Introduction to Promise function
Promise is a built in function with 2 arguments:
basic syntax of the promise() function is:

    var promise1 = new Promise(function(resolve, reject) {
        setTimeout(function() {
            resolve('AfterTimeout');
        }, 1000);
    });
    promise1.then(function(value) {
        console.log(value);

    });
    console.log(promise1);
In this sample, the Promise function will run a setTimeout function
the Promise function is asyncronous, therefore any action run by it will hapan after the regular code.

Promise with await

Vid 27 ~ 30:00

    const getUser = cb => {
        setTimeout(() => {
            cb({
                name: 'Max'
            })
        }, 2000)
    }

    getUser(user => {
        console.log(user.name);
    });

Loading external script

the next example shows how to add a link to external script in code, using promise function

    function loadScript(src) {
        const SCR = 'https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.2.0/lodash.js'
        return new Promise((resolve, reject) => {
            let script = document.createElement('script');
            script.src = src;

            script.onload = () => resolve(script);
            script.onerror = () => 
                reject(new Error('Script load error:' + src));
            document.head.append(script);
        });
    }

    let promise =
    loadScript('SCR');
    promise.then(
        script => alert(`${script.src} is loaded!`),
        error => alert(`Error: ${error.message}`)
    );
and with async & await

    (
      async() => {                  
        try {
          let script =
            await loadScript(SCR);
              alert(`${script.src} is loaded!`);
        } catch (error) {
          error => alert(`Error: ${error.message}`);
        }
      }
    )();    
    //the func is wrapped with () and ran by ()
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