using

In JavaScript, using is a contextual keyword that declares a block-scoped constant whose value is disposed of automatically when its scope exits. The value must be an object with a [Symbol.dispose]() method, or null or undefined, and any other value throws a TypeError. Like const, a using binding can’t be reassigned, and it has the same temporal dead zone as let.

When the scope exits, normally or through an error, the disposers run in reverse order of declaration. A disposer that throws while another error is propagating produces a SuppressedError. The declaration works in blocks, function bodies, for and for...of heads, and at the top level of a module, but not at the top level of a script. Elsewhere, using is an ordinary identifier.

The asynchronous counterpart is await using, and DisposableStack manages disposal by hand.

using <name> = <value>[, <name2> = <value2>, ...];

for (using <name> of <iterable>) {
  <statements>
}

> function openConnection(name) {
…   console.log(`Opening ${name}`);
…   return {
…     [Symbol.dispose]() {
…       console.log(`Closing ${name}`);
…     },
…   };
… }
undefined
> {
…   using primary = openConnection("primary");
…   using replica = openConnection("replica");
…   console.log("Running queries");
… }
Opening primary
Opening replica
Running queries
Closing replica
Closing primary
undefined

> {
…   using connection = { [Symbol.dispose]: () => console.log("Closed") };
…   throw new Error("Query failed");
… }
Closed
Uncaught Error: Query failed