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ECMAScript Explicit Resource Management

NOTE: This proposal has subsumed the Async Explicit Resource Management proposal. This proposal repository should be used for further discussion of both sync and async of explicit resource management.

This proposal intends to address a common pattern in software development regarding the lifetime and management of various resources (memory, I/O, etc.). This pattern generally includes the allocation of a resource and the ability to explicitly release critical resources.

For example, ECMAScript Generator Functions and Async Generator Functions expose this pattern through the return method, as a means to explicitly evaluate finally blocks to ensure user-defined cleanup logic is preserved:

// sync generators
function * g() {
  const handle = acquireFileHandle(); // critical resource
  try {
    ...
  }
  finally {
    handle.release(); // cleanup
  }
}

const obj = g();
try {
  const r = obj.next();
  ...
}
finally {
  obj.return(); // calls finally blocks in `g`
}

// async generators
async function * g() {
  const handle = acquireStream(); // critical resource
  try {
    ...
  }
  finally {
    await stream.close(); // cleanup
  }
}

const obj = g();
try {
  const r = await obj.next();
  ...
}
finally {
  await obj.return(); // calls finally blocks in `g`
}

As such, we propose the adoption of a novel syntax to simplify this common pattern:

// sync disposal
function * g() {
  using handle = acquireFileHandle(); // block-scoped critical resource
} // cleanup

{
  using obj = g(); // block-scoped declaration
  const r = obj.next();
} // calls finally blocks in `g`

// async disposal
async function * g() {
  using stream = acquireStream(); // block-scoped critical resource
  ...
} // cleanup

{
  await using obj = g(); // block-scoped declaration
  const r = await obj.next();
} // calls finally blocks in `g`

In addition, we propose the addition of two disposable container objects to assist with managing multiple resources:

Status

Stage: 3
Champion: Ron Buckton (@rbuckton)
Last Presented: March, 2023 (slides, notes #1, notes #2)

For more information see the TC39 proposal process.

Authors

Motivations

This proposal is motivated by a number of cases:

Prior Art

<!-- Links to similar concepts in existing languages, prior proposals, etc. -->

Definitions

Syntax

using Declarations

// a synchronously-disposed, block-scoped resource
using x = expr1;            // resource w/ local binding
using y = expr2, z = expr4; // multiple resources

Grammar

Please refer to the specification text for the most recent version of the grammar.

await using Declarations

// an asynchronously-disposed, block-scoped resource
await using x = expr1;            // resource w/ local binding
await using y = expr2, z = expr4; // multiple resources

An await using declaration can appear in the following contexts:

await using in for-of and for-await-of Statements

for (await using x of y) ...

for await (await using x of y) ...

You can use an await using declaration in a for-of or for-await-of statement inside of an async context to explicitly bind each iterated value as an async disposable resource. for-await-of does not implicitly make a non-async using declaration into an async await using declaration, as the await markers in for-await-of and await using are explicit indicators for distinct cases: for await only indicates async iteration, while await using only indicates async disposal. For example:


// sync iteration, sync disposal
for (using x of y) ; // no implicit `await` at end of each iteration

// sync iteration, async disposal
for (await using x of y) ; // implicit `await` at end of each iteration

// async iteration, sync disposal
for await (using x of y) ; // implicit `await` at end of each iteration

// async iteration, async disposal
for await (await using x of y) ; // implicit `await` at end of each iteration

While there is some overlap in that the last three cases introduce some form of implicit await during execution, it is intended that the presence or absence of the await modifier in a using declaration is an explicit indicator as to whether we are expecting the iterated value to have an @@asyncDispose method. This distinction is in line with the behavior of for-of and for-await-of:

const iter = { [Symbol.iterator]() { return [].values(); } };
const asyncIter = { [Symbol.asyncIterator]() { return [].values(); } };

for (const x of iter) ; // ok: `iter` has @@iterator
for (const x of asyncIter) ; // throws: `asyncIter` does not have @@iterator

for await (const x of iter) ; // ok: `iter` has @@iterator (fallback)
for await (const x of asyncIter) ; // ok: `asyncIter` has @@asyncIterator

using and await using have the same distinction:

const res = { [Symbol.dispose]() {} };
const asyncRes = { [Symbol.asyncDispose]() {} };

using x = res; // ok: `res` has @@dispose
using x = asyncRes; // throws: `asyncRes` does not have @@dispose

await using x = res; // ok: `res` has @@dispose (fallback)
await using x = asyncres; // ok: `asyncRes` has @@asyncDispose

