Can you explain the distinction between incorporating and excluding the keyword "await" in the following code snippets?

Currently, I'm diving into an MDN article that covers async/await. I've grasped the rationale behind using the async keyword preceding functions, yet there's a bit of uncertainty regarding the await keyword. Although I've researched "await" extensively and comprehend the general idea, examples still leave me puzzled. Take this simple snippet of code (featured in the MDN article) utilizing async/await.

async function hello() {
  return greeting = await Promise.resolve("Hello");
};

hello().then(value => console.log(value));

It's no surprise that this snippet logs "Hello" to the console. However, even if we remove "await," the result remains consistent.

async function hello() {
  return greeting = Promise.resolve("Hello"); // without await
};

hello().then(value => console.log(value));

Could someone shed light on what exactly the await keyword followed by Promise.resolve accomplishes? Why does the output stay unchanged when it's not included? Appreciate any insights.

Answer №1

Modern JavaScript engines have made it so that there is no distinction between using return await somePromise and just return somePromise at the top level of a function. However, if this code were within a control block like a try/catch, the difference would be noticeable.

This particular change occurred somewhat recently. Prior to ES2020, using return await would add an additional async "tick" into the process. This was optimized out as part of a normative change in ES2020 specifically for native promises being awaited (there may still be a slight difference with non-native thenables).

To clarify, consider the following two examples:

async function example() {
    try {
        return await somethingReturningAPromise();
    } catch (e) {
        // handle error/rejection
    }
}

versus

async function example() {
    try {
        return somethingReturningAPromise();
    } catch (e) {
        // handle error/rejection
    }
}

In the first example, any rejection from somethingReturningAPromise() will be caught in the catch block due to the await. In the second example, the promise is simply returned without going through the catch.

When not within a control structure, the use of await is mainly a stylistic choice.

Answer №2

It's important to remember that async/await is essentially a more user-friendly interface for handling promises.

An async function always returns a promise automatically (as soon as the first await or return statement is encountered) which will be resolved once the function finishes executing. The value of the resolved promise will match the returned value of the function.

async enables you to utilize await within the asynchronous scope, where await only makes sense when preceding a promise (such as another async function call or a promise created on-the-fly with new Promise()). Using await before non-promise values won't cause any issues.

await signifies a pause in execution and instructs the engine to wait until the promise is fulfilled before continuing. The resulting value of the fulfilled promise will then be given back or 'returned' by await.

For practical illustration:

In your original code snippet:

async function hello() {
  const greeting = await Promise.resolve("Hello"); // HERE I CAN USE await BECAUSE I'M INSIDE AN async SCOPE.
  console.log('I will be printed after above promise is resolved.');
  return greeting;
};

hello().then(value => console.log(value));

In a streamlined version of your code, the async keyword could be omitted entirely:

function hello() {
  return Promise.resolve("Hello");
};

hello().then(value => console.log(value));

If you have any uncertainties, feel free to reach out.

Similar questions

If you have not found the answer to your question or you are interested in this topic, then look at other similar questions below or use the search

Hiding and showing div elements using CSS, JavaScript, and PHP

Here is the current code snippet: <? while ($row1 = mysql_fetch_object($result1)) { echo '<a href="#" onclick="showhide("'.$row1->id.'");">Name</a>'; while ($row2 = mysql_fetch_object($result2)) { ...

Using Internet Explorer to watch full-screen HTML 5 videos

I am in the process of developing my own HTML 5 Browser Player. Everything seems to be working well, except for getting the full screen feature to function properly in IE 10. Chrome, Safari, and Firefox are all doing great with it. Unfortunately, my JavaS ...

Issue: The module '[object Object]' could not be located

const express = require('express'); const app = express(); const jade = require('jade'); const path = require('path'); const server = require('http').createServer(app); const io = require('socket.io').liste ...

Differences in HTML animations can be seen when comparing Google Chrome to Microsoft Edge. Looking for a workaround for autoplay to ensure

My intro video animation is facing recording difficulties due to the autoplay policy in Google Chrome. It seems nearly impossible to capture it accurately. If only autoplay could function for an offline HTML file, my issue would be resolved. Unfortunately ...

What is the correct way to adjust the style.top attribute of an element using JavaScript?

