图像不会使用承诺下载

Question

>我需要下载所有图像并用它们生成word文档。 使用 nodeJS 和 Meteor

WebApp.connectHandlers.use('/download', async function (req, res, next) {
// ...
const images = [];
await lines.forEach(async (line, k) => {
if (line.type && line.type === 'image') {
images.push({
id: line.id,
file: line.id + '.jpg',
});
download_image(line.imageUrl, line.id + '.jpg');
}
});
// ...
// Then I use images[] to insert them into a Word document.
});
const download_image = (url, image_path) =>
axios({
url,
responseType: 'stream',
}).then(
(response) =>
new Promise((resolve, reject) => {
response.data
.pipe(fs.createWriteStream(image_path))
.on('finish', () => resolve())
.on('error', (e) => reject(e));
})
);

问题是在我将图像插入Word文档之前没有下载图像。

如何在图像完成下载之前停止/等待？我不太擅长承诺。想念她的是什么？

谢谢！

Answer 1

使用带有async函数的.forEach(或类似的数组方法(是常见的错误。async function只是意味着它返回承诺，并且await的工作方式与将承诺与then链接在一起的方式相同。因此，此行wait lines.forEach(async (line, k) => {只会创建并返回一堆承诺，但它不会等待里面的所有承诺完成。

WebApp.connectHandlers.use('/download', async function (req, res, next) {
// ...
const images = [];
const promises = [];
lines.forEach((line, k) => {
if (line.type && line.type === 'image') {
images.push({
id: line.id,
file: line.id + '.jpg',
});
promises.push(download_image(line.imageUrl, line.id + '.jpg'));
}
});
// here you get array with all the images downloaded
const downloadedImages = await Promise.all(promises);
// this line will be executed after you download all images
// ...
});
// This function would work same with or without the `async` keyword 
// (because async function return promise - you are returning the promise. 
// Async function allows to use await, but you are not using await in this function).
// However it is good practice to have all functions that returns promise 
// marked as `async` so you know that you receive promise from it.
const download_image = async (url, image_path) =>
// Dont forget to return your promise otherwise you cannot await it
return axios({
url,
responseType: 'stream',
}).then(
(response) =>
new Promise((resolve, reject) => {
response.data
.pipe(fs.createWriteStream(image_path))
.on('finish', () => resolve())
.on('error', (e) => reject(e));
})
);