Javascript - Async, Await, Promises

1.callback_hell.js

const fs = require("fs");
const path = "./temp.txt";

const errorCallback = (err) => {
    console.log(err);
    console.log("Failed writing to the files");
}

const doneCallback2 = (err) => {
    if(err) {
        errorCallback(err);
    } else {
        console.log("Wrote to the file temp2 successfully");
    }
}

const doneCallback = (err) => {
    if (err) {
        errorCallback(err);
    } else {
        console.log("Wrote to the file successfully");
        console.log("Step 2. Write to temp2.txt");
        fs.writeFile("./temp1.txt", "Hello world "+ new Date(),doneCallback2 )
    }
};

console.log("Step 1. Write to temp.txt");
fs.writeFile(path, "Hello world "+ new Date(), doneCallback);

2.sync.js

const fs = require("fs");

function main() {
try {
    console.log("Writing to file 1");
    fs.writeFileSync("./temp.txt", "Hello World "+new Date());
    console.log("Wrote to file 1 successfully");
    console.log("Writing to file 2");
    fs.writeFileSync("./temp2.txt", "Hello World "+new Date());
    console.log("Wrote to file 2 successfully");
} catch (err) {
    console.log(err);
    console.log("Failed writing to the files");
}
}

main();
console.log("After main");

3.promises.js

const fs = require("fs");

const writeFilePromise = async (filePath, data) => {
    return new Promise((resolve, reject) => {
        fs.writeFile(filePath, data, (err) => {
            if (err) {
                reject(err);
            } else {
                resolve();
            }
        })
    })
}

async function main() {
    try {
        console.log("Writing to file 1");
        await writeFilePromise("./temp.txt", "Hello World "+new Date());
        console.log("Wrote to file 1 successfully");
        console.log("Writing to file 2");
        await writeFilePromise("./temp2.txt", "Hello World "+new Date());
        console.log("Wrote to file 2 successfully");
    } catch (err) {
        console.log(err);
        console.log("Failed writing to the files");
    }
}

main();
console.log("After main");

4.fs.promises.js

const fs = require("fs").promises;
async function main() {
    try {
        console.log("Writing to file 1");
        await fs.writeFile("./temp.txt", "Hello World "+new Date());
        console.log("Wrote to file 1 successfully");
        console.log("Writing to file 2");
        await fs.writeFile("./temp2.txt", "Hello World "+new Date());
        console.log("Wrote to file 2 successfully");
    } catch (err) {
        console.log(err);
        console.log("Failed writing to the files");
    }
}
    
    main();
    console.log("After main");

5.multiple-promises.js

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

/**
 * Returns a promise that resolves after n seconds.
 * @param {number} n The no of seconds to wait
 * @returns Promise<void>
 */
async function wait(n = 1) {
    return new Promise(resolve => setTimeout(resolve, n * 1000))
}

function logTime() {
    console.log(new Date().toLocaleDateString('en-US', {second: '2-digit'}));
}
async function _main() {
    logTime();
    // await wait();
    // await Promise.all([wait(), wait(), wait(), wait(), wait()])
    for (let i = 0; i < 5; i++) {
        await wait();
    }
    logTime();
}

async function main() {
    const query = { name: "rajiv"};
    const users = await Users.find(query).exec();
    if (users.length == 0) {
        throw new Error("No user can be found with the given name");
    }
    const [user] = users;
    // const posts = await Posts.find({"userId": user.id}).exec();
    // const comments = await Comments.find({"userId": user.id}).exec();
    const [posts, comments ] = await Promise.all([
        Posts.find({"userId": user.id}).exec(),
        Comments.find({"userId": user.id}).exec()
    ]);
    return { posts, comments };
}

main();

6.arrays-promises.js

function logTime() {
    console.log(new Date().toLocaleDateString('en-US', {second: '2-digit'}));
}

async function wait(n = 1) {
    return new Promise(resolve => setTimeout(resolve, n * 1000))
}

async function getData(userId) {
    await wait();
    return { name : "Name "+ userId};
}

const timeout = 3000;
async function getDataWithTimeout(userId) {

    return new Promise((resolve, reject) => {
        let promisePending = true;
        const timeoutCallback = () => {
            if (promisePending) {
                reject(new Error("Timed out"));
                promisePending = false;
            }
        }
        setTimeout(timeoutCallback, timeout);
        http.get("url", (err, response) => {
            if (!promisePending) {
                return;
            }
            promisePending = false;
            if (err) {
                reject(err);
            } else {
                resolve(response);
            }
        })
    })
}

async function main() {
    const userIds = [2, 4, 7];
    logTime();
    const data = await getData(2);
    // const data = await Promise.all(userIds.map(userId => getData(userId)));
    console.log(JSON.stringify(data));
    logTime();
}

main();