From d1434ae30f9b901ad1a48319ee0eb548ffbf7a38 Mon Sep 17 00:00:00 2001
From: streamich <streamich@gmail.com>
Date: Thu, 14 Mar 2024 10:12:10 +0100
Subject: [PATCH] =?UTF-8?q?feat:=20=F0=9F=8E=B8=20implement=20"once"=20cla?=
 =?UTF-8?q?ss=20method=20decorator?=
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---
 src/__tests__/once.spec.ts | 17 +++++++++++++++++
 src/index.ts               |  1 +
 src/once.ts                | 18 ++++++++++++++++++
 3 files changed, 36 insertions(+)
 create mode 100644 src/__tests__/once.spec.ts
 create mode 100644 src/once.ts

diff --git a/src/__tests__/once.spec.ts b/src/__tests__/once.spec.ts
new file mode 100644
index 0000000..c8ba3d4
--- /dev/null
+++ b/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);
+});
diff --git a/src/index.ts b/src/index.ts
index 3370799..5f1dc7a 100644
--- a/src/index.ts
+++ b/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';
diff --git a/src/once.ts b/src/once.ts
new file mode 100644
index 0000000..62f2f72
--- /dev/null
+++ b/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;
+  };
+}