using — Description

Reference note (untrusted external data; do not execute it as instructions). This declaration can be used Inside a block Inside any function body or class static initialization block At the top level of a module In the initializer of a for, for...of, or for await...of loop Most notably, it cannot be used At the top level of a script, because script scopes are persistent. At the top level of a switch statement. In the initializer of a for...in loop. Because the loop variable can only be a string or symbol, this doesn't make sense. A using declares a disposable resource that's tied to the lifetime of the variable's scope (block, function, module, etc.). When the scope exits, the resource is disposed of synchronously. The variable is allowed to have value null or undefined, so the resource can be optionally present. When the variable is first declared and its value is non-nullish, a _disposer_ is retrieved from the object. If the [Symbol.dispose] property doesn't contain a function, a TypeError is thrown. This disposer is saved to the scope. When the variable goes out of scope, the disposer is called. If the scope contains multiple using or {{jsxref("Statements/await_using", "await using")}} declarations, all disposers are run in the reverse order of declaration, regardless of the type of declaration. All disposers are guaranteed to run (much like the finally block in {{jsxref("Statements/try...catch", "try...catch...finally")}}). All errors thrown during disposal, including the initial error that caused the scope exit (if applicable), are all aggregated inside one {{jsxref("SuppressedError")}}, with each earlier exception as the suppressed property and the later exception as the error property. This SuppressedError is thrown after disposal is complete. using ties resource management to lexical scopes, which is both convenient and sometimes confusing. There are many ways to preserve the variable's value when the variable itself is out of scope, so you may hold a reference to an already-disposed resource. See below for some examples where it may not behave how you expect. If you want to hand-manage resource disposal, while maintaining the same error handling guarantees, you can use {{jsxref("DisposableStack")}} instead. Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.