承诺过早解决问题

Question

我有一个问题，我的Promise.all解决得太早了。对于一个测试，我想要console.log数组的长度，它是从承诺映射中被推入的，但是它很遗憾地返回了0。我肯定这很简单..。

fromPath(job.data.path, { density: 100, format: "png" }).bulk(-1, true).then(output => {
  Promise.all(output.map(page => 
    jimp.read(Buffer.from(page.base64, 'base64')).then(img => 
      {
        img.invert().getBase64Async(jimp.AUTO).then(data => imageArray.push(data.replace('data:image/png;base64,', ''))).catch(err => console.log(err))
      }
      ).catch(err => console.log(err))
      )).catch(err => console.log(err))
}
  // This returns early
).then(console.log(imageArray.length)).then(done()).catch(err => console.log(err));

任何帮助都将不胜感激。

Answer 1

那里有很多问题。它们主要属于以下几类：

不从实现处理程序返回承诺链的结果，这意味着履行处理程序所处的链将不会链接到您在其中创建的承诺链--调用函数并将其返回值传递到then，而不是将函数传递到。

请参阅该代码的最小、最少更改的内联注释：

fromPath(job.data.path, { density: 100, format: "png" }).bulk(-1, true)
.then(output => {
    // Return the result of `Promise.all`
    return Promise.all(
        output.map(
            page => jimp.read(Buffer.from(page.base64, 'base64'))
                .then(img => {
                    // Return the ersult of this promise chain
                    return img.invert().getBase64Async(jimp.AUTO)
                    .then(data => imageArray.push(data.replace('data:image/png;base64,', '')))
                    // This error handler does nothing useful, since we now return
                    // the promise chain
                    // .catch(err => console.log(err))
                })
                // Recommend **not** handling errors at this level, but using
                // `Promise.allSettled` instead
                .catch(err => console.log(err))
        )
    )
    // This will only be reached if a *synchronous* error occurs calling (for instance)
    // `getBase64Async`, since you've already caught errors above
    .catch(err => console.log(err))
})
// Use a function here, don't call `console.log` directly
.then(() => console.log(imageArray.length))
//    ^^^^^^
// Pass `done` directly to `then`, don't call it and pass its return value
// (e.g., remove the `()`)
.then(done) // Or `.then(() => done)` if you want `done` called with no arguments
.catch(err => console.log(err));

但是，如果我不得不不使用async函数，我可能会这样做：

fromPath(job.data.path, { density: 100, format: "png" }).bulk(-1, true)
.then(output => Promise.allSettled(
    output.map(
        page => jimp.read(Buffer.from(page.base64, 'base64'))
            .then(img => img.invert().getBase64Async(jimp.AUTO))
            .then(data => {
                imageArray.push(data.replace('data:image/png;base64,', ''));
            })
    )
))
.then(results => {
    // Show errors
    const errors = results.filter(({status}) => status === "rejected");
    for (const {reason} of errors) {
        console.error(reason);
    }
    // Done
    done();
});