async_error_handling.ts

class MyError extends Error {
}

async function doSomething() {
    return Promise.resolve({value: 5});
}

async function doSomethingElse(value: number): Promise<string> {
    return Promise.reject(new Error("myMessage"));
}

// several try/catch blocks, requiring let declarations
async function separateTries() {
    let first: number;
    try {
        first = (await doSomething()).value;
    } catch (error) {
        throw new MyError(error.message + "internalStuff");
    }

    if (first > 10) {
        throw new MyError("internal check failed" + "internalStuff");
    }
    let second: string;

    try {
        second = await doSomethingElse(first);
    } catch (error) {
        throw new MyError(error.message + "internalStuff");
    }
    // do something with second
    const result = second + "!!!111elf"

    return result;
}

// one try/catch block. requires specific handling of intermediate check
async function oneTry() {

    try {
        const first = (await doSomething()).value;

        if (first > 10) {
            // i can return a rejected Promise directly
            return Promise.reject(new MyError("internal check failed" + "internalStuff"));
            // i can throw an error so the local catch handles it correctly
            // throw new MyError("internal check failed");
        }

        const second = await doSomethingElse(first);
        // do something with second
        const result = second + "!!!111elf"

        return result;
    } catch (error) {
        throw new MyError(error.message + "internalStuff");
    }
}

// mix async/await and promise chaining
async function mixWithChaining() {

    function helper(error: Error): never {
        throw new MyError(error.message + "internalStuff");
    }

    const first = (await doSomething().catch(helper)).value;

    if (first > 10) {
        throw new MyError("internal check failed" + "internalStuff");
    }

    const second = await doSomethingElse(first).catch(helper);
    // do something with second
    const result = second + "!!!111elf"

    return result;
}