[英]JS: Promise doesn't return value
I need to get a value of an asynchronous function. 我需要获取一个异步函数的值。 I tried to use Promise, but that does not work:
我尝试使用Promise,但这不起作用:
const res = new Promise(function (resolve, reject) {
gm(readStream).size({ bufferStream: true }, function (err, size) {
if (!err) resolve(size)
})
})
console.log(res)
The result I get is Promise { <pending> }
我得到的结果是
Promise { <pending> }
Promises are an abstraction for callbacks, not magic. 承诺是回调的抽象,而不是魔术。 They can't make asynchronous code synchronous.
他们无法使异步代码同步。
The correct solution is: 正确的解决方案是:
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)
})
You could also use async / await
here: 您也可以
async / await
这里使用async / await
:
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)}`);
}
Or, if you're using NodeJS (Version 8+), you might be able to adapt your function to use util.promisify
. 或者,如果您使用的是NodeJS(版本8以上),则可以使您的函数适应使用
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. 其他
Promise
库(例如Bluebird
)也提供了此类功能,可以轻松地将“标准”节点样式的功能(具有以err, data
回调err, data
作为参数的回调的功能)转换为可兑现的等价物。
Or just use the callback. 或者只是使用回调。
Your code should be looked like this: 您的代码应如下所示:
const res = new Promise(function (resolve, reject) {
gm(readStream).size({ bufferStream: true }, function (err, size) {
if (!err) resolve(size)
else reject(err)
})
})
function onResolved(data) {
console.log('data: ', data);
}
function onRejected(err) {
console.log('err:', err);
}
res.then(onResolved, onRejected);
Promise does not make your code synchronous, it lets you control when you want to get the result, not immediately like callback
style. Promise不会使您的代码同步,它使您可以控制何时要获得结果,而不必像
callback
样式那样立即进行控制。
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