a test to use scan as a reduce that just accumulate final result

src/functional.js

@@ -164,17 +164,38 @@ export const filter = (...args) => {
   return immediate ? run(items) : run
 }
 
+/**
+ * Accumulate values while scanning an iterable.
+ *
+ * By default `scan` behaves like the original implementation and returns a
+ * `{ results, errors, failure }` object containing every intermediate result.
+ *
+ * When the caller only cares about the final accumulated value (e.g. using
+ * `scan` as a pure reduce), set `storePartialResults: false`. In that mode the
+ * function returns `{ value, errors, failure }`, where `value` is the last
+ * successful accumulator.
+ *
+ * @param {AsyncIterable|Array} iterable - Source of values.
+ * @param {(accumulator, item) => Promise<Accumulator>} scanner
+ * @param {*} initialValue.
+ * @param {{
+ *  strategy?: StrategyFn, onError?, onFailure?, storePartialResults?: boolean
+ * }} opts
+ */
 // eslint-disable-next-line complexity
 export const scan = async (iterable, scanner, initialValue, opts = {}) => {
-  const {strategy = failFast, onError, onFailure} = opts
+  const {
+    strategy = failFast, onError, onFailure, storePartialResults = true,
+  } = opts
   const results = []
   let acc = initialValue
   const errors = []
 
   for await (const item of iterable) {
     try {
       acc = await scanner(acc, item)
-      results.push(acc)
+      if (storePartialResults)
+        results.push(acc)
     } catch (error) {
       const strategyName = strategy.name ?? strategy
 
@@ -186,12 +207,9 @@ export const scan = async (iterable, scanner, initialValue, opts = {}) => {
         if (onFailure) {
           onFailure({item, error})
         }
-        errors.push({item, error})
-        return {
-          results,
-          errors,
-          failure: {item, error},
-        }
+        return storePartialResults
+          ? {results, errors, failure: {item, error}}
+          : {result: acc, errors, failure: {item, error}}
       }
 
       if (strategyName === 'skip') {
@@ -209,7 +227,9 @@ export const scan = async (iterable, scanner, initialValue, opts = {}) => {
     onFailure(true)
   }
 
-  return {results, errors, failure}
+  return storePartialResults
+    ? {results, errors, failure}
+    : {result: acc, errors, failure}
 }
 
 export const pipe = (...fns) => input =>

tests/scan-reduce.test.js

@@ -0,0 +1,95 @@
+import {test, expect} from 'vitest'
+import {scan, failFast} from '$src/functional'
+
+test('returns final value when storePartialResults is false', async () => {
+  const tracks = [
+    {duration: 5},
+    {duration: 7},
+    {duration: 10},
+  ]
+
+  const {result: totalDuration} = await scan(
+    tracks,
+    (accumulator, {duration}) => accumulator + duration,
+    0,
+    {storePartialResults: false},
+  )
+
+  expect(totalDuration).toBe(22)
+})
+
+test('failFast stops on error with storePartialResults false', async () => {
+  const tracks = [
+    {duration: 5},
+    {duration: 7},
+    {duration: 10},
+  ]
+
+  const {result, failure, errors} = await scan(
+    tracks,
+    (accumulator, {duration}) => {
+      if (duration > 7)
+        throw new Error('Duration too long')
+      return accumulator + duration
+    },
+    0,
+    {strategy: failFast, storePartialResults: false},
+  )
+
+  expect(result).toBe(12) // 5 + 7 before error
+  expect(failure).toEqual({
+    item: {duration: 10},
+    error: new Error('Duration too long'),
+  })
+  // failFast doesn't collect errors in the errors array
+  expect(errors).toHaveLength(0)
+})
+
+test('failFast throws when accessing missing property', async () => {
+  const tracks = [
+    {duration: 5},
+    {}, // Missing duration property - will cause error when trying to access it
+    {duration: 10},
+  ]
+
+  const {result, failure} = await scan(
+    tracks,
+    (accumulator, item) => {
+      // Explicitly throw on missing property to test failFast behavior
+      if (item.duration === undefined)
+        throw new Error('Missing duration')
+      return accumulator + item.duration
+    },
+    0,
+    {strategy: failFast, storePartialResults: false},
+  )
+
+  expect(result).toBe(5) // Only first item processed
+  expect(failure.item).toEqual({})
+  expect(failure.error.message).toBe('Missing duration')
+})
+
+test(
+  'empty iterable returns initial value with storePartialResults false',
+  async () => {
+    const {result: total} = await scan(
+      [],
+      (accumulator, item) => accumulator + item,
+      42,
+      {storePartialResults: false},
+    )
+
+    expect(total).toBe(42)
+  }
+)
+
+test('default behavior unchanged with storePartialResults true', async () => {
+  const {results} = await scan(
+    [1, 2, 3],
+    (acc, item) => acc + item,
+    0,
+    {storePartialResults: true},
+  )
+
+  expect(results).toEqual([1, 3, 6])
+})