More functions (#3)

src/identity.ts

@@ -0,0 +1 @@
+export const identity = <T>(x: T): T => x;

src/last.test.ts

@@ -0,0 +1,13 @@
+import { it, expect } from "vitest";
+import { last } from "./last.js";
+
+for (const [xs, y] of [
+  [[], undefined],
+  [[1], 1],
+  [[1, 2], 2],
+  [[1, 2, 3], 3],
+] satisfies [unknown[], unknown][]) {
+  it(`finds the last element in an array ${JSON.stringify(xs)}`, () => {
+    expect(last(xs)).toBe(y);
+  });
+}

src/main.ts

@@ -1 +1,4 @@
+export * from "./identity.js";
+export * from "./last.js";
+export * from "./once.js";
 export * from "./unique.js";

src/once.test.ts

@@ -0,0 +1,17 @@
+import { it, expect } from "vitest";
+import { once } from "./once.js";
+
+it("calls a function twice", () => {
+  let x = 0;
+  const f = once(() => {
+    x++;
+  });
+
+  f();
+
+  expect(x).toBe(1);
+
+  f();
+
+  expect(x).toBe(1);
+});

src/once.ts

@@ -0,0 +1,15 @@
+export const once = <T extends unknown[], S>(
+  f: (...args: T) => S,
+): ((...args: T) => S) => {
+  let called = false;
+  let value: S;
+
+  return (...args: T): S => {
+    if (!called) {
+      called = true;
+      value = f(...args);
+    }
+
+    return value;
+  };
+};