This results in a matrix of behaviors based on the presence of each await marker:

const res = { [Symbol.dispose]() {} };
const asyncRes = { [Symbol.asyncDispose]() {} };
const iter = { [Symbol.iterator]() { return [res, asyncRes].values(); } };
const asyncIter = { [Symbol.asyncIterator]() { return [res, asyncRes].values(); } };

for (using x of iter) ;
// sync iteration, sync disposal
// - `iter` has @@iterator: ok
// - `res` has @@dispose: ok
// - `asyncRes` does not have @@dispose: *error*

for (using x of asyncIter) ;
// sync iteration, sync disposal
// - `asyncIter` does not have @@iterator: *error*

for (await using x of iter) ;
// sync iteration, async disposal
// - `iter` has @@iterator: ok
// - `res` has @@dispose (fallback): ok
// - `asyncRes` has @@asyncDispose: ok

for (await using x of asyncIter) ;
// sync iteration, async disposal
// - `asyncIter` does not have @@iterator: error

for await (using x of iter) ;
// async iteration, sync disposal
// - `iter` has @@iterator (fallback): ok
// - `res` has @@dispose: ok
// - `asyncRes` does not have @@dispose: error

for await (using x of asyncIter) ;
// async iteration, sync disposal
// - `asyncIter` has @@asyncIterator: ok
// - `res` has @@dispose: ok
// - `asyncRes` does not have @@dispose: error

for await (await using x of iter) ;
// async iteration, async disposal
// - `iter` has @@iterator (fallback): ok
// - `res` has @@dispose (fallback): ok
// - `asyncRes` does has @@asyncDispose: ok

for await (await using x of asyncIter) ;
// async iteration, async disposal
// - `asyncIter` has @@asyncIterator: ok
// - `res` has @@dispose (fallback): ok
// - `asyncRes` does has @@asyncDispose: ok

Or, in table form:

SyntaxIterationDisposal
for (using x of y)@@iterator@@dispose
for (await using x of y)@@iterator@@asyncDispose/@@dispose
for await (using x of y)@@asyncIterator/@@iterator@@dispose
for await (await using x of y)@@asyncIterator/@@iterator@@asyncDispose/@@dispose

Semantics

using Declarations

using Declarations with Explicit Local Bindings

UsingDeclaration :
  `using` BindingList `;`

LexicalBinding :
    BindingIdentifier Initializer

When a using declaration is parsed with BindingIdentifier Initializer, the bindings created in the declaration are tracked for disposal at the end of the containing Block or Module (a using declaration cannot be used at the top level of a Script):

{
  ... // (1)
  using x = expr1;
  ... // (2)
}

The above example has similar runtime semantics as the following transposed representation:

{
  const $$try = { stack: [], error: undefined, hasError: false };
  try {
    ... // (1)

    const x = expr1;
    if (x !== null && x !== undefined) {
      const $$dispose = x[Symbol.dispose];
      if (typeof $$dispose !== "function") {
        throw new TypeError();
      }
      $$try.stack.push({ value: x, dispose: $$dispose });
    }

    ... // (2)
  }
  catch ($$error) {
    $$try.error = $$error;
    $$try.hasError = true;
  }
  finally {
    while ($$try.stack.length) {
      const { value: $$expr, dispose: $$dispose } = $$try.stack.pop();
      try {
        $$dispose.call($$expr);
      }
      catch ($$error) {
        $$try.error = $$try.hasError ? new SuppressedError($$error, $$try.error) : $$error;
        $$try.hasError = true;
      }
    }
    if ($$try.hasError) {
      throw $$try.error;
    }
  }
}

If exceptions are thrown both in the block following the using declaration and in the call to [Symbol.dispose](), all exceptions are reported.

using Declarations with Multiple Resources

A using declaration can mix multiple explicit bindings in the same declaration:

{
  ...
  using x = expr1, y = expr2;
  ...
}

These bindings are again used to perform resource disposal when the Block or Module exits, however in this case [Symbol.dispose]() is invoked in the reverse order of their declaration. This is approximately equivalent to the following:

{
  ... // (1)
  using x = expr1;
  using y = expr2;
  ... // (2)
}

Both of the above cases would have similar runtime semantics as the following transposed representation:

{
  const $$try = { stack: [], error: undefined, hasError: false };
  try {
    ... // (1)

    const x = expr1;
    if (x !== null && x !== undefined) {
      const $$dispose = x[Symbol.dispose];
      if (typeof $$dispose !== "function") {
        throw new TypeError();
      }
      $$try.stack.push({ value: x, dispose: $$dispose });
    }

    const y = expr2;
    if (y !== null && y !== undefined) {
      const $$dispose = y[Symbol.dispose];
      if (typeof $$dispose !== "function") {
        throw new TypeError();
      }
      $$try.stack.push({ value: y, dispose: $$dispose });
    }

