feat: Add selectFirst

.github/contributing.md

@@ -16,8 +16,8 @@ Scan through the [existing issues](https://github.com/aleclarson/radashi/issues)
 
 ## You want to write code?
 
-- To get started, run `yarn` in the project's root directory to install the dependencies.
-- You can run the unit tests with `yarn test`. They require Node v16+. You can run `nvm use` in the root directory to change to the correct Node version. The tests should pass (duh) and maintain 100% code coverage.
+- To get started, run `pnpm i` in the project's root directory to install the dependencies.
+- You can run the unit tests with `pnpm test`. They require Node v16+. You can run `nvm use` in the root directory to change to the correct Node version. The tests should pass (duh) and maintain 100% code coverage.
 - To get familiar with the existing code I would recommend looking through the docs and the codebase in parallel. For each function in the docs, find the implementation in the source and skim over the code.
 - When coding, try not to create internal APIs (any function or module that is not exported to be used by the client).
 - Also, try not to use functions from other Radashi modules. This is a softer rule but we want to make it easy for readers to understand a function without having to navigate the codebase. As a utility library users should ideally be able to copy/paste a function from Radashi into their project. Most Radashi functions should be easy to write in isolation.

docs/array/select.mdx

@@ -7,6 +7,7 @@ description: Filter and map an array
 ## Basic usage
 
 Applies a filter and a map operation at once and in one pass.
+If the filter is omitted, returns all non-nullish mapped values.
 
 ```ts
 import * as _ from 'radashi'

docs/array/selectFirst.mdx

@@ -0,0 +1,38 @@
+---
+title: selectFirst
+group: 'Array'
+description: Array find + map
+---
+
+## Basic usage
+
+Returns the mapped value for the first element that satisfies the specified condition--else undefined.
+If the filter is omitted, returns the first non-nullish mapped value.
+
+```ts
+import * as _ from 'radashi'
+
+const fish = [
+  {
+    name: 'Marlin',
+    weight: 105,
+    source: 'ocean',
+  },
+  {
+    name: 'Bass',
+    weight: 8,
+    source: 'lake',
+  },
+  {
+    name: 'Trout',
+    weight: 13,
+    source: 'lake',
+  },
+]
+
+_.selectFirst(
+  fish,
+  f => f.weight,
+  f => f.source === 'lake',
+) // => 8
+```

src/array/select.ts

@@ -1,6 +1,7 @@
 /**
  * Select performs a filter and a mapper inside of a reduce, only
- * iterating the list one time.
+ * iterating the list one time. If condition is omitted, will
+ * select all mapped values that are non-nullish.
  *
  * ```ts
  * select(

src/array/selectFirst.ts

@@ -0,0 +1,29 @@
+/**
+ * Select performs a find + map operation, short-circuiting on the first
+ * element that satisfies the prescribed condition. If condition is omitted,
+ * will select the first mapped value which is non-nullish.
+ *
+ * ```ts
+ * selectFirst(
+ *   [1, 2, 3, 4],
+ *   x => x * x,
+ *   x => x > 2
+ * )
+ * // => 9
+ * ```
+ */
+export function selectFirst<T, U>(
+  array: readonly T[],
+  mapper: (item: T, index: number) => U,
+  condition?: (item: T, index: number) => boolean,
+): U | undefined {
+  if (!array) {
+    return undefined
+  }
+  let foundIndex = -1
+  const found = array.find((item, index) => {
+    foundIndex = index
+    return condition ? condition(item, index) : mapper(item, index) != null
+  })
+  return found === undefined ? undefined : mapper(found, foundIndex)
+}

src/array/tests/select.test.ts

@@ -52,7 +52,7 @@ describe('select function', () => {
     )
     expect(result).toEqual(['c2', 'd3'])
   })
-  test('works without a condition callback', () => {
+  test('works without a condition callback, filtering nullish mapped values', () => {
     const list = [{ a: 1 }, { b: 2 }, { a: 3 }, { a: null }, { a: undefined }]
     const result = _.select(list, obj => obj.a)
     expect(result).toEqual([1, 3])

src/array/tests/selectFirst.test.ts

@@ -0,0 +1,60 @@
+import * as _ from 'radashi'
+
+const cast = <T = any[]>(value: any): T => value
+
+describe('selectFirst function', () => {
+  test('does not fail on bad input', () => {
+    expect(
+      _.selectFirst(
+        cast(null),
+        x => x,
+        x => x,
+      ),
+    ).toBeUndefined()
+    expect(
+      _.selectFirst(
+        cast(undefined),
+        x => x,
+        x => x,
+      ),
+    ).toBeUndefined()
+  })
+  test('returns mapped result of first value that meets the condition', () => {
+    const list = [
+      { group: 'a', word: 'hello' },
+      { group: 'b', word: 'bye' },
+      { group: 'a', word: 'oh' },
+      { group: 'b', word: 'hey' },
+      { group: 'c', word: 'ok' },
+    ]
+    const result = _.selectFirst(
+      list,
+      x => x.word,
+      x => x.group === 'b',
+    )
+    expect(result).toEqual('bye')
+  })
+  test('does not fail on empty input list', () => {
+    const list: any[] = []
+    const result = _.selectFirst(
+      list,
+      (x: any) => x.word,
+      x => x.group === 'a',
+    )
+    expect(result).toBeUndefined()
+  })
+  test('works with index', () => {
+    const letters = ['a', 'b', 'c', 'd']
+    const result = _.selectFirst(
+      letters,
+      (l, idx) => `${l}${idx}`,
+      (_, idx) => idx > 1,
+    )
+    expect(result).toEqual('c2')
+  })
+  test('works without a condition callback, filtering nullish mapped values', () => {
+    const list = [{ a: null }, { a: undefined }, { b: 2 }, { a: 1 }, { a: 3 }]
+    const result = _.selectFirst(list, el => el.a)
+    expect(result).toEqual(1)
+  })
+})

src/mod.ts

@@ -19,6 +19,7 @@ export * from './array/range'
 export * from './array/replace'
 export * from './array/replaceOrAppend'
 export * from './array/select'
+export * from './array/selectFirst'
 export * from './array/shift'
 export * from './array/sift'
 export * from './array/sort'