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Javascript – Why error in the "synchronous" part of promise/async function is handled differently

Compare these 2 snippets:

try {
    console.log('start')
    const test = (() => {
        console.log('inside')
        const a = null;
        a.c = 1
    })()
    console.log('end')
} catch (error) {
    console.log('nvmnghia');
    console.error(error);
}

// prints:
// start
// inside
// nvmnghia
// TypeError: set props of null

try {
    console.log('start')
    const test = (async () => { // Note the async without await
        console.log('inside')
        const a = null;
        a.c = 1
    })()
    console.log('end')
} catch (error) {
    console.log('nvmnghia');
    console.error(error);
}

// prints:
// start
// inside
// end
// undefined, due to normal return
// Uncaught (in promise) TypeError: ...
//   and no nvmnghia, which means the error wasn't caught indeed

Why does it behave this way? I’m surpise cuz my mental model for async/promise has always been that the part before the first await is run synchronously i.e. NOT scheduled in a microtask.

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2

Answers


  1. 0

    When your code is run synchronously, the errors get caught immediately by try/catch. When it is async, even if there is no await used, the function returns a promse, and errors are then instead handled by the error of the promise. These errors don’t get caught by a try/catch, so you’ll need to use .catch() or await.

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  2. 0

    JS engine handles asynchronous request in form of events. So, catch block can’t handle async error. You can handle that error in promise catch like below:

    try {
    console.log('start')
    const test = (async () => { // Note the async without await
        console.log('inside')
        const a = null;
        a.c = 1
    })().catch(err => console.log("Catch from promise block:", err));
    console.log('end')
} catch (error) {
    console.log('nvmnghia');
    console.error(error);
}

    You will see Catch from promise block: in output before error.

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