    ... // (2)
  }
  catch ($$error) {
    $$try.error = $$error;
    $$try.hasError = true;
  }
  finally {
    while ($$try.stack.length) {
      const { value: $$expr, dispose: $$dispose } = $$try.stack.pop();
      try {
        $$dispose.call($$expr);
      }
      catch ($$error) {
        $$try.error = $$try.hasError ? new SuppressedError($$error, $$try.error) : $$error;
        $$try.hasError = true;
      }
    }
    if ($$try.hasError) {
      throw $$try.error;
    }
  }
}

Since we must always ensure that we properly release resources, we must ensure that any abrupt completion that might occur during binding initialization results in evaluation of the cleanup step. When there are multiple declarations in the list, we track each resource in the order they are declared. As a result, we must release these resources in reverse order.

using Declarations and null or undefined Values

This proposal has opted to ignore null and undefined values provided to the using declarations. This is similar to the behavior of using in C#, which also allows null. One primary reason for this behavior is to simplify a common case where a resource might be optional, without requiring duplication of work or needless allocations:

if (isResourceAvailable()) {
  using resource = getResource();
  ... // (1)
  resource.doSomething()
  ... // (2)
}
else {
  // duplicate code path above
  ... // (1) above
  ... // (2) above
}

Compared to:

using resource = isResourceAvailable() ? getResource() : undefined;
... // (1) do some work with or without resource
resource?.doSomething();
... // (2) do some other work with or without resource

using Declarations and Values Without [Symbol.dispose]

If a resource does not have a callable [Symbol.dispose] member, a TypeError would be thrown immediately when the resource is tracked.

using Declarations in for-of and for-await-of Loops

A using declaration may occur in the ForDeclaration of a for-of or for-await-of loop:

for (using x of iterateResources()) {
  // use x
}

In this case, the value bound to x in each iteration will be synchronously disposed at the end of each iteration. This will not dispose resources that are not iterated, such as if iteration is terminated early due to return, break, or throw.

using declarations may not be used in in the head of a for-in loop.

await using Declarations

await using Declarations with Explicit Local Bindings

UsingDeclaration :
  `await` `using` BindingList `;`

LexicalBinding :
    BindingIdentifier Initializer

When an await using declaration is parsed with BindingIdentifier Initializer, the bindings created in the declaration are tracked for disposal at the end of the containing async function body, Block, or Module:

{
  ... // (1)
  await using x = expr1;
  ... // (2)
}

The above example has similar runtime semantics as the following transposed representation:

{
  const $$try = { stack: [], error: undefined, hasError: false };
  try {
    ... // (1)

    const x = expr1;
    if (x !== null && x !== undefined) {
      let $$dispose = x[Symbol.asyncDispose];
      if (typeof $$dispose !== "function") {
        $$dispose = x[Symbol.dispose];
      }
      if (typeof $$dispose !== "function") {
        throw new TypeError();
      }
      $$try.stack.push({ value: x, dispose: $$dispose });
    }

    ... // (2)
  }
  catch ($$error) {
    $$try.error = $$error;
    $$try.hasError = true;
  }
  finally {
    while ($$try.stack.length) {
      const { value: $$expr, dispose: $$dispose } = $$try.stack.pop();
      try {
        await $$dispose.call($$expr);
      }
      catch ($$error) {
        $$try.error = $$try.hasError ? new SuppressedError($$error, $$try.error) : $$error;
        $$try.hasError = true;
      }
    }
    if ($$try.hasError) {
      throw $$try.error;
    }
  }
}

If exceptions are thrown both in the statements following the await using declaration and in the call to [Symbol.asyncDispose](), all exceptions are reported.

await using Declarations with Multiple Resources

An await using declaration can mix multiple explicit bindings in the same declaration:

{
  ...
  await using x = expr1, y = expr2;
  ...
}

These bindings are again used to perform resource disposal when the Block or Module exits, however in this case each resource's [Symbol.asyncDispose]() is invoked in the reverse order of their declaration. This is approximately equivalent to the following:

{
  ... // (1)
  await using x = expr1;
  await using y = expr2;
  ... // (2)
}

Both of the above cases would have similar runtime semantics as the following transposed representation:

{
  const $$try = { stack: [], error: undefined, hasError: false };
  try {
    ... // (1)

    const x = expr1;
    if (x !== null && x !== undefined) {
      let $$dispose = x[Symbol.asyncDispose];
      if (typeof $$dispose !== "function") {
        $$dispose = x[Symbol.dispose];
      }
      if (typeof $$dispose !== "function") {
        throw new TypeError();
      }
      $$try.stack.push({ value: x, dispose: $$dispose });
    }

    const y = expr2;
    if (y !== null && y !== undefined) {
      let $$dispose = y[Symbol.asyncDispose];
      if (typeof $$dispose !== "function") {
        $$dispose = y[Symbol.dispose];
      }
      if (typeof $$dispose !== "function") {
        throw new TypeError();
      }
      $$try.stack.push({ value: y, dispose: $$dispose });
    }

