为什么退出过程在循环结束之前执行？

Question

I came from Ruby, so I spent a day to figure out why process.exit(1) executes before loop end. Any ideas what I need to change to make it work?

export default async () => {
    const workbook = Xlsx.readFile(path.join(__dirname, './excel.xlsx'));
    const data = Xlsx.utils.sheet_to_json(workbook.Sheets.Extract, {
        header: getHeaderFor(workbook.Sheets.Extract),
    });
    await data.map(async (product, index) => {
        if (index < 6) return;
        await processProduct(product);
    });
    process.exit(1);
};

Answer 1

I think that If you replace:

await data.map(async (product, index) => {
    if (index < 6) return;
    await processProduct(product);
});

with

for (let i = 6; i < data.length; i++) {
  await processProduct(data[i]);
}

your problem will be solved.

Some explanation: javascript's map function is creating new array from source array when it is calling a callback function for each element of the source array and pushing the returned value of the callback into the result array. In this case await data.map(... is invalid statement, because the result of map is array. To fix this it is possible to wrap the result in await Promise.all(data.map(...)) or to use a normal for loop. You can use the first case when you don't care about concurrency and the second case when you want to achieve sequential order.

Answer 2

While @codtex's answer will work, it will also result in poor performance since each iteration of the loop will wait until the previous iteration. What you really want to do is collect all of the Promises, and then wait for them to finish:

const promises = data.map((product, index) => {
  if (index < 6) return;
  return processProduct(product);
});

await Promise.all(promises);