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Description

Bug Report

When using the "ES5" target, async/await code generation does not correctly handle breaking from labelled blocks.

🔎 Search Terms

break block label async

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about everything

⏯ Playground Link

Playground link with relevant code

💻 Code

Compiler options: --lib ES2015,dom --target ES5

async function foo() {
    tmp: {
        for (let x = 0; x < 4; x++)
            if (await Promise.resolve(true))
                break tmp;
        console.log("Impossible");
        throw 0;
    }
    console.log("Success");
}

foo();

🙁 Actual behavior

"Impossible" is logged, because the break statement seems to short-circuit the inner loop

🙂 Expected behavior

"Success" is logged, because the break statement should short-circuit the outer block

Interestingly, it does seem to handle breaking from outer loops (eg adding a superfluous loop, tmp: for (let nvm = 0; nvm < 1; nvm++) ...), but not plain blocks.

Contributor guide

Research direction: Inspect the async code generation logic in the TypeScript compiler, specifically how labelled blocks are handled when breaking from them in async functions targeting ES5. Look at the emit for labelled statements and the break statement's handling in async contexts.
Tech stack: typescript
Domain: developer experience
Issue type: Bug
Difficulty: 2
Estimated time: Half day
Activity status: Active
Clarity: Clear
Prerequisites: TypeScript
Newbie friendliness: 30