    ... // (2)
  }
  catch ($$error) {
    $$try.error = $$error;
    $$try.hasError = true;
  }
  finally {
    while ($$try.stack.length) {
      const { value: $$expr, dispose: $$dispose } = $$try.stack.pop();
      try {
        await $$dispose.call($$expr);
      }
      catch ($$error) {
        $$try.error = $$try.hasError ? new SuppressedError($$error, $$try.error) : $$error;
        $$try.hasError = true;
      }
    }
    if ($$try.hasError) {
      throw $$try.error;
    }
  }
}

Since we must always ensure that we properly release resources, we must ensure that any abrupt completion that might occur during binding initialization results in evaluation of the cleanup step. When there are multiple declarations in the list, we track each resource in the order they are declared. As a result, we must release these resources in reverse order.

await using Declarations and null or undefined Values

This proposal has opted to ignore null and undefined values provided to await using declarations. This is consistent with the proposed behavior for the using declarations in this proposal. Like in the sync case, this allows simplifying a common case where a resource might be optional, without requiring duplication of work or needless allocations:

if (isResourceAvailable()) {
  await using resource = getResource();
  ... // (1)
  resource.doSomething()
  ... // (2)
}
else {
  // duplicate code path above
  ... // (1) above
  ... // (2) above
}

Compared to:

await using resource = isResourceAvailable() ? getResource() : undefined;
... // (1) do some work with or without resource
resource?.doSomething();
... // (2) do some other work with or without resource

await using Declarations and Values Without [Symbol.asyncDispose] or [Symbol.dispose]

If a resource does not have a callable [Symbol.asyncDispose] or [Symbol.asyncDispose] member, a TypeError would be thrown immediately when the resource is tracked.

await using Declarations in for-of and for-await-of Loops

An await using declaration may occur in the ForDeclaration of a for-await-of loop:

for await (await using x of iterateResources()) {
  // use x
}

In this case, the value bound to x in each iteration will be asynchronously disposed at the end of each iteration. This will not dispose resources that are not iterated, such as if iteration is terminated early due to return, break, or throw.

await using declarations may not be used in in the head of a for-of or for-in loop.

Implicit Async Interleaving Points ("implicit await")

The await using syntax introduces an implicit async interleaving point (i.e., an implicit await) whenever control flow exits an async function body, Block, or Module containing an await using declaration. This means that two statements that currently execute in the same microtask, such as:

async function f() {
  {
    a();
  } // exit block
  b(); // same microtask as call to `a()`
}

will instead execute in different microtasks if an await using declaration is introduced:

async function f() {
  {
    await using x = ...;
    a();
  } // exit block, implicit `await`
  b(); // different microtask from call to `a()`.
}

It is important that such an implicit interleaving point be adequately indicated within the syntax. We believe that the presence of await using within such a block is an adequate indicator, since it should be fairly easy to recognize a Block containing an await using statement in well-formatted code.

It is also feasible for editors to use features such as syntax highlighting, editor decorations, and inlay hints to further highlight such transitions, without needing to specify additional syntax.

Further discussion around the await using syntax and how it pertains to implicit async interleaving points can be found in #1.

Examples

The following show examples of using this proposal with various APIs, assuming those APIs adopted this proposal.

WHATWG Streams API

{
  using reader = stream.getReader();
  const { value, done } = reader.read();
} // 'reader' is disposed

NodeJS FileHandle

{
  using f1 = await fs.promises.open(s1, constants.O_RDONLY),
        f2 = await fs.promises.open(s2, constants.O_WRONLY);
  const buffer = Buffer.alloc(4092);
  const { bytesRead } = await f1.read(buffer);
  await f2.write(buffer, 0, bytesRead);
} // 'f2' is disposed, then 'f1' is disposed

NodeJS Streams

{
  await using writable = ...;
  writable.write(...);
} // 'writable.end()' is called and its result is awaited

Logging and tracing

// audit privileged function call entry and exit
function privilegedActivity() {
  using activity = auditLog.startActivity("privilegedActivity"); // log activity start
  ...
} // log activity end

Async Coordination

import { Semaphore } from "...";
const sem = new Semaphore(1); // allow one participant at a time

export async function tryUpdate(record) {
  using lck = await sem.wait(); // asynchronously block until we are the sole participant
  ...
} // synchronously release semaphore and notify the next participant

Three-Phase Commit Transactions

// roll back transaction if either action fails
async function transfer(account1, account2) {
  await using tx = transactionManager.startTransaction(account1, account2);
  await account1.debit(amount);
  await account2.credit(amount);

