Throttle amount of promises open at a given time

Question

The following Typescript performs each call to doSomething(action) one at a time. (Meaning the second item in the list does not get a call made until the first

Answer 1

EDIT

Jeff Bowman has vastly improved his answer to resolve meaningful values. Feel free to view the history of this answer to understand why the resolved values are so important/useful.

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throttlep

This solution closely mimics the native Promise.all

How it's the same …

• Resolves promises as quickly as possible
• Resolves an array of values in the same order as the inputs
• Rejects as soon as it encounters one reject

How it's different …

• Number parameter limits the number of simultaneously-running Promises
• Array input accepts promise creators (thunks); not actual Promises

// throttlep :: Number -> [(* -> Promise)]
const throttlep = n=> Ps=>
  new Promise ((pass, fail)=> {
    // r is the number of promises, xs is final resolved value
    let r = Ps.length, xs = []
    // decrement r, save the resolved value in position i, run the next promise
    let next = i=> x=> (r--, xs[i] = x, run(Ps[n], n++))
    // if r is 0, we can resolve the final value xs, otherwise chain next
    let run = (P,i)=> r === 0 ? pass(xs) : P().then(next(i), fail)
    // initialize by running the first n promises
    Ps.slice(0,n).forEach(run)
  })

// -----------------------------------------------------
// make sure it works

// delay :: (String, Number) -> (* -> Promise)
const delay = (id, ms)=>
  new Promise (pass=> {
    console.log (`running: ${id}`)
    setTimeout(pass, ms, id)
  })

// ps :: [(* -> Promise)]
let ps = new Array(10)
for (let i = 0; i < 10; i++) {
  ps[i] = () => delay(i, Math.random() * 3000)
}

// run a limit of 3 promises in parallel
// the first error will reject the entire pool
throttlep (3) (ps) .then (
  xs => console.log ('result:', xs),
  err=> console.log ('error:', err.message)
)

Console output

Inputs are run in order; Resolved results are in the same order as the inputs

running: 0
running: 1
running: 2
=> Promise {}
running: 3
running: 4
running: 5
running: 6
running: 7
running: 8
running: 9
result: [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

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Practical use

Let's look at a more practical code example. This code is tasked with fetching a set of images from a server. This is how we might use throttlep to throttle the amount of simultaneous requests to 3 at a time

// getImage :: String -> Promise
let getImage = url=> makeRequest(url).then(data => data.base64, reqErrorHandler)

// actions :: [(* -> Promise)]
let actions = [
  ()=> getImage('one.jpg'),
  ()=> getImage('two.jpg'),
  ()=> getImage('three.jpg'),
  ()=> getImage('four.jpg'),
  ()=> getImage('five.jpg')
]

// throttle the actions then do something...
throttlep (3) (actions) .then(results => {
  // results are guaranteed to be ordered the same as the input array
  console.log(results)
  // [, , , , ]
})