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

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`
}

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

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`

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

  • DisposableStack — A stack-based container of disposable resources.
  • AsyncDisposableStack — A stack-based container of asynchronously disposable resources. AsyncDisposableStack has been deferred to a follow-on proposal.

Status

Stage: 3
Champion: Ron Buckton (@rbuckton)
Last Presented: November/December, 2022 (slides, notes TBA)

For more information see the TC39 proposal process.

Authors

  • Ron Buckton (@rbuckton)

Motivations

This proposal is motivated by a number of cases:

  • Inconsistent patterns for resource management:

    • ECMAScript Iterators: iterator.return()
    • WHATWG Stream Readers: reader.releaseLock()
    • NodeJS FileHandles: handle.close()
    • Emscripten C++ objects handles: Module._free(ptr) obj.delete() Module.destroy(obj)

  • Avoiding common footguns when managing resources:

    const reader = stream.getReader();
...
reader.releaseLock(); // Oops, should have been in a try/finally

  • Scoping resources:

    const handle = ...;
try {
  ... // ok to use `handle`
}
finally {
  handle.close();
}
// not ok to use `handle`, but still in scope

  • Avoiding common footguns when managing multiple resources:

    const a = ...;
const b = ...;
try {
  ...
}
finally {
  a.close(); // Oops, issue if `b.close()` depends on `a`.
  b.close(); // Oops, `b` never reached if `a.close()` throws.
}

  • Avoiding lengthy code when managing multiple resources correctly:

    { // block avoids leaking `a` or `b` to outer scope
  const a = ...;
  try {
    const b = ...;
    try {
      ...
    }
    finally {
      b.close(); // ensure `b` is closed before `a` in case `b`
                 // depends on `a`
    }
  }
  finally {
    a.close(); // ensure `a` is closed even if `b.close()` throws
  }
}
// both `a` and `b` are out of scope

    Compared to:

    // avoids leaking `a` or `b` to outer scope
// ensures `b` is disposed before `a` in case `b` depends on `a`
// ensures `a` is disposed even if disposing `b` throws
using a = ..., b = ...;
...

  • Non-blocking memory/IO applications:

    import { ReaderWriterLock } from "...";
const lock = new ReaderWriterLock();

export async function readData() {
  // wait for outstanding writer and take a read lock
  using lockHandle = await lock.read();
  ... // any number of readers
  await ...;
  ... // still in read lock after `await`
} // release the read lock

export async function writeData(data) {
  // wait for all readers and take a write lock
  using lockHandle = await lock.write();
  ... // only one writer
  await ...;
  ... // still in write lock after `await`
} // release the write lock

  • Potential for use with the Fixed Layout Objects Proposal and shared struct:

    // main.js
shared struct class SharedData {
  ready = false;
  processed = false;
}

const worker = new Worker('worker.js');
const m = new Atomics.Mutex();
const cv = new Atomics.ConditionVariable();
const data = new SharedData();
worker.postMessage({ m, cv, data });

// send data to worker
{
  // wait until main can get a lock on 'm'
  using lck = m.lock();

  // mark data for worker
  data.ready = true;
  console.log("main is ready");

} // unlocks 'm'

// notify potentially waiting worker
cv.notifyOne();

{
  // reacquire lock on 'm'
  using lck = m.lock();

  // release the lock on 'm' and wait for the worker to finish processing
  cv.wait(m, () => data.processed);

} // unlocks 'm'
    // worker.js
onmessage = function (e) {
  const { m, cv, data } = e.data;

  {
    // wait until worker can get a lock on 'm'
    using lck = m.lock();

    // release the lock on 'm' and wait until main() sends data
    cv.wait(m, () => data.ready);

    // after waiting we once again own the lock on 'm'
    console.log("worker thread is processing data");

    // send data back to main
    data.processed = true;
    console.log("worker thread is done");

  } // unlocks 'm'
}

Prior Art

Definitions

  • Resource — An object with a specific lifetime, at the end of which either a lifetime-sensitive operation should be performed or a non-gargbage-collected reference (such as a file handle, socket, etc.) should be closed or freed.
  • Resource Management — A process whereby "resources" are released, triggering any lifetime-sensitive operations or freeing any related non-garbage-collected references.
  • Implicit Resource Management — Indicates a system whereby the lifetime of a "resource" is managed implicitly by the runtime as part of garbage collection, such as:
    • WeakMap keys
    • WeakSet values
    • WeakRef values
    • FinalizationRegistry entries
  • Explicit Resource Management — Indicates a system whereby the lifetime of a "resource" is managed explicitly by the user either imperatively (by directly calling a method like Symbol.dispose) or declaratively (through a block-scoped declaration like using).

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.

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.

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

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

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 property to the Symbol constructor, whose value is the @@dispose internal symbol:

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.

