Awaiting on async method call in JS

Have you ever been in a situation where you have spent hours together trying to debug as to why that database query is not awaited even after using the await for an async method.

Lately, I ran into an issue where the database call was passed and the method run finished with the undefined in the database response even after using await. After spending hours trying to fix it, one article took me back top using a Promise in javascript. Unfortunately this did not end the story as little was known to me that the two methods resolve and reject passed as params to the lambda call can be utilized to fetch the return response from that call like below.

export function getDataFromDb() {
    var response = new Promise((resolve, reject) => {
        .... 
        // lines to fetch data from db
        ....

        if(error) {
            reject(error);
        }

        resolve(dbData);
    })

    return response
}

....
....
// make async call
var result = await getDataFromDb();
....

The resolve() call actually returns the response from the database call or similar.

Promise actually helps in awaiting the response from this or similar async calls and it also gives the benefit of reject the response, if an error was found.

Another or the synchronous way of calling the promise method is by using the then and/ or the catch methods of the promise. The then call's parameter should have the return value of the promise's method passed to the resolve method and the catch call's parameter is the parameter passed to the reject method.

// make sync call
getDataFromDb()
  .then(res => { console.log("Res from resolve is " + res) })
  .catch(err => { console.error("Error from reject is " + err.msg) }

Having shown both the ways, it is for one to decide which way of calling the promise is better 😂 ...

How Promise works

Promise was apparently called or used as theneables in JS back in the day, I am assuming it to be before ES6 or 2015.

To begin with let's understand Promise's signture as of today.

var Promise: PromiseConstructor
new <string>(executor: (resolve: (value: string | PromiseLike<string>) => void, reject: (reason?: any) => void) => void) => Promise<string>

Depending on the type passed, resolve must be called with the same type, in this case it is string, whereas the reject can be of any, may be new Error().

Promise takes in two methods as parameters. One to be called if the promise was completed successfully and the other to be called if not. The two paramters are called the resolve and the reject calls. The parameter passed to the resolve method is the return value of the promise.

Why use Promise's

Promise's are more or less another way of using the try - catch block if one looks closely in an asynchronous way. Is it possible to run atry-catch in an asynchronous way? Using promises's again and again can help improve one's ability to understand their use cases better for sure.