feat: add rejectLate option

README.md

@@ -10,18 +10,24 @@ const promiseCallLimit = require('promise-call-limit')
 const things = getLongListOfThingsToFrobulate()
 
 // frobulate no more than 4 things in parallel
-promiseCallLimit(things.map(thing => () => frobulateThing(thing)), 4)
+promiseCallLimit(things.map(thing => () => frobulateThing(thing)), {
+limit: 4 })
   .then(results => console.log('frobulated 4 at a time', results))
 ```
 
 ## API
 
-### promiseCallLimit(queue Array<() => Promise>, limit = defaultLimit)
-
-The default limit is the number of CPUs on the system - 1, or 1.
-
-The reason for subtracting one is that presumably the main thread is taking
-up a CPU as well, so let's not be greedy.
+### promiseCallLimit(queue Array<() => Promise>, opts<Object>)
+
+opts can contain:
+  - limit: specified concurrency limit. Defaults to the number of
+    CPUs on the system minus one. Presumably the main thread is taking
+    up a CPU as well, so let's not be greedy.  In the case where there
+    is only one cpu the limit will default to 1.
+  - rejectLate: if true, then any rejection will not prevent the rest of
+    the queue from running.  Any subsequent rejections will be ignored,
+    and the first rejection will be what the function finally rejects
+    with.
 
 Note that the array should be a list of Promise-_returning_ functions, not
 Promises themselves.  If you have a bunch of Promises already, you're best

index.js

@@ -5,20 +5,24 @@ const os = require('os')
 
 /* istanbul ignore next - version-specific workaround */
 const defLimit = 'availableParallelism' in os
-  ? os.availableParallelism()
-  : Math.max(1, os.cpus().length)
+  ? Math.max(1, os.availableParallelism() - 1)
+  : Math.max(1, os.cpus().length - 1)
 
-const callLimit = (queue, limit = defLimit) => new Promise((res, rej) => {
+const callLimit = (queue, { limit = defLimit, rejectLate } = {}) => new Promise((res, rej) => {
   let active = 0
   let current = 0
   const results = []
 
+  // Whether or not we rejected, distinct from the rejection just in case the rejection itself is falsey
   let rejected = false
+  let rejection
   const reject = er => {
     if (rejected)
       return
     rejected = true
-    rej(er)
+    rejection = er
+    if (!rejectLate)
+      rej(rejection)
   }
 
   let resolved = false
@@ -31,22 +35,27 @@ const callLimit = (queue, limit = defLimit) => new Promise((res, rej) => {
 
   const run = () => {
     const c = current++
-    if (c >= queue.length) {
-      return resolve()
-    }
+    if (c >= queue.length)
+      return rejected ? reject() : resolve()
 
     active ++
     results[c] = queue[c]().then(result => {
       active --
       results[c] = result
+      return result
+    }, (er) => {
+      active --
+      reject(er)
+    }).then(result => {
+      if (rejected && active === 0)
+        return rej(rejection)
       run()
       return result
-    }, reject)
+    })
   }
 
-  for (let i = 0; i < limit; i++) {
+  for (let i = 0; i < limit; i++)
     run()
-  }
 })
 
 module.exports = callLimit

test/basic.js

@@ -27,7 +27,7 @@ t.test('two by two', t => {
       calledTre = true
       res(3)
     }, 50)),
-  ], 2).then(res => t.strictSame(res, [1, 2, 3]))
+  ], { limit: 2 }).then(res => t.strictSame(res, [1, 2, 3]))
 })
 
 t.test('rejection', t => t.rejects(callLimit([
@@ -39,3 +39,28 @@ t.test('triple rejection', t => t.rejects(callLimit([
   () => Promise.reject(new Error('poop')),
   () => Promise.reject(new Error('poop')),
 ]), { message: 'poop' }))
+
+t.test('late rejection', async t => {
+  const results = []
+  await t.rejects(callLimit([
+    () => new Promise(resolve => setTimeout(() => {
+      results.push('first success')
+      resolve('ok')
+    }, 50)),
+    () => new Promise((_, reject) => {
+      setTimeout(() => {
+        results.push('slow rejection')
+        reject(new Error('slow rejection'))
+      }, 100)
+    }),
+    () => {
+      results.push('fast rejection')
+      return Promise.reject(new Error('fast rejection'))
+    },
+    () => new Promise(resolve => setTimeout(() => {
+      results.push('second success')
+      resolve('ok 2')
+    }, 50)),
+  ], { limit: 2, rejectLate: true }), { message: 'fast rejection' })
+  t.match(results, ['first success', 'fast rejection', 'slow rejection', 'second success'])
+})