  // mark transaction success if we reach this point
  tx.succeeded = true;
} // await transaction commit or rollback

Shared Structs

main_thread.js

// main_thread.js
shared struct Data {
  mut;
  cv;
  ready = 0;
  processed = 0;
  // ...
}

const data = Data();
data.mut = Atomics.Mutex();
data.cv = Atomics.ConditionVariable();

// start two workers
startWorker1(data);
startWorker2(data);

worker1.js

const data = ...;
const { mut, cv } = data;

{
  // lock mutex
  using lck = Atomics.Mutex.lock(mut);

  // NOTE: at this point we currently own the lock

  // load content into data and signal we're ready
  // ...
  Atomics.store(data, "ready", 1);

} // release mutex

// NOTE: at this point we no longer own the lock

// notify worker 2 that it should wake
Atomics.ConditionVariable.notifyOne(cv);

{
  // reacquire lock on mutex
  using lck = Atomics.Mutex.lock(mut);

  // NOTE: at this point we currently own the lock

  // release mutex and wait until condition is met to reacquire it
  Atomics.ConditionVariable.wait(mut, () => Atomics.load(data, "processed") === 1);

  // NOTE: at this point we currently own the lock

  // Do something with the processed data
  // ...

} // release mutex

// NOTE: at this point we no longer own the lock

worker2.js

const data = ...;
const { mut, cv } = data;

{
  // lock mutex
  using lck = Atomics.Mutex.lock(mut);

  // NOTE: at this point we currently own the lock

  // release mutex and wait until condition is met to reacquire it
  Atomics.ConditionVariable.wait(mut, () => Atomics.load(data, "ready") === 1);

  // NOTE: at this point we currently own the lock

  // read in values from data, perform our processing, then indicate we are done
  // ...
  Atomics.store(data, "processed", 1);

} // release mutex

// NOTE: at this point we no longer own the lock

API

Additions to Symbol

This proposal adds the dispose and asyncDispose properties to the Symbol constructor, whose values are the @@dispose and @@asyncDispose internal symbols:

Well-known Symbols

Specification Name[[Description]]Value and Purpose
@@dispose"Symbol.dispose"A method that explicitly disposes of resources held by the object. Called by the semantics of using declarations and by DisposableStack objects.
@@asyncDispose"Symbol.asyncDispose"A method that asynchronosly explicitly disposes of resources held by the object. Called by the semantics of await using declarations and by AsyncDisposableStack objects.

TypeScript Definition

interface SymbolConstructor {
  readonly asyncDispose: unique symbol;
  readonly dispose: unique symbol;
}

The SuppressedError Error

If an exception occurs during resource disposal, it is possible that it might suppress an existing exception thrown from the body, or from the disposal of another resource. Languages like Java allow you to access a suppressed exception via a getSuppressed() method on the exception. However, ECMAScript allows you to throw any value, not just Error, so there is no convenient place to attach a suppressed exception. To better surface these suppressed exceptions and support both logging and error recovery, this proposal seeks to introduce a new SuppressedError built-in Error subclass which would contain both the error that was most recently thrown, as well as the error that was suppressed:

class SuppressedError extends Error {
  /**
   * Wraps an error that suppresses another error, and the error that was suppressed.
   * @param {*} error The error that resulted in a suppression.
   * @param {*} suppressed The error that was suppressed.
   * @param {string} message The message for the error.
   * @param {{ cause?: * }} [options] Options for the error.
   */
  constructor(error, suppressed, message, options);

  /**
   * The name of the error (i.e., `"SuppressedError"`).
   * @type {string}
   */
  name = "SuppressedError";

  /**
   * The error that resulted in a suppression.
   * @type {*}
   */
  error;

  /**
   * The error that was suppressed.
   * @type {*}
   */
  suppressed;

  /**
   * The message for the error.
   * @type {*}
   */
  message;
}

We've chosen to use SuppressedError over AggregateError for several reasons:

Built-in Disposables

%IteratorPrototype%.@@dispose()

We also propose to add Symbol.dispose to the built-in %IteratorPrototype% as if it had the following behavior:

%IteratorPrototype%[Symbol.dispose] = function () {
  this.return();
}

%AsyncIteratorPrototype%.@@asyncDispose()

We propose to add Symbol.asyncDispose to the built-in %AsyncIteratorPrototype% as if it had the following behavior:

%AsyncIteratorPrototype%[Symbol.asyncDispose] = async function () {
  await this.return();
}

Other Possibilities

We could also consider adding Symbol.dispose to such objects as the return value from Proxy.revocable(), but that is currently out of scope for the current proposal.

