asyncFilter()

utils/helpers.js

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+/**
+ * @format
+ */
+
+/**
+ * @template T
+ * @typedef {(value: T) => Promise<boolean>} AsyncFilterCallback
+ */
+
+/**
+ * @template T
+ * @param {T[]} arr
+ * @param {AsyncFilterCallback<T>} cb
+ * @returns {Promise<T[]>}
+ */
+const asyncFilter = async (arr, cb) => {
+  const results = await Promise.all(arr.map(item => cb(item)))
+
+  return arr.filter((_, i) => results[i])
+}
+
+module.exports = {
+  asyncFilter
+}

utils/helpers.spec.js

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+/**
+ * @format
+ */
+
+const { asyncFilter } = require('./helpers')
+
+const numbers = [1, 2, 3, 4]
+const odds = [1, 3]
+const evens = [2, 4]
+
+describe('asyncFilter()', () => {
+  it('returns an empty array when given one', async () => {
+    expect.hasAssertions()
+    const result = await asyncFilter([], () => true)
+
+    expect(result).toStrictEqual([])
+  })
+
+  it('rejects', async () => {
+    expect.hasAssertions()
+
+    const result = await asyncFilter(numbers, () => false)
+
+    expect(result).toStrictEqual([])
+  })
+
+  it('rejects via calling with the correct value', async () => {
+    expect.hasAssertions()
+
+    const result = await asyncFilter(numbers, v => v % 2 !== 0)
+
+    expect(result).toStrictEqual(odds)
+  })
+
+  it('filters via calling with the correct value', async () => {
+    expect.hasAssertions()
+
+    const result = await asyncFilter(numbers, v => v % 2 === 0)
+
+    expect(result).toStrictEqual(evens)
+  })
+
+  it('handles promises', async () => {
+    expect.hasAssertions()
+
+    const result = await asyncFilter(numbers, v => Promise.resolve(v % 2 === 0))
+    expect(result).toStrictEqual(evens)
+  })
+})

utils/index.js

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+/**
+ * @format
+ */
+
+const { asyncFilter } = require('./helpers')
+
+module.exports = {
+  asyncFilter
+}