Javascript - async await

// 1. async functions always return a Promise
// 2. await will return resolved data of a promise, it can be a 'data or an 'error'.

// An async function: getDataPromise()
const getDataPromise = (num) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            typeof num === 'number' ? resolve(num * 2) : reject("Number must be provided")
        }, 2000)
    })
}

// 1. Call getDataPromise() directly within another funcion
const processData1 = () => {
    getDataPromise(2).then(data => {
        result = data;
        console.log('Data1: ' + data);
    }).catch(err => {
        console.log("Error1: " + err)
    })
}
processData1();

// 2. Return getDataPromise() then call it outside of a function.
const processData2 = () => {
    return getDataPromise(2);
}
processData2().then(data => {
    console.log('Data2: ' + data);
}).catch(err => {
    console.log("Error2: " + err)
});

// 3. Call getDataPromise() with await in a async function
const processData3 = async () => {
    // result can be the 'data' or the 'error'
    const result = await getDataPromise('2');
    return result;
}

processData3().then(data => {
    console.log('Data3: ' + data);
}).catch(err => {
    console.log('Error3: ' + err);
});

/////////////////////////////////////////////////////////////////////////////
// NOTE: await won’t work in the top-level code. But we can use it like this
/////////////////////////////////////////////////////////////////////////////
(async () => {
  let response = await fetch('/article/promise-chaining/user.json');
  let user = await response.json();
  ...
})();

/////////////////////////////////////////////////////////////////////////////
// NOTE: Thenable object also support async and await.
/////////////////////////////////////////////////////////////////////////////
class Thenable {
    constructor(num) {
        this.num = num;
    }
    then(resolve1, reject) {
        console.log(resolve1);
        // resolve with this.num*2 after 1000ms
        setTimeout(() => resolve1(this.num * 2), 1000); // (*)
    }
};

// 1. Call thenable object
// var obj = new Thenable(2);
// obj.then((data) => { 
//     console.log("resolve: " + data);
// }, err => {
//     console.log("reject: " + err)
// })

// 2. Use async and await
async function f() {
    // waits for 1 second, then result becomes 2
    let result = await new Thenable(2);
    console.log(result);
}
f();