lib: add defaultValue and name options to AsyncLocalStorage

The upcoming `AsyncContext` specification adds a default value and name
to async storage variables. This adds the same to `AsyncLocalStorage`
to promote closer alignment with the pending spec.

```js
const als = new AsyncLocalStorage({
  name: 'foo',
  defaultValue: 123,
});

console.log(als.name);  // 'foo'
console.log(als.getStore());  // 123
```

Refs: https://github.com/tc39/proposal-async-context/blob/master/spec.html

Author: jasnell

doc/api/async_context.md

@@ -116,13 +116,16 @@ Each instance of `AsyncLocalStorage` maintains an independent storage context.
 Multiple instances can safely exist simultaneously without risk of interfering
 with each other's data.
 
-### `new AsyncLocalStorage()`
+### `new AsyncLocalStorage([options])`
 
 <!-- YAML
 added:
  - v13.10.0
  - v12.17.0
 changes:
+ - version: REPLACEME
+   pr-url: https://github.com/nodejs/node/pull/00000
+   description: Add `defaultValue` and `name` options.
  - version:
     - v19.7.0
     - v18.16.0
@@ -135,6 +138,10 @@ changes:
    description: Add option onPropagate.
 -->
 
+* `options` {Object}
+  * `defaultValue` {any} The default value to be used when no store is provided.
+  * `name` {string} A name for the `AsyncLocalStorage` value.
+
 Creates a new instance of `AsyncLocalStorage`. Store is only provided within a
 `run()` call or after an `enterWith()` call.
 
@@ -286,6 +293,16 @@ emitter.emit('my-event');
 asyncLocalStorage.getStore(); // Returns the same object
 ```
 
+### `asyncLocalStorage.name`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* {string|undefined}
+
+The name of the `AsyncLocalStorage` instance if provided.
+
 ### `asyncLocalStorage.run(store, callback[, ...args])`
 
 <!-- YAML

lib/internal/async_local_storage/async_context_frame.js

@@ -4,10 +4,29 @@ const {
   ReflectApply,
 } = primordials;
 
+const {
+  validateObject,
+  validateString,
+} = require('internal/validators');
+
 const AsyncContextFrame = require('internal/async_context_frame');
 const { AsyncResource } = require('async_hooks');
 
 class AsyncLocalStorage {
+  #defaultValue = undefined;
+  #name = undefined;
+
+  constructor(options = {}) {
+    validateObject(options, 'options');
+    this.#defaultValue = options.defaultValue;
+
+    if (options.name !== undefined) {
+      validateString(options.name, 'options.name');
+      this.#name = options.name;
+    }
+  }
+
+  get name() { return this.#name; }
   static bind(fn) {
     return AsyncResource.bind(fn);
   }
@@ -40,7 +59,11 @@ class AsyncLocalStorage {
   }
 
   getStore() {
-    return AsyncContextFrame.current()?.get(this);
+    const frame = AsyncContextFrame.current();
+    if (!frame?.has(this)) {
+      return this.#defaultValue;
+    }
+    return frame?.get(this);
   }
 }
 

lib/internal/async_local_storage/async_hooks.js

@@ -9,6 +9,11 @@ const {
   Symbol,
 } = primordials;
 
+const {
+  validateObject,
+  validateString,
+} = require('internal/validators');
+
 const {
   AsyncResource,
   createHook,
@@ -27,11 +32,23 @@ const storageHook = createHook({
 });
 
 class AsyncLocalStorage {
-  constructor() {
+  #defaultValue = undefined;
+  #name = undefined;
+
+  constructor(options = {}) {
     this.kResourceStore = Symbol('kResourceStore');
     this.enabled = false;
+    validateObject(options, 'options');
+    this.#defaultValue = options.defaultValue;
+
+    if (options.name !== undefined) {
+      validateString(options.name, 'options.name');
+      this.#name = options.name;
+    }
   }
 
+  get name() { return this.#name; }
+
   static bind(fn) {
     return AsyncResource.bind(fn);
   }
@@ -109,8 +126,12 @@ class AsyncLocalStorage {
   getStore() {
     if (this.enabled) {
       const resource = executionAsyncResource();
+      if (!(this.kResourceStore in resource)) {
+        return this.#defaultValue;
+      }
       return resource[this.kResourceStore];
     }
+    return this.#defaultValue;
   }
 }
 

test/parallel/test-als-defaultvalue-original.js

@@ -0,0 +1,44 @@
+// Flags: --no-async-context-frame
+'use strict';
+
+require('../common');
+
+const {
+  AsyncLocalStorage,
+} = require('async_hooks');
+
+const {
+  strictEqual,
+  throws,
+} = require('assert');
+
+// ============================================================================
+// The defaultValue option
+const als1 = new AsyncLocalStorage();
+strictEqual(als1.getStore(), undefined, 'value should be undefined');
+
+const als2 = new AsyncLocalStorage({ defaultValue: 'default' });
+strictEqual(als2.getStore(), 'default', 'value should be "default"');
+
+const als3 = new AsyncLocalStorage({ defaultValue: 42 });
+strictEqual(als3.getStore(), 42, 'value should be 42');
+
+const als4 = new AsyncLocalStorage({ defaultValue: null });
+strictEqual(als4.getStore(), null, 'value should be null');
+
+throws(() => new AsyncLocalStorage(null), {
+  code: 'ERR_INVALID_ARG_TYPE',
+});
+
+// ============================================================================
+// The name option
+
+const als5 = new AsyncLocalStorage({ name: 'test' });
+strictEqual(als5.name, 'test');
+
+const als6 = new AsyncLocalStorage();
+strictEqual(als6.name, undefined);
+
+throws(() => new AsyncLocalStorage({ name: 1 }), {
+  code: 'ERR_INVALID_ARG_TYPE',
+});

test/parallel/test-als-defaultvalue.js

@@ -0,0 +1,44 @@
+// Flags: --async-context-frame
+'use strict';
+
+require('../common');
+
+const {
+  AsyncLocalStorage,
+} = require('async_hooks');
+
+const {
+  strictEqual,
+  throws,
+} = require('assert');
+
+// ============================================================================
+// The defaultValue option
+const als1 = new AsyncLocalStorage();
+strictEqual(als1.getStore(), undefined, 'value should be undefined');
+
+const als2 = new AsyncLocalStorage({ defaultValue: 'default' });
+strictEqual(als2.getStore(), 'default', 'value should be "default"');
+
+const als3 = new AsyncLocalStorage({ defaultValue: 42 });
+strictEqual(als3.getStore(), 42, 'value should be 42');
+
+const als4 = new AsyncLocalStorage({ defaultValue: null });
+strictEqual(als4.getStore(), null, 'value should be null');
+
+throws(() => new AsyncLocalStorage(null), {
+  code: 'ERR_INVALID_ARG_TYPE',
+});
+
+// ============================================================================
+// The name option
+
+const als5 = new AsyncLocalStorage({ name: 'test' });
+strictEqual(als5.name, 'test');
+
+const als6 = new AsyncLocalStorage();
+strictEqual(als6.name, undefined);
+
+throws(() => new AsyncLocalStorage({ name: 1 }), {
+  code: 'ERR_INVALID_ARG_TYPE',
+});