如何获取异步的值并传递给 function？

Question

I need to get the value of mtime and return as result of lastSave function but i get undefined. I know I can use fs.statSync("./Data.xlsx") to get this value. I'm using fs.stat() as example for async inside a function.

const lastSave = async (file) => {
    const momento = await fs.stat(file, function (err, stats) {
        var mtime = stats.mtime;
        console.log("interno", mtime);    // this works
        return mtime
    });
    return momento
}

console.log(lastSave('./Data.xlsx'))  // this writes "undefined"

Answer 1

Use the sync variant of the function.

const fs = require('fs');

const lastSave = file => {
    return fs.statSync(file).mtime;
}

console.log(lastSave('./Data.xlsx'));

If there is no sync version wrap the call in a async function and call it:

const fs = require('fs');

const lastSave = file => {
    return new Promise(done => fs.stat(file, function (err, stats) {
        var mtime = stats.mtime;
        console.log("interno", mtime);    // this works
        done(mtime);
    }));
}

(async () => {
    console.log(await lastSave('./Data.xlsx'));
})();

You can use the promise-based variant:

const fs = require('fs/promises');

const lastSave = async file => {
    return (await fs.stat(file)).mtime;
}

(async () => {
    console.log(await lastSave('./Data.xlsx'));
})();

Alternatively you can promisify your async function:

const fs = require('fs');
const util = require('util');

const lastSave = async file => {
    return (await util.promisify(fs.stat)(file)).mtime;
}

(async () => {
    console.log(await lastSave('./Data.xlsx'));
})();

Alternatives for

(async () => {
    console.log(await lastSave('./Data.xlsx'));
})();

are

lastSave('./Data.xlsx').then(console.log);

and

lastSave('./Data.xlsx').then(result => { console.log(result); });