The Common Disposable and AsyncDisposable Interfaces

The Disposable Interface

An object is disposable if it conforms to the following interface:

PropertyValueRequirements
@@disposeA function that performs explicit cleanup.The function should return undefined.

TypeScript Definition

interface Disposable {
  /**
   * Disposes of resources within this object.
   */
  [Symbol.dispose](): void;
}

The AsyncDisposable Interface

An object is async disposable if it conforms to the following interface:

PropertyValueRequirements
@@asyncDisposeAn async function that performs explicit cleanup.The function should return a Promise.

TypeScript Definition

interface AsyncDisposable {
  /**
   * Disposes of resources within this object.
   */
  [Symbol.asyncDispose](): Promise<void>;
}

The DisposableStack and AsyncDisposableStack container objects

This proposal adds two global objects that can act as containers to aggregate disposables, guaranteeing that every disposable resource in the container is disposed when the respective disposal method is called. If any disposable in the container throws an error during dispose, it would be thrown at the end (possibly wrapped in a SuppressedError if multiple errors were thrown):

class DisposableStack {
  constructor();

  /**
   * Gets a value indicating whether the stack has been disposed.
   * @returns {boolean}
   */
  get disposed();

  /**
   * Alias for `[Symbol.dispose]()`.
   */
  dispose();

  /**
   * Adds a resource to the top of the stack. Has no effect if provided `null` or `undefined`.
   * @template {Disposable | null | undefined} T
   * @param {T} value - A `Disposable` object, `null`, or `undefined`.
   * @returns {T} The provided value.
   */
  use(value);

  /**
   * Adds a non-disposable resource and a disposal callback to the top of the stack.
   * @template T
   * @param {T} value - A resource to be disposed.
   * @param {(value: T) => void} onDispose - A callback invoked to dispose the provided value.
   * @returns {T} The provided value.
   */
  adopt(value, onDispose);

  /**
   * Adds a disposal callback to the top of the stack.
   * @param {() => void} onDispose - A callback to evaluate when this object is disposed.
   * @returns {void}
   */
  defer(onDispose);

  /**
   * Moves all resources currently in this stack into a new `DisposableStack`.
   * @returns {DisposableStack} The new `DisposableStack`.
   */
  move();

  /**
   * Disposes of resources within this object.
   * @returns {void}
   */
  [Symbol.dispose]();

  [Symbol.toStringTag];
}

AsyncDisposableStack is the async version of DisposableStack and is a container used to aggregate async disposables, guaranteeing that every disposable resource in the container is disposed when the respective disposal method is called. If any disposable in the container throws an error during dispose, or results in a rejected Promise, it would be thrown at the end (possibly wrapped in a SuppressedError if multiple errors were thrown):

These classes provided the following capabilities:

NOTE: DisposableStack is inspired by Python's ExitStack.

NOTE: AsyncDisposableStack is inspired by Python's AsyncExitStack.

Aggregation

The DisposableStack and AsyncDisposableStack classes provid the ability to aggregate multiple disposable resources into a single container. When the DisposableStack container is disposed, each object in the container is also guaranteed to be disposed (barring early termination of the program). If any resource throws an error during dispose, it will be collected and rethrown after all resources are disposed. If there were multiple errors, they will be wrapped in nested SuppressedError objects.

For example:

// sync
const stack = new DisposableStack();
const resource1 = stack.use(getResource1());
const resource2 = stack.use(getResource2());
const resource3 = stack.use(getResource3());
stack[Symbol.dispose](); // disposes of resource3, then resource2, then resource1

// async
const stack = new AsyncDisposableStack();
const resource1 = stack.use(getResource1());
const resource2 = stack.use(getResource2());
const resource3 = stack.use(getResource3());
await stack[Symbol.asyncDispose](); // dispose and await disposal result of resource3, then resource2, then resource1

If all of resource1, resource2 and resource3 were to throw during disposal, this would produce an exception similar to the following:

new SuppressedError(
  /*error*/ exception_from_resource3_disposal,
  /*suppressed*/ new SuppressedError(
    /*error*/ exception_from_resource2_disposal,
    /*suppressed*/ exception_from_resource1_disposal
  )
)

Interoperation and Customization

The DisposableStack and AsyncDisposableStack classes also provide the ability to create a disposable resource from a simple callback. This callback will be executed when the stack's disposal method is executed.

