try/using semantics

async-try-using.js

Symbol.asyncDispose = Symbol.for("Symbol.asyncDispose");
function __asyncUsing(obj) {
    let state = obj !== null && obj !== undefined ? 0 : 2;
    return {
        [Symbol.asyncIterator]() { return this; },
        async next() {
            if (state) return this.return();
            state = 1;
            return { value: obj };
        },
        async return() {
            if (state < 2) {
                state = 2;
                const asyncDispose = obj[Symbol.asyncDispose];
                if (asyncDispose) {
                    await asyncDispose.call(obj);
                }
                else {
                    obj[Symbol.dispose]();
                }
            }
            return { done: true };
        }
    };
}

class AsyncDisposable {
    async [Symbol.asyncDispose]() {
        console.log("start asyncDispose");
        await Promise.resolve();
        console.log("end asyncDispose");
    }
}


async function main() {
    console.log("before disposed");
    for await (const _ of __asyncUsing(new AsyncDisposable())) {
        console.log("do some work");
    }
    console.log("after disposed");
}

main();

sync-try-using.js

Symbol.dispose = Symbol.for("Symbol.dispose");

function __using(obj) {
    let state = obj !== null && obj !== undefined ? 0 : 2;
    return {
        [Symbol.iterator]() { return this; },
        next() {
            if (state) return this.return();
            state = 1;
            return { value: obj };
        },
        return() {
            if (state < 2) {
                state = 2;
                obj[Symbol.dispose]();
            }
            return { done: true };
        }
    };
}

class Disposable {
    [Symbol.dispose]() {
        console.log("disposed");
    }
}


console.log("before disposed");
for (const _ of __using(new Disposable())) {
    console.log("do some work");
}
console.log("after disposed");