how to pipe functions, when a promise in the promise in the middle checks authorization?

Question

i'm trying to compose some functions together:

compose = (...fns) => fns.reduce((f, g) => (...args) => f(g(...args)));

checkAuthorization returns a promise that check if a user is authorized. buildParams receives someRequestData, and pipes the result to searchItem.

    checkAuthorization()
      .then(() => {
            compose(
                searchItem,
                buildParams
            )(someRequestData)
    }, (e) => {
       handleError(e)
    })

I think it's OK, but I wish to have a more elegant look for readability, something like:

    compose(
      searchItem,
      checkAuthorization
      buildParams
    )(someRequestData)

so what will happen is: 1) build params 2) checkAuth 3) search item

Any suggestions?

Answer 1

No, that's not possible, since checkAuthorisation does not receive and pass through the params. And even if you would rewrite it to do that, it still would be weird and a reader would assume that you're building the params whose authorisation should be checked. So don't do that - you have a non-linear flow, and trying to force it into some linear composition is no good.

Btw, I would recommend to avoid compose when you're calling the function immediately anyway:

checkAuthorization().then(() =>
  searchItem(buildParams(someRequestData))
, e =>
   handleError(e)
);

or maybe

checkAuthorization().then( compose(searchItem, buildParams, ()=>someRequestData)
                         , handleError ); //                ^^^^ "const"

Answer 2

Here's a composer to handle both sync functions and Promises. Looks like it works correctly maintaining the order:

// Async pipe try. Pass functions left to right
const pipePromises = (...fns) => x => fns.reduce((p, fn) => p.then(fn), Promise.resolve(x));
  
// functions for the test
const me = x => new Promise(function(resolve, reject) {
  setTimeout(() => resolve(x), 10)
})

const double = x => new Promise(function(resolve, reject) {
  setTimeout(() => resolve(x * 2), 30)
})

const inc = x => new Promise(function(resolve, reject) {
  setTimeout(() => resolve(x + 1), 400)
})

const log = x => { console.log('log: ', x); return x }

const syncTriple = x => x * 3; // sync function

// let's call our chain
pipePromises( 
  me, log, // 3
  double, log, // 6
  syncTriple, log, // 18 -- SYNC
  inc, log, // 19
  double, log, // 38
  inc, log, // 39
  syncTriple, log, // 117 -- SYNC
  inc, log // 118
)(3) // 3