrote it since I was tired of checking type errors manually using the same old if/else or switch/case methods, so instead of writing :

if (myCheckingFunc(value) && myOtherFunc(value)) {
  doStuff()
} else {
  doOtherStuff()
}

You could just write :

Promise.resolve(value)
  .then(PCO.myCheckingFunc)
  .then(PCO.myOtherFunc)
  .then(doStuff)
  .catch(doOtherStuff)

It still is pretty minimal, but I do think this can be of help to ensure a layer of dynamical verification without having to cope with if/else forests or huge monolithic switch/case blocks.

Create an assertion

To create a custom assertion function, you only have to use the PCO.assert function.

function validateObject(x) {
  return Array.isArray(x)
    && x.length > 3
    && x.length < 9
}

Promise.resolve(15)
  .then(x => PCO.assert(x, validateObject))
  .then(doOtherStuff)
  .catch(computeError)

Common assertion functions

• PCO.isset Check if an element isn't undefined

• PCO.exists Check if an element isn't undefined and is non-null

• PCO.fullString Check if an element is a non-empty Sstring (trims whitespaces)

• PCO.isObject Check if an element is a non-array Object

• PCO.isArray Check if an element is a valid Array

• PCO.isNumber Check if an element is a valid Number

• PCO.isInteger Check if an element is a valid Integer

Multiple assertions

If you need multiple validations at once, you can use two important functions :

• PCO.any This functions acts as an "or" operator. It will proceed if any of the given assertions is true.

Example

Promise.resolve("Coucou")
  .then(x => PCO.any(x, [PCO.fullString, PCO.isset]))
  .then(doStuff)
  .catch(computeError)

// This example should not trigger the .catch() block

• PCO.all This function acts as an "and" operator. It will proceed if all of the given assertions are true.

Example

Promise.resolve(15)
  .then(x => PCO.all(x, [PCO.fullString, PCO.isset]))
  .then(doStuff)
  .catch(computeError)

// This example should trigger the .catch() block !

Important !

PCO.any and PCO.all both require as a second argument an array of assertions. You can use standard PCO assertion functions as well as custom assertion functions (this might be useful for dataset validation for instance).

Verify assertion on multiple values

You can use PCO.traverse using an array of values instead of a single value to check every value inside the array with your assertion.

Example

Promise.resolve(["foo", "bar", null])
  .then(x => PCO.traverse(x, PCO.exists))
  .then(doStuff)
  .catch(computeError)

// This exemple should trigger the .catch() block !