Javascript Promise

Create a promise and consume a promise

example one

let p = new Promise((resolve, reject) => {
    let n = Math.floor(Math.random() * 100);
    let good = n % 2 == 0 ? true : false;

    if (good) {
        setTimeout(resolve, 100, { "dd": 1212 });
    } else {
        setTimeout(reject, 200, "bad2");
    }
});

p.then((v) => console.log("suc", v))
    .catch((v) => console.log("failed:", v));

//this is identical as the above
p.then((v) => console.log("suc", v), (v) => console.log("failed:", v));

example two

import { readFile as _readFile } from 'fs';

let p = new Promise((resolve, reject) => {
  _readFile("./test.js", 'utf8', (err, data) => {
    if (err) {
      reject(err);
    } else {
      resolve(data);
    }
  })
});

p.then((data) => console.log("good", data))
  .catch((err) => console.log("err", err))

Use await to consume a function which return a promise

example one

function myCreate() {
    return new Promise((resolve, reject) => {
        let n = Math.floor(Math.random() * 100);
        let good = n % 2 == 0 ? true : false;

        if (good) {
            setTimeout(resolve, 100, { "dd": 1212 });
        } else {
            setTimeout(reject, 200, "bad2");
        }
    });
}

//Any exception thrown in the try block (or any rejected promise when using await) will jump to the catch.
try {
    const ans = await myCreate();
    console.log("ans", ans);
} catch (err) {
    console.log("err:", err);
}

use async function to consume the promise

function myCreate() {
    return new Promise((resolve, reject) => {
        let n = Math.floor(Math.random() * 100);
        let good = n % 2 == 0 ? true : false;

        if (good) {
            setTimeout(resolve, 100, { "dd": 1212 });
        } else {
            setTimeout(reject, 200, "bad2");
        }
    });
}

async function test() {
    try {
        const ans = await myCreate();
        console.log("ans", ans);
    } catch (err) {
        console.log("err:", err);
    }
}

test();

You still can use then to consume a promise returned by a function.

function myCreate() {
    return new Promise((resolve, reject) => {
        let n = Math.floor(Math.random() * 100);
        let good = n % 2 == 0 ? true : false;

        if (good) {
            setTimeout(resolve, 100, { "dd": 1212 });
        } else {
            setTimeout(reject, 200, "bad2");
        }
    });
}

myCreate().then((ans) => console.log("ans", ans), (err) => console.log("err:", err));