Added at, first and last

README.md

@@ -44,6 +44,17 @@ n(1)  // => false
 n(2)  // => true
 ```
 
+**at** gets an array's item by position:
+
+```typescript
+const getthird = at(2);
+getthird([2, 4, 8, 16]);      // => 8
+getthird([2, 4]);             // => undefined
+const getsecondlast = at(-2);
+getsecondlast([1, 2, 3, 4]);  // => 3
+getsecondlast([1]);           // => undefined
+```
+
 **attr** gets an object's attribute:
 
 ```typescript
@@ -80,6 +91,15 @@ bool({x: 1});  // => true
 [-2, 8, -7].sort(cmp({ key: Math.abs }))         // => [-2, -7, 8]
 ```
 
+**first** and **last** return the first or last item of an array:
+
+```typescript
+first([1, 2, 3])  // => 1
+first([])         // => undefined
+last([1, 2, 3])   // => 3
+last([])          // => undefined
+```
+
 **identity** returns its argument untouched:
 
 ```typescript

package.json

@@ -1,6 +1,6 @@
 {
   "name": "tk-functional",
-  "version": "0.2.1",
+  "version": "0.2.2",
   "description": "Typescript/Javascript helpers for functional-style programming",
   "source": "src/index.ts",
   "main": "dist/tk-functional.umd.js",

src/index.test.ts

@@ -1,4 +1,4 @@
-import { always, and, attr, bool, cmp, fmap, identity, is, isinstance, never, nn, nnu, nop, not, nu, or, partial, pipe } from "./index";
+import { always, and, at, attr, bool, cmp, first, fmap, identity, is, isinstance, last, never, nn, nnu, nop, not, nu, or, partial, pipe } from "./index";
 
 describe(nop, () => {
   it("does nothing", () => {
@@ -136,6 +136,36 @@ describe(attr, () => {
   });
 });
 
+describe(at, () => {
+  it("gets an index in an array", () => {
+    const second = at(1);
+    expect(second([1, 4, 8])).toBe(4);
+    expect(second([1])).toBeUndefined();
+  });
+
+  it("handles negative indices from the end", () => {
+    const second_from_end = at(-2);
+    expect(second_from_end([1, 4, 6, 8])).toBe(6);
+    expect(second_from_end([1])).toBeUndefined();
+  });
+});
+
+describe(first, () => {
+  it("gets the first item in an array", () => {
+    expect(first([1, 4, 8])).toBe(1);
+    expect(first([1])).toBe(1);
+    expect(first([])).toBeUndefined();
+  });
+});
+
+describe(last, () => {
+  it("gets the last item in an array", () => {
+    expect(last([1, 4, 8])).toBe(8);
+    expect(last([1])).toBe(1);
+    expect(last([])).toBeUndefined();
+  });
+});
+
 describe(nn, () => {
   it("checks for null", () => {
     expect(nn(undefined)).toBeUndefined();

src/index.ts

@@ -111,6 +111,27 @@ export function attr<K extends string>(name: K): <V, O extends Record<K, V>>(obj
   return obj => obj[name];
 }
 
+/**
+ * Index getter
+ */
+export function at<T>(idx: number): (array: ReadonlyArray<T>) => T | undefined {
+  if (idx < 0) {
+    return array => array[array.length + idx];
+  } else {
+    return array => array[idx];
+  }
+}
+
+/**
+ * Array first item getter
+ */
+export const first: <T>(array: ReadonlyArray<T>) => T | undefined = at(0);
+
+/**
+ * Array last item getter
+ */
+export const last: <T>(array: ReadonlyArray<T>) => T | undefined = at(-1);
+
 /**
  * Check that a value is not null, throwing an error if it is
  */