可以等待/异步在javascript中阻塞非阻塞进程

Question

I read this article from node

https://nodejs.org/en/docs/guides/blocking-vs-non-blocking/

It says the code below is a process blocker:

const fs = require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

what if I add await? will the code above becomes non-blocking or it will stay in its true nature?

Example code:

const fs = require('fs');
const data = await fs.readFileSync('/file.md'); // no more blocking
console.log(data);

Thank you

Answer 1

No, the code can't run since await must use in async function .

And await should use for function that return promise.

the code means:

// await new Promise(...)
// console.log(...)

new Promise().then((...) => console.log(...))

If you should non-block function, you should use fs.readFile instead.

Answer 2

Blocking means that the whole application is blocked.

So, all SetInterval , Promise , Events or whatever async callback are paused until that sync function ends its execution.

It's the same of what you get when you use a for..loop .

NodeJS provides you some non-blocking file system methods with callbacks, just change your code like this.

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
    if (err) throw err;
    console.log(data)
});

There is the only way.

Answer 3

await operator wait a promise, and wrapped in async function

you should code like this

const fs = require("fs");

function readFile(fileName) {
  return new Promise((resolve, reject) => {
    fs.readFile(fileName, (err, data) => {
      if (err) reject(err);
      resolve(data);
    });
  });
}

async function f1() {
  try {
    var x = await readFile("foo.json");
    console.log(x);
  } catch (e) {
    console.log(e); // 30
  }
}

f1();