承诺过早解决

Question

我遇到了一个问题，我的 Promise.all 解决得太早了。对于测试，我想 console.log 从 promise 映射中推送的数组的长度，但遗憾的是它返回 0。我敢肯定这很简单......

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));

任何帮助将不胜感激。

Answer 1

那里有很多问题。主要分为以下几类：

• 不从履行处理程序返回承诺链的结果，这意味着履行处理程序所在的链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给then，而不是将函数传递给then

在该代码的最小更改中查看内联 cmets：

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));

FWIW，不过，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();
});