A playground explaining promise things in ES6

promises.js

/*
Promises are fun by Olly Shaw
https://twitter.com/olly_shaw
https://github.com/ollyjshaw

To run this download the file and do something like 
node promises.js

logging will be out of order (Promises are asychronous). But hopefully you can find your way.
*/

let greetingPromise = Promise.resolve("hi")

//it's a promise
console.log(greetingPromise) //Promise { 'hi' }

//will print the greeting out
greetingPromise.then((value) => console.log(value)) //hi

//you can throw an error but it must be caught in the chain
greetingPromise.then((value) => {
  console.log(value)
  throw Error("foobar mk1")
}).catch(err => console.warn(`caught an error: ${err}`)) //caught an error: Error: foobar mk1

//not being in a chain will bubble the error up and give a
//  UnhandledPromiseRejectionWarning: 
//  DeprecationWarning: Unhandled promise rejections are deprecated. 
//  In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
greetingPromise.then((value) => {
  console.log(value)
  throw Error("foobar")
}) //UnhandledPromiseRejectionWarning

//Try catch will not save you here, this will still give a UnhandledPromiseRejectionWarning
try {
  greetingPromise.then(() => {
    throw Error("foobar")
  })
} catch (error){
  console.log("I caught the error", error)
} //UnhandledPromiseRejectionWarning

//Start off some number chains
let numberPromise = Promise.resolve(42)

//chaining requires that a function is given to the then method
//even if no value is returned or used.
greetingPromise.then((greetValue) => {
  console.log(`greetValue is: ${greetValue}`) //hi
}).then(()=>numberPromise).then(numberValue => {
  console.log(`numberValue is ${numberValue}`) //42
})

//if you just put the reference to a promise into the then, what happens is 
//the promise reference is wrapped in a thenable. The thenable will just produce an `undefined`
greetingPromise.then((greetValue) => {
  console.log(`greetValue is: ${greetValue}`) //hi
}).then(numberPromise).then(thenableThing => {
  console.log(`thenableThing is ${thenableThing}`) //undefined
})

//if you return a value from a then, it is passed down the promise chain
greetingPromise.then(() => {
  return 3
}).then(fromThenBlockNumberValue => {
  console.log(`fromThenBlockNumberValue is ${fromThenBlockNumberValue}`) //3
})

// If you want to pass a parameter to a promise, simply wrap a it in a function
function doubler(foo) {
  return Promise.resolve(foo*2)
}

//A function returning a promise can then really unlock some cool stuff.
numberPromise.then((value)=> doubler(value)).then(doubledValue => {
  console.log(`doubledValue is ${doubledValue}`) //84
})

//Beware the use of blocks in the then
numberPromise.then((value)=> {
  doubler(value)
}).then(nothingWasReturnedFromBlock => {
  console.log(`nothingWasReturnedFromBlock is ${nothingWasReturnedFromBlock}`) //undefined
})

//if you need a block you must return
numberPromise.then((value)=> {
  return doubler(value)
}).then(returnedFromBlock => {
  console.log(`returnedFromBlock is ${returnedFromBlock}`) //84
})

//If you have an error in the chain, further promise chaining is skipped and
//the catch clause is hit.
numberPromise.then(() => {
  throw Error("foobar mk2")
}).then(()=>greetingPromise).then(() => {
  console.log("You won't see this")
}).then(() => {
  console.log("Or this")
}).then(() => {
  throw Error("This Error won't be seen either")
}).catch(err => console.warn(`dealing with the error: ${err}`)) //dealing with the error: Error: foobar mk2