Node.js中Promise同步与异步场景下的错误处理差异探究

我正在探究Node.js里Promise执行器内抛出错误、代码异常（bug）与reject()的错误处理差异。不少文章提到Promise存在隐式try/catch，能把异常和抛出的错误自动转为reject，但实际测试发现并非所有场景都如此。

测试代码 #

function delay(ms) {
    return new Promise((resolve, reject) => {
        setTimeout(value => {
            resolve();
        }, ms);
    });
}

function f(delayMs, action) {
    return new Promise((resolve, reject) => {
        if (delayMs == 0) {
            console.log(`f() forcing error type ${action} sync...`);
            if (action == 'exception') {
                console.xxx();
            } else if (action == 'throw') {
                throw new Error('thown error sync');
            } else {
                reject('reject sync');
            }
        } else {
            delay(delayMs).
                then(() => {
                    console.log(`f() forcing error type ${action} after delay...`);
                    if (action == 'exception') {
                        console.yyy();
                    } else if (action == 'throw') {
                        throw new Error('thrown error after delay');
                    } else  {
                        reject('reject after delay');
                    }
                });
        }
    });
}

function main(delayMs, action) {
    f(delayMs, action).
        then(value => {
            console.log('Should not see this');
        }).
        catch(err => {
            if (typeof err == 'object') console.log('In main catch:', err.message);
            else console.log('In main catch:', err);
        }).
        finally(() => {
            console.log('In main finally');
        });
}

main(0, 'exception');
main(0, 'throw');
main(0, 'reject');
main(500, 'reject');
main(1000, 'exception');
main(1500, 'throw');

测试输出结果 #

f() forcing error type exception sync...
f() forcing error type throw sync...
f() forcing error type reject sync...
In main catch: console.xxx is not a function
In main catch: thown error sync
In main catch: reject sync
In main finally
In main finally
In main finally
f() forcing error type reject after delay...
In main catch: reject after delay
In main finally
f() forcing error type exception after delay...
/mnt/c/Users/test/promise-stackoverflow-2.js:25
                        console.yyy();
                                ^

TypeError: console.yyy is not a function
    at /mnt/c/Users/test/promise-stackoverflow-2.js:25:33

Node.js v18.12.1

测试结论 #

• 同步场景（delayMs=0）：代码异常、主动抛出错误、调用reject()这三种情况，均能被main函数的catch捕获，符合隐式try/catch的描述。
• 异步场景（依赖另一个Promise）：调用reject()仍能被main的catch捕获，但代码异常和主动抛出错误会触发顶层未捕获异常，直接导致进程退出，无法被外层catch处理。

疑问与讨论 #

我发现可以给delay()返回的Promise添加catch处理，并在其中调用外层的reject()，从而把错误传递到main的catch中，但不确定这种做法是否是必须的？

另外，不少文章认为async/await更便于错误处理，但我觉得单纯使用Promise链式调用更适合做严格的错误控制。

内容的提问来源于stack exchange，提问作者gschro