In order to correct a JavaScript source code, I need to adjust the style.top property of a div element based on an integer parameter from a function called index. Here is the current implementation: div.style.top = (index * 22 + 2)+"px"; However, for lar ...

Loading content dynamically into a div from an external or internal source can greatly enhance user experience on a website. By

As I work on my website, I am incorporating a div structure as shown below: <div class="container"> <div class="one-third column"> <a id="tab1" href="inc/tab1.html"><h2>tab1</h2></a> </div> & ...

A guide on organizing nested object data in react-native

{ "crocodile":{ "age_in_years":"20", "name":"snapjaw", "country":"australia" }, "jaguar":{ "age_in_years":"8", "name&q ...

How can TypeScript rules be incorporated into a Next.js project without compromising next/core-web-vitals?

In my current NextJS project which is in typescript, I have the following configuration in my .eslintrc.json: { "extends": "next/core-web-vitals" } Now, I want to include additional typescript rules, such as enforcing the rule of n ...

Waiting for a webpage to fully load with JavaScript in Selenium

I am facing an issue where I need to wait for the page to fully load before executing a certain action. It is important for me that the loading circle on the browser tab stops spinning before proceeding. The current Ajax function I am using does not work c ...

Issue with konvaJS when trying to simultaneously resize, drag, and apply filters to an image

Looking for help with resizing, dragging, and filtering images using Konvajs 2d canvas library? If the images are not resizing properly after applying a filter, can someone assist me? Note: Please be aware that when using Google image URLs, there may be c ...

Is there a way to have a button function as a submit button for a form even when it is located in a separate component within a react application?

I am in the process of creating a user-friendly "My Account" page using react, where users can easily update their account information. I have divided my components into two sections: the navbar and the form itself. However, I am facing an issue with the s ...

click event not triggering custom hook

I have developed a custom hook for fetching data and am struggling to implement it successfully. Below is my custom hook: import { useReducer } from "react"; import axios from "axios"; const dataFetchReducer = (state, action) => { ...

Calculate the occurrences of numbers within a string in an array of objects containing a specific string each time an object is removed

I am facing an issue where I need to re-number strings based on their type, regardless of their previous number. For example, if I delete Oranges 2 from [Oranges 1, Oranges 2, Oranges 3], it should become [Oranges 1, Oranges 2]. This means the numbers shou ...

What is the purpose of specifying http://localhost:3000 when accessing API routes in Next.js?

I created an API route within the pages directory of my NextJS project. It is functioning properly as I am able to retrieve data by directly accessing the URL like http://localhost:3000/api/tv/popular. My goal is to fetch this data using getStaticProps and ...

RadAjaxNamespace is not found within the codebase

While testing the application on my development machine, I encountered an error message in the browser debug console stating "RadAjaxNamespace is not defined". Interestingly, this error does not appear when accessing the production server. Upon comparing t ...

Utilize the Webstorm debugger in conjunction with node.js for seamless debugging

Several weeks ago, I attempted to get the webstorm debugger up and running, but unfortunately, it didn't work. Now, I'm giving it another shot, but I'm faced with the same outcome. I am following the instructions outlined here: http://www.j ...

Exploring Angular 9: Harnessing the Power of Fork Join with an Array of

I have a challenge where I need to send multiple API requests to an endpoint by iterating over an array of values To handle this, I decided to use rxjs library and specifically the forkJoin method //array to keep observables propOb: Observable<any>[ ...

Execute function when button is clicked in ExpressJS

I am looking to execute a function on my node server when a button on my website is clicked: What I currently have: Index.html (I have not included the entire content for simplicity) <button id="tv">tv</button> Client.js (Client side) const ...

What is the best way to achieve a stylish Bootstrap modal design with a blurred and transparent header, as well as a left sidebar that seamlessly blends into

Is it feasible to create a modal with a blurred (transparent) background for the header section, allowing the site to show through? Additionally, can a sidebar on the left side of the modal also be transparent and blurred, revealing the site underneath? C ...

Transforming a base64 encoded string into a byte array

I have implemented a form where users can upload images to the page using an <input id = "fileLoader" type = "file" />. With JavaScript, I convert these uploaded images to base64 and send them to the server. On the server side, I need to decode this ...