The ability to create a disposable resource from a callback has several benefits:

Assist in Complex Construction

A user-defined disposable class might need to allocate and track multiple nested resources that should be disposed when the class instance is disposed. However, properly managing the lifetime of these nested resources in the class constructor can sometimes be difficult. The move method of DisposableStack/AsyncDisposableStack helps to more easily manage lifetime in these scenarios:

// sync
class PluginHost {
  #disposed = false;
  #disposables;
  #channel;
  #socket;

  constructor() {
    // Create a DisposableStack that is disposed when the constructor exits.
    // If construction succeeds, we move everything out of `stack` and into
    // `#disposables` to be disposed later.
    using stack = new DisposableStack();

    // Create an IPC adapter around process.send/process.on("message").
    // When disposed, it unsubscribes from process.on("message").
    this.#channel = stack.use(new NodeProcessIpcChannelAdapter(process));

    // Create a pseudo-websocket that sends and receives messages over
    // a NodeJS IPC channel.
    this.#socket = stack.use(new NodePluginHostIpcSocket(this.#channel));

    // If we made it here, then there were no errors during construction and
    // we can safely move the disposables out of `stack` and into `#disposables`.
    this.#disposables = stack.move();

    // If construction failed, then `stack` would be disposed before reaching
    // the line above. Event handlers would be removed, allowing `#channel` and
    // `#socket` to be GC'd.
  }

  loadPlugin(file) {
    // A disposable should try to ensure access is consistent with its "disposed" state, though this isn't strictly
    // necessary since some disposables could be reusable (i.e., a Connection with an `open()` method, etc.).
    if (this.#disposed) throw new ReferenceError("Object is disposed.");
    // ...
  }

  [Symbol.dispose]() {
    if (!this.#disposed) {
      this.#disposed = true;
      const disposables = this.#disposables;

      // NOTE: we can free `#socket` and `#channel` here since they will be disposed by the call to
      // `disposables[Symbol.dispose]()`, below. This isn't strictly a requirement for every Disposable, but is
      // good housekeeping since these objects will no longer be useable.
      this.#socket = undefined;
      this.#channel = undefined;
      this.#disposables = undefined;

      // Dispose all resources in `disposables`
      disposables[Symbol.dispose]();
    }
  }
}

// async
const privateConstructorSentinel = {};

class AsyncPluginHost {
  #disposed = false;
  #disposables;
  #channel;
  #socket;

  /** @private */
  constructor(arg) {
    if (arg !== privateConstructorSentinel) throw new TypeError("Use AsyncPluginHost.create() instead");
  }
  
  // NOTE: there's no such thing as an async constructor
  static async create() {
    const host = new AsyncPluginHost(privateConstructorSentinel);

    // Create an AsyncDisposableStack that is disposed when the constructor exits.
    // If construction succeeds, we move everything out of `stack` and into
    // `#disposables` to be disposed later.
    await using stack = new AsyncDisposableStack();


    // Create an IPC adapter around process.send/process.on("message").
    // When disposed, it unsubscribes from process.on("message").
    host.#channel = stack.use(new NodeProcessIpcChannelAdapter(process));

    // Create a pseudo-websocket that sends and receives messages over
    // a NodeJS IPC channel.
    host.#socket = stack.use(new NodePluginHostIpcSocket(host.#channel));

    // If we made it here, then there were no errors during construction and
    // we can safely move the disposables out of `stack` and into `#disposables`.
    host.#disposables = stack.move();

    // If construction failed, then `stack` would be asynchronously disposed before reaching
    // the line above. Event handlers would be removed, allowing `#channel` and
    // `#socket` to be GC'd.
    return host;
  }

  loadPlugin(file) {
    // A disposable should try to ensure access is consistent with its "disposed" state, though this isn't strictly
    // necessary since some disposables could be reusable (i.e., a Connection with an `open()` method, etc.).
    if (this.#disposed) throw new ReferenceError("Object is disposed.");
    // ...
  }

  async [Symbol.asyncDispose]() {
    if (!this.#disposed) {
      this.#disposed = true;
      const disposables = this.#disposables;

      // NOTE: we can free `#socket` and `#channel` here since they will be disposed by the call to
      // `disposables[Symbol.asyncDispose]()`, below. This isn't strictly a requirement for every disposable, but is
      // good housekeeping since these objects will no longer be useable.
      this.#socket = undefined;
      this.#channel = undefined;
      this.#disposables = undefined;

      // Dispose all resources in `disposables`
      await disposables[Symbol.asyncDispose]();
    }
  }
}

Subclassing Disposable Classes

You can also use a DisposableStack to assist with disposal in a subclass constructor whose superclass is disposable:

class DerivedPluginHost extends PluginHost {
  constructor() {
    super();

    // Create a DisposableStack to cover the subclass constructor.
    using stack = new DisposableStack();

    // Defer a callback to dispose resources on the superclass. We use `defer` so that we can invoke the version of
    // `[Symbol.dispose]` on the superclass and not on this or any subclasses.
    stack.defer(() => super[Symbol.dispose]());

