How does `using` / `await using` with `Symbol.dispose` work in TypeScript 5.2+?

Q: How does `using` / `await using` with `Symbol.dispose` work in TypeScript 5.2+?

**Target Audience:** Developers who write cleanup code (closing connections, releasing locks, deleting temp files) and want to understand TypeScript's `using` declarations from zero. **Difficulty:** Medium **TypeScript version:** Checked with TypeScript 5.4.5 `--strict` (the page compiler; `using` has been supported since 5.2). The page compiles to ES2022, so it runs TypeScript's downlevel helpers. As a cross-check, both examples were also compiled to ESNext (native `using` syntax) and run in Bun 1.4.2, which has the feature built in: the output was identical. > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output was produced by actually running the code; the **Verified** section quotes it in full. Managing *many* resources as one unit (`DisposableStack`, rollback on failed setup) has its own page: [How do you type and compose multiple disposables with `Dis

The problem

Question presented to candidate: "TypeScript 5.2 added using and await using. What do they do, how does an object opt in with Symbol.dispose, in what order does cleanup run, and what do you need in your config and runtime to use them today?"

What a strong answer should cover:

using x = expr declares a const-like, block-scoped binding whose [Symbol.dispose]() is called when the block exits, at the end of the block, on return and on throw; several bindings dispose in reverse order (last in, first out)
The types: Disposable ([Symbol.dispose](): void) and AsyncDisposable ([Symbol.asyncDispose](): PromiseLike<void>); the initializer must be one of those or null/undefined (TS2850), and await using only works in async code (TS2852)
await using awaits Symbol.asyncDispose (falling back to Symbol.dispose) before leaving the block
Errors: a body error is rethrown after cleanup; if cleanup also throws, both are wrapped in a SuppressedError (error = the newer, suppressed = the earlier)
Setup: lib must include esnext or esnext.disposable; for older targets TypeScript emits helper code (__addDisposableResource, __disposeResources); runtimes without the built-ins need at least Symbol.dispose / Symbol.asyncDispose polyfills

Code / implementation expected: Yes — a small resource class with [Symbol.dispose], two using bindings, and the order of the log lines.

using
resource-management

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Solution

Target Audience: Developers who write cleanup code (closing connections, releasing locks, deleting temp files) and want to understand TypeScript's using declarations from zero. Difficulty: Medium TypeScript version: Checked with TypeScript 5.4.5 --strict (the page compiler; using has been supported since 5.2). The page compiles to ES2022, so it runs TypeScript's downlevel helpers. As a cross-check, both examples were also compiled to ESNext (native using syntax) and run in Bun 1.4.2, which has the feature built in: the output was identical.

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. All output was produced by actually running the code; the Verified section quotes it in full.

Managing many resources as one unit (DisposableStack, rol

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11 min read · 11 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

TypeScriptusing: reverse-order disposal on return and throw

Practice

TypeScriptawait using, mixed disposables and SuppressedError

Practice

Discussion

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