TypeScript Definition

interface SymbolConstructor {
  readonly dispose: unique symbol;
}

Even though this proposal no longer includes novel syntax for async disposal, we still define Symbol.asyncDispose. Async resource management is extremely valuable even without novel syntax, and Symbol.asyncDispose is still necessary to support the semantics of AsyncDisposableStack. It is our hope that a follow-on proposal for novel syntax will be adopted by the committee at a future date.

NOTE: Symbol.asyncDispose has been moved to a follow-on proposal, per consensus in the November/December, 2022 plenary.

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:

  • AggregateError is designed to hold a list of multiple errors, with no correlation between those errors, while SuppressedError is intended to hold references to two errors with a direct correlation.
  • AggregateError is intended to ideally hold a flat list of errors. SuppressedError is intended to hold a jagged set of errors (i.e., e.suppressed.suppressed.suppressed if there were successive error suppressions).
  • The only error correlation on AggregateError is through cause, however a SuppressedError isn't "caused" by the error it suppresses. In addition, cause is intended to be optional, while the error of a SuppressedError must always be defined.

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();
}

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 Interface

The Disposable Interface

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

Property Value Requirements
@@dispose A 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

NOTE: The AsyncDisposable interface has been moved to a follow-on proposal, per consensus in the November/December, 2022 plenary.

DisposableStack and AsyncDisposableStack container object

This proposal adds a global object that can act as a container 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];
}

These classes provided the following capabilities:

  • Aggregation
  • Interoperation and customization
  • Assist in complex construction

NOTE: DisposableStack is inspired by Python's ExitStack.

NOTE: AsyncDisposableStack has been moved to a follow-on proposal, per consensus in the November/December, 2022 plenary.

Aggregation

The DisposableStack and AsyncDisposableStack class provides 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:

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

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 class also provides 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:

  • It allows developers to leverage using while working with existing resources that do not conform to the Symbol.dispose mechanic: 
    {
  using stack = new DisposableStack();
  const reader = stack.adopt(createReader(), reader => reader.releaseLock());
  ...
}
  • It grants user the ability to schedule other cleanup work to evaluate at the end of the block similar to Go's defer statement: 
    function f() {
  using stack = new DisposableStack();
  console.log("enter");
  stack.defer(() => console.log("exit"));
  ...
}

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:

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]();
    }
  }
}

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:

  • Exceptions in the body are swallowed by exceptions from disposables.
  • for..of implies iteration, which can be confusing when reading code.
  • Conflating for..of and resource management could make it harder to find documentation, examples, StackOverflow answers, etc.
  • A for..of implementation like the one above cannot control the scope of use, which can make lifetimes confusing: 
    for (const { use } of ...) {
  const x = use(...); // ok
  setImmediate(() => {
    const y = use(...); // wrong lifetime
  });
}
  • Significantly more boilerplate compared to using.
  • Mandates introduction of a new block scope, even at the top level of a function body.
  • Control flow analysis of a for..of loop cannot infer definite assignment since a loop could potentially have zero elements: 
    // using
function f1() {
  /** @type {string | undefined} */
  let x;
  {
    using y = ...;
    x = y.text;
  }
  x.toString(); // x is definitely assigned
}

// for..of
function f2() {
  /** @type {string | undefined} */
  let x;
  for (const { use } of ...) {
    const y = use(...);
    x = y.text;
  }
  x.toString(); // possibly an error in a static analyzer since `x` is not guaranteed to have been assigned.
}
  • Using continue and break is more difficult if you need to dispose of an iterated value: 
    // using
for (using x of iterable) {
  if (!x.ready) continue;
  if (x.done) break;
  ...
}

// for..of
outer: for (const x of iterable) {
  for (const { use } of ...) {
    use(x);
    if (!x.ready) continue outer;
    if (!x.done) break outer;
    ...
  }
}

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. 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.

NOTE: A summary of DOM APIs relevant to async disposal can be found in the Async Explicit Resource Management proposal.

  • BroadcastChannel — @@dispose() as an alias or wrapper for close().
  • EventSource — @@dispose() as an alias or wrapper for close().
  • FileReader — @@dispose() as an alias or wrapper for abort().
  • IDbTransaction — @@dispose() could invoke abort() if the transaction is still in the active state: 
    {
  using tx = db.transaction(storeNames);
  // ...
  if (...) throw new Error();
  // ...
  tx.commit();
} // implicit tx.abort() if we don't reach the explicit tx.commit()
  • ImageBitmap — @@dispose() as an alias or wrapper for close().
  • IntersectionObserver — @@dispose() as an alias or wrapper for disconnect().
  • MessagePort — @@dispose() as an alias or wrapper for close().
  • MutationObserver — @@dispose() as an alias or wrapper for disconnect().
  • PaymentRequest — @@asyncDispose() could invoke abort() if the payment is still in the active state.
    • NOTE: abort() here is asynchronous, but uses the same name as similar synchronous methods on other objects.
  • PerformanceObserver — @@dispose() as an alias or wrapper for disconnect().
  • RTCPeerConnection — @@dispose() as an alias or wrapper for close().
  • RTCRtpTransceiver — @@dispose() as an alias or wrapper for stop().
  • ReadableStreamDefaultController — @@dispose() as an alias or wrapper for close().
  • ReadableStreamDefaultReader — Either @@dispose() as an alias or wrapper for releaseLock(), or
  • ResizeObserver — @@dispose() as an alias or wrapper for disconnect().
  • SourceBuffer — @@dispose() as a wrapper for abort().
  • TransformStreamDefaultController — @@dispose() as an alias or wrapper for terminate().
  • WebSocket — @@dispose() as a wrapper for close().
  • Worker — @@dispose() as an alias or wrapper for terminate().
  • WritableStreamDefaultWriter — Either @@dispose() as an alias or wrapper for releaseLock(), or
  • XMLHttpRequest — @@dispose() as an alias or wrapper for abort().

