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fix Promise.prototype.finally to correctly propagate onFinally errors #1725

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Merged
Perryvw merged 2 commits into from
May 31, 2026
Merged

fix Promise.prototype.finally to correctly propagate onFinally errors #1725

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4b3162b
(lualib): update Promise.prototype.finally to correctly handle onFina…
pilaoda May 28, 2026
71fc6cc
(test): add tests for async/await try-catch-finally re-throw cases
pilaoda May 28, 2026
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21 changes: 9 additions & 12 deletions src/lualib/Promise.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -125,20 +125,17 @@ export class __TS__Promise<T> implements Promise<T> {
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/finally
// Delegates to .then() so that a new Promise is returned (per ES spec §27.2.5.3)
// and the original fulfillment value / rejection reason is preserved.
// reference: https://github.com/tc39/proposal-promise-finally/blob/fd934c0b42d59bf8d9446e737ba14d50a9067216/polyfill.js#L34-L41
public finally(onFinally?: () => void): Promise<T> {
if (typeof onFinally !== "function") {
return this.then(onFinally, onFinally);
}
return this.then(
onFinally
? (value: T): T => {
onFinally();
return value;
}
: undefined,
onFinally
? (reason: any): never => {
onFinally();
throw reason;
}
: undefined
x => new __TS__Promise(resolve => resolve(onFinally())).then(() => x),
e =>
new __TS__Promise(resolve => resolve(onFinally())).then(() => {
throw e;
})
);
}

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98 changes: 98 additions & 0 deletions test/unit/builtins/async-await.spec.ts
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Expand Up @@ -1126,6 +1126,104 @@ describe("try/catch in async function", () => {
.setTsHeader(promiseTestLib)
.expectToEqual(["finally", "ok"]);
});

test.each([0, 1, 2])("async re-throw (%p)", i => {
util.testModule`
const i: number = ${i};
async function foo() {
try {
try {
if (i === 0) { throw "z"; }
} catch (e) {
throw "a";
} finally {
if (i === 1) { throw "b"; }
}
} catch (e) {
throw (e as string).toUpperCase();
} finally {
throw "C";
}
}
export let result: string = "x";
async function run() {
try {
await foo();
} catch (e) {
result = (e as string)[(e as string).length - 1];
}
}
run();
`.expectToEqual({ result: "C" });
});

test("async: catch re-throws, finally still runs", () => {
util.testModule`
const foo = async () => {
throw "original";
};

let finallyCalled = false;
let caughtError: any = false;

const run = async () => {
try {
await foo();
} catch (e) {
throw "re-thrown: " + e;
} finally {
finallyCalled = true;
}
};

run().catch(e => { caughtError = e; });

export const result = { finallyCalled, caughtError };
`.expectToEqual({
result: {
finallyCalled: true,
caughtError: "re-thrown: original",
},
});
});

test("async: finally throw overrides catch throw", () => {
util.testModule`
const run = async () => {
try {
throw "try-error";
} catch (e) {
throw "catch-error";
} finally {
throw "finally-error";
}
};

let caughtError: any = false;
run().catch(e => { caughtError = e; });

export const result = caughtError;
`.expectToEqual({ result: "finally-error" });
});

test("async: finally return overrides catch throw", () => {
util.testFunction`
async function run() {
try {
throw "try-error";
} catch (e) {
throw "catch-error";
} finally {
return "finally-return";
}
}

let result: any;
run().then(v => { result = v; }).catch(e => { result = "rejected: " + e; });

return result;
`.expectToEqual("finally-return");
});
});

describe("async generators are unsupported", () => {
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40 changes: 40 additions & 0 deletions test/unit/builtins/promise.spec.ts
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Expand Up @@ -1370,4 +1370,44 @@ describe("Promise.finally", () => {
return result.value;
`.expectToEqual(99);
});

test("finally throw overrides fulfillment value", () => {
util.testModule`
let result: any;
Promise.resolve("ok")
.finally(() => { throw "finally-error"; })
.then(v => { result = v; }, e => { result = e; });
export const output = result;
`.expectToEqual({ output: "finally-error" });
});

test("finally throw overrides rejection reason", () => {
util.testModule`
let result: any;
Promise.reject("original")
.finally(() => { throw "finally-error"; })
.then(v => { result = v; }, e => { result = e; });
export const output = result;
`.expectToEqual({ output: "finally-error" });
});

test("finally returning rejected promise overrides fulfillment", () => {
util.testModule`
let result: any;
Promise.resolve("ok")
.finally(() => Promise.reject("finally-rejected") as any)
.then(v => { result = v; }, e => { result = e; });
export const output = result;
`.expectToEqual({ output: "finally-rejected" });
});

test("finally returning rejected promise overrides rejection", () => {
util.testModule`
let result: any;
Promise.reject("original")
.finally(() => Promise.reject("finally-rejected") as any)
.then(v => { result = v; }, e => { result = e; });
export const output = result;
`.expectToEqual({ output: "finally-rejected" });
});
});
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