JS:承诺不返回值 [英] JS: Promise doesn't return value

问题描述

我需要获取异步函数的值.我尝试使用 Promise，但这不起作用:

I need to get a value of an asynchronous function. I tried to use Promise, but that does not work:

const res = new Promise(function (resolve, reject) {
  gm(readStream).size({ bufferStream: true }, function (err, size) {
    if (!err) resolve(size)
  })
})
console.log(res)

我得到的结果是 Promise { <pending>}

推荐答案

Promise 是回调的抽象，而不是魔法.他们无法使异步代码同步.

Promises are an abstraction for callbacks, not magic. They can't make asynchronous code synchronous.

正确的解决方法是:

const res = new Promise(function (resolve, reject) {
  gm(readStream).size({ bufferStream: true }, function (err, size) {
    if (err) reject(err);
    else resolve(size);

  })
});

res.then(function(promiseResolutionValue) {
    console.log(res)
})

您也可以在此处使用 async/await:

You could also use async / await here:

const getSize = readStream => {
    return new Promise(function (resolve, reject) {
    gm(readStream).size({ bufferStream: true }, function (err, size) {
      if (err) reject(err);
      else resolve(size);
    })
  });
}


let printSize = async readStream => {
  console.log(`Size is ${await getSize(readStream)}`);
}

或者，如果您使用的是 NodeJS(版本 8+)，您或许可以调整您的函数以使用 util.promisify.
其他 Promise 库，例如 Bluebird，也提供了这样的函数，可以轻松地转换标准"节点样式的函数(具有 err、data 作为参数)转换为承诺返回的等价物.

Or, if you're using NodeJS (Version 8+), you might be able to adapt your function to use util.promisify.
Other Promise libraries, such as Bluebird, also offer such functions, to easily convert 'standard' node-style functions (functions that have a callback with err, data as arguments) into promise-returning equivalents.

或者只是使用回调.

这篇关于JS:承诺不返回值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！