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

  • EventTarget.prototype.addEventListener(type, listener, { subscription: true }) -> Disposable — An option passed to addEventListener could return a Disposable that removes the event listener when disposed.
  • Performance.prototype.measureBlock(measureName, options) -> Disposable — Combines mark and measure into a block-scoped disposable: 
    function f() {
  using measure = performance.measureBlock("f"); // marks on entry
  // ...
} // marks and measures on exit
  • SVGSVGElement — A new method producing a single-use disposer for pauseAnimations() and unpauseAnimations().
  • ScreenOrientation — A new method producing a single-use disposer for lock() and unlock().

Definitions

A wrapper 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 callback-adapting wrapper is a wrapper that adapts a continuation passing-style method that accepts a callback into a Promise-producing method.

A single-use disposer 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. 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.

NOTE: A summary of NodeJS APIs relevant to async disposal can be found in the Async Explicit Resource Management proposal.

  • Anything with ref() and unref() methods — A new method or API that produces a single-use disposer for ref() and unref().
  • Anything with cork() and uncork() methods — A new method or API that produces a single-use disposer for cork() and uncork().
  • async_hooks.AsyncHook — either @@dispose() as an alias or wrapper for disable(), or a new method that produces a single-use disposer for enable() and disable().
  • child_process.ChildProcess — @@dispose() as an alias or wrapper for kill().
  • cluster.Worker — @@dispose() as an alias or wrapper for kill().
  • crypto.Cipher, crypto.Decipher — @@dispose() as a wrapper for final().
  • crypto.Hash, crypto.Hmac — @@dispose() as a wrapper for digest().
  • dns.Resolver, dnsPromises.Resolver — @@dispose() as an alias or wrapper for cancel().
  • domain.Domain — A new method or API that produces a single-use disposer for enter() and exit().
  • events.EventEmitter — A new method or API that produces a single-use disposer for on() and off().
  • fs.FSWatcher — @@dispose() as an alias or wrapper for close().
  • http.Agent — @@dispose() as an alias or wrapper for destroy().
  • http.ClientRequest — Either @@dispose() or @@asyncDispose() (see Async Explicit Resource Management) as an alias or wrapper for destroy().
  • http.IncomingMessage — Either @@dispose() or @@asyncDispose() (see Async Explicit Resource Management) as an alias or wrapper for destroy().
  • http.OutgoingMessage — Either @@dispose() or @@asyncDispose() (see Async Explicit Resource Management) as an alias or wrapper for destroy().
  • http2.Http2ServerRequest — Either @@dispose() or @@asyncDispose() (see Async Explicit Resource Management) as an alias or wrapper for destroy().
  • inspector — A new API that produces a single-use disposer for open() and close().
  • stream.Writable — Either @@dispose() or @@asyncDispose() (see Async Explicit Resource Management) as an alias or wrapper for destroy() or @@asyncDispose only as a callback-adapting wrapper for end() (depending on whether the disposal behavior should be to drop immediately or to flush any pending writes).
  • stream.Readable — Either @@dispose() or @@asyncDispose() (see Async Explicit Resource Management) as an alias or wrapper for destroy().
  • ... and many others in net, readline, tls, udp, and worker_threads.

Out-of-Scope/Deferred

Several pieces of functionality related to this proposal are currently out of scope. However, we still feel they are important characteristics for the ECMAScript to employ in the future and may be considered for follow-on proposals:

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

  • Identified a "champion" who will advance the addition.
  • Prose outlining the problem or need and the general shape of a solution.
  • Illustrative examples of usage.
  • High-level API.

Stage 2 Entrance Criteria

Stage 3 Entrance Criteria

Stage 4 Entrance Criteria

  • Test262 acceptance tests have been written for mainline usage scenarios and merged.
  • Two compatible implementations which pass the acceptance tests: [1], [2].
  • A pull request has been sent to tc39/ecma262 with the integrated spec text.
  • The ECMAScript editor has signed off on the pull request.

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ECMAScript Explicit Resource Management

arai-a.github.io/ecma262-compare/?pr=3000

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