    // If any operations throw during subclass construction, the instance will still be disposed, and superclass
    // resources will be freed
    doSomethingThatCouldPotentiallyThrow();

    // As the last step before exiting, empty out the DisposableStack so that we don't dispose ourselves.
    stack.move();
  }
}

Here, we can use stack to track the result of super() (i.e., the this value). If any exception occurs during subclass construction, we can ensure that [Symbol.dispose]() is called, freeing resources. If the subclass also needs to track its own disposable resources, this example is modified slightly:

class DerivedPluginHostWithOwnDisposables extends PluginHost {
  #logger;
  #disposables;

  constructor() {
    super()

    // Create a DisposableStack to cover the subclass constructor.
    using stack = new DisposableStack();

    // Defer a callback to dispose resources on the superclass. We use `defer` so that we can invoke the version of
    // `[Symbol.dispose]` on the superclass and not on this or any subclasses.
    stack.defer(() => super[Symbol.dispose]());

    // Create a logger that uses the file system and add it to our own disposables.
    this.#logger = stack.use(new FileLogger());

    // If any operations throw during subclass construction, the instance will still be disposed, and superclass
    // resources will be freed
    doSomethingThatCouldPotentiallyThrow();

    // Persist our own disposables. If construction fails prior to the call to `stack.move()`, our own disposables
    // will be disposed before they are set, and then the superclass `[Symbol.dispose]` will be invoked.
    this.#disposables = stack.move();
  }

  [Symbol.dispose]() {
    this.#logger = undefined;

    // Dispose of our resources and those of our superclass. We do not need to invoke `super[Symbol.dispose]()` since
    // that is already tracked by the `stack.defer` call in the constructor.
    this.#disposables[Symbol.dispose]();
  }
}

In this example, we can simply add new resources to the stack and move its contents into the subclass instance's this.#disposables. In the subclass [Symbol.dispose]() method we don't need to call super[Symbol.dispose]() since that has already been tracked by the stack.defer call in the constructor.

Relation to Iterator and for..of

Iterators in ECMAScript also employ a "cleanup" step by way of supplying a return method. This means that there is some similarity between a using declaration and a for..of statement:

// using
function f() {
  using x = ...;
  // use x
} // x is disposed


// for..of
function makeDisposableScope() {
  const resources = [];
  let state = 0;
  return {
    next() {
      switch (state) {
        case 0:
          state++;
          return {
            done: false,
            value: {
              use(value) {
                resources.unshift(value);
                return value;
              }
            }
          };
        case 1:
          state++;
          for (const value of resources) {
            value?.[Symbol.dispose]();
          }
        default:
          state = -1;
          return { done: true };
      }
    },
    return() {
      switch (state) {
        case 1:
          state++;
          for (const value of resources) {
            value?.[Symbol.dispose]();
          }
        default:
          state = -1;
          return { done: true };
      }
    },
    [Symbol.iterator]() { return this; }
  }
}

function f() {
  for (const { use } of makeDisposableScope()) {
    const x = use(...);
    // use x
  } // x is disposed
}

However there are a number drawbacks to using for..of as an alternative:

Relation to DOM APIs

This proposal does not necessarily require immediate support in the HTML DOM specification, as existing APIs can still be adapted by using DisposableStack or AsyncDisposableStack. However, there are a number of APIs that could benefit from this proposal and should be considered by the relevant standards bodies. The following is by no means a complete list, and primarily offers suggestions for consideration. The actual implementation is at the discretion of the relevant standards bodies.

In addition, several new APIs could be considered that leverage this functionality:

Definitions

A <dfn><a name="wrapper"></a>wrapper</dfn> for x() is a method that invokes x(), but only if the object is in a state such that calling x() will not throw as a result of repeated evaluation.

A <dfn><a name="adapter"></a>callback-adapting wrapper</dfn> is a wrapper that adapts a continuation passing-style method that accepts a callback into a Promise-producing method.

A <dfn><a name="disposer"></a>single-use disposer</dfn> for x() and y() indicates a newly constructed disposable object that invokes x() when constructed and y() when disposed the first time (and does nothing if the object is disposed more than once).

Relation to NodeJS APIs

This proposal does not necessarily require immediate support in NodeJS, as existing APIs can still be adapted by using DisposableStack or AsyncDisposableStack. However, there are a number of APIs that could benefit from this proposal and should be considered by the NodeJS maintainers. The following is by no means a complete list, and primarily offers suggestions for consideration. The actual implementation is at the discretion of the NodeJS maintainers.

Meeting Notes

TODO

The following is a high-level list of tasks to progress through each stage of the TC39 proposal process:

Stage 1 Entrance Criteria

Stage 2 Entrance Criteria

Stage 3 Entrance Criteria

Stage 4 Entrance Criteria

Implementations

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