[英]Why await doesn't wait for callback?
When using await on a function with callback, such as fs.writeFile, await doesn't wait for the code inside the callback to execute.在带有回调的 function 上使用 await 时,例如 fs.writeFile,await 不会等待回调中的代码执行。 For example:
例如:
const fs = require("fs")
async function test() {
for (let i = 0; i < 3; i++) {
console.log(`processing ${i}`)
const fileName = `${i}.json`
await fs.writeFile(fileName, JSON.stringify(i), err => {
if (err) throw err
console.log(`file ${i} is written`)
})
console.log(`${i} is done.`)
}
}
test()
The above code produces:上面的代码产生:
processing 0
0 is done.
processing 1
1 is done.
processing 2
2 is done.
file 1 is written
file 0 is written
file 2 is written
instead of:代替:
processing 0
file 0 is written
0 is done.
processing 1
file 1 is written
1 is done.
processing 2
file 2 is written
2 is done.
Could anyone please explain why await fails to let it finish writing the file before continuing?谁能解释为什么 await 在继续之前无法让它完成写入文件?
This is because writeFile
doesn't return a Promise
and await
waits for a Promise resolution before suspending the execution.这是因为
writeFile
不返回Promise
并且await
在暂停执行之前等待 Promise 分辨率。 You can either upgrade to the latest version of writeFile
or write a function which returns a promise
:您可以升级到最新版本的
writeFile
或编写返回 promise 的promise
:
const fs = require("fs")
async function test() {
for (let i = 0; i < 3; i++) {
console.log(`processing ${i}`)
const fileName = `${i}.json`
await writeToFile(fileName, i);
console.log(`${i} is done.`)
}
}
function writeToFile(fileName, index) {
return new Promise((resolve, reject) => {
fs.writeFile(fileName, JSON.stringify(index), err => {
if (err) {
// denotes failure
reject(err)
}
console.log(`file ${index} is written`);
// denotes success
resolve(true)
})
});
}
test()
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