feat: 🎸 implement "once" class method decorator

src/__tests__/once.spec.ts

@@ -0,0 +1,17 @@
+import {once} from '../once';
+
+test('limits function invocation to one time only an memoizes the first result', async () => {
+  let executions = 0;
+  class A {
+    @once
+    test(value: number) {
+      executions++;
+      return value;
+    }
+  }
+  const a = new A();
+  expect(a.test(123)).toBe(123);
+  expect(a.test(1)).toBe(123);
+  expect(a.test(2)).toBe(123);
+  expect(executions).toBe(1);
+});

src/index.ts

@@ -2,6 +2,7 @@ export * from './base64';
 export * from './Cache';
 export * from './codeMutex';
 export * from './concurrency';
+export {once} from './once';
 export {concurrency as concurrencyDecorator} from './concurrencyDecorator';
 export * from './dataUri';
 export * from './Defer';

src/once.ts

@@ -0,0 +1,18 @@
+/**
+ * A class method decorator that limits a method to be called only once. All
+ * subsequent calls will return the result of the first call.
+ */
+export function once<This, Args extends any[], Return>(
+  target: (this: This, ...args: Args) => Return,
+  context?: ClassMethodDecoratorContext<This, (this: This, ...args: Args) => Return>,
+) {
+  let called = false;
+  let res: unknown;
+  return function (this: This, ...args: Args): Return {
+    if (!called) {
+      called = true;
+      res = target.call(this, ...args);
+    }
+    return res as Return;
+  };
+}