Semicolon changes output of code

[negative0]

• Version:
  v12.18.3 and v14.5.0

• Platform:
  Linux pop-os 5.11.0-7612-generic Failed assert in AsyncWrap::MakeCallback(uint32_t, int, Handle) from test-timers-unref.js on ARM #13161721575720.04~97a8d1a-Ubuntu SMP Thu Apr 1 21:15:20 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

• Subsystem:
  Not sure

What steps will reproduce the bug?

const f1 = async (abc) => {
        console.log(abc);
        return [10, 12];
}

const f2 = async () => {
        let match = {a: 10};
        let a,b;
        match = {
                ...match,
                b: 20
        }
        [a,b] = await f1(match);
}

f2();

and

const f1 = async (abc) => {
        console.log(abc);
        return [10, 12];
}

const f2 = async () => {
        let match = {a: 10};
        let a,b;
        match = {
                ...match,
                b: 20
        };
        [a,b] = await f1(match);
}

f2();

These codes give different outputs. The only difference is a semicolon after match

How often does it reproduce? Is there a required condition?

Every time. No required condition, I think.

What is the expected behavior?

{ a: 10, b: 20 }

What do you see instead?

{ a: 10 }

Additional information

[Ayase-252]

It's definitely not a bug in Node.js but a quirk of JavaScript.

By the spec, the semicolon ; is important in this case. Without the ;, the {}[expression] is intepreted as property access. So the first snippet is equivalent with

const f2 = async () => {
        let match = {a: 10};
        let a,b;
        // match is `[10, 12]`, as you can verify.
        match = ({
                ...match,
                b: 20
        }[a,b] = await f1(match));
}

[aduh95]

You can use tools such as https://astexplorer.net/ to try to decipher what JS you are writing. To get the expected output, you need to either:

• use semicolons
• use let or const keyword when doing the assignment: const [a,b] = await f1(match)
• send a proposal to TC39 for changing the ECMAScript spec.

Closing this now, but feel free to keep asking questions if you need more help.

EDIT: FYI, here's how Prettier styles it with the no-semi option: https://prettier.io/playground/#N4Igxg9gdgLgprEAuc0DOMAEAzAjJgXkwEM0BPKMTACmICMwBKQgPk2AB0pMfefIoaCABs4AOmEQA5rQaMA3Fz58ATnBgBXFdwDauAAwAaTLgBMAXUVQAvly4CMOU4RLlKNZgTaduynqKwAW2IYMAALF2BiJBN9ays-f3USQzoExODQiKIfRLzMMULM8MMlfL86GNN9Mr942uUdYlTzF2IAd2IASyw8amKwhS5bKC5sU2ohqBBDEAgABxgu9GRQYhUVCHaABXWENGQQYmFOsgPZuhViMABrdQBleeuuqClkGBUNOFm4QLo4AAmAMBABliK8NMQpHAAGIQFSZJavQ7EDQwCAzEBhGCBYQAdTCPTgaCeYDg932PS6ADcemRDmA0OcQC80HAVDBtlcpMFkNhjmzZgArNAADwAQldbg9iIE4CCXnA+QLviARaL7i8pKIAIoaCDwZXCQUgJ4qNkqQ50ehwYSY+YqF4wPFdAEwMLIAAcRlNmzZeKu80ODuJ7OpStmAEd9fAuQsDihSABaKBwQGAzFqaNdNRcqG8pD842qtmBLpGk1oLW6mNKwsq2Ywegut0epCmRtXLrCLUAYQggQLIGJAFZMRo2QAVegJosm6lfACSUGBsHuYEdiwAgiv7jAyKIK3BrNYgA