13+ Best AJAX Interview Questions For Engineers
We've compiled a comprehensive list of AJAX interview questions and answers to help you in your upcoming technical interviews. These questions demand a practical understanding of how AJAX is used. AJAX is commonly covered in technical interviews as it relates to common web development practices for modern user-interface applications. Especially those that rely on Frontend frameworks like React, Angular, Vue, etc. AJAX requests are a way for user-interface applications to communicate with a server-side application via web-requests. Almost every modern user-interface application uses AJAX requests to some extent.
Why is AJAX covered in technical interviews?
AJAX is covered in technical interviews because it is an essential tool used by Frontend engineers (and by proxy, backend engineers) to send requests between a user-interface and server-side application. AJAX uses HTTP requests to communicate, so using AJAX properly requires a strong understanding of the HTTP protocol, its various methods, and its proper uses and best practices. All of this is central to being viewed as a strong candidate for web application development positions.
15 AJAX Interview Questions & Answers
1. Describe in practical terms what AJAX is used for in a web application.
2. What are the http methods used in AJAX calls, and what do they do?
There are five http methods used in AJAX calls, they are GET, POST, PUT, PATCH, DELETE.
PATCH requests are used to update some entity in the server-side application; however, unlike the AJAX PUT method which is meant to replace an existing entity, a PATCH request only updates part of the existing entity. The JSON payload for a PATCH request will not contain the full set of properties of the entity to be updated. Instead, it will carry only the properties which are the focus of the update. The request body for an AJAX PATCH request might look like, e.g.
5. What is an HTTP request vs an AJAX request?
An AJAX request is asynchronous, meaning that the time the result will return for the request is indeterminate. You can’t guarantee when an AJAX response will come, or if it will at all; an AJAX request could fail! Other processes and code can run while the result of the request is being awaited. If several AJAX requests are initiated at the same time, for example perhaps two separate AJAX requests load in a user’s personal data and a list of a user’s friends, the result of those two requests can be displayed in whichever order they return. Loading indicators can be rendered on the screen, and the user can still use the application while the AJAX requests are pending.
7. Depending on the method by which you make AJAX requests, for example using ES6 fetch() vs jQuery $.ajax(), your code would rely on either promises or callbacks, respectively. Describe what a callback function would be used for in the context of an AJAX request.
8. What is a promise? Describe what a promise would be used for in the context of an AJAX request.
As noted earlier, AJAX requests are asynchronous processes, meaning that it is indeterminate when the AJAX request will be complete. The completion depends on many factors which can not be accounted for, for example a given user’s internet connection speed. Because AJAX requests are asynchronous, a promise can be defined such that it will have a desired value, after the AJAX call has completed.
9. What is one advantage of using promises rather than callbacks when dealing with asynchronous functionality like AJAX requests?
Using callback functions in the context of making AJAX requests is not necessarily a bad practice; however, there are situations where callback functions can lead to unreadable and difficult to understand code. Specifically when callback functions are nested.
An example of AJAX requests using nested callbacks:
If you read carefully through the above code, you’ll see that we are making 3 AJAX requests.
1) First fetching a list of users.
2) A second AJAX request which depends upon the result of the first. In this case, we take the first user from the list of users, and then gets more details about that user.
3) We search for all the users who share the firstName of the user in the second request.
Hard to follow! See below how promises can clean up the above code.
10. How can AJAX have disadvantages or provide obstacles in terms of SEO? What are some solutions to these problems?
Content-driven websites which rely on search rankings in Google or other search engines must be careful in their usage of AJAX. While AJAX is a straightforward way to build more dynamic-feeling and interactive user interfaces, improper usage can cause a content-dependent site to be virtually unfindable on the internet!
The reason this can happen, is because when Google builds its indexes of searchable websites, it uses computer algorithms to “crawl” the web, landing on webpages, and digesting the content that is on the webpage. When a site uses AJAX to load in its content, Google’s crawling algorithm will land on a page, and see no content. The content might be on its way, but the crawler does not know that it should wait for an AJAX request to complete, so this page looks completely blank!
Fortunately, there are some solutions that allow a website to combine the interactivity of an AJAX-driven user interface and the searchability of its content. These solutions are called server-side rendered frontend frameworks, React Next is one such example.
$.ajaxSetup() takes all the same options as a regular $.ajax() call; however, the options passed in will serve as defaults for all future $.ajax() calls, (until $.ajaxSetup() is called again). Our solution will look like:
Now, all of our future $.ajax() calls will include the “yes-pls” header.
When a web browser sees that you are trying to make an AJAX request between two different URLs (for example, a GET request), the browser first sends what is called a “preflight” OPTIONS request. This request will ask the server being contacted if it has explicitly allowed the sending server to make the GET request. If the webserver was not specificall configured to allow GET requests from the sending hostname, then the OPTIONS request will fail and the GET request will not occur.
13. In many cases, AJAX requests are made to an API that exists at some web address; however, what if you wanted to load a static file with AJAX. Would this be possible?
It is possible to load in data from a static file. Instead of using an API’s URL for the URL property of your AJAX request, simply point directly to the static file’s address. This could be a JSON file for example.
14. Bob is a web developer working on an interface that shows a list of users and their associated data like their name, phone number, and address. Bob makes an AJAX call to retrieve the list of users, but he notices that the data he receives does not have the address field that is required.
He decides that, since there are only 4 users in the list, he will make a separate AJAX request for each user, to an endpoint where he can retrieve the address as needed.
Is Bob’s approach to solving this problem OK? If not, why?
Bob did not make the best solution to the problem. The issue is that Bob relies on the assumption that there are only 4 users in the system. If the system grows, to say 200 users, loading Bob’s list will require 200 AJAX requests to be made to the backend. Just to retrieve address information! The proper approach is for Bob to communicate his need for address information to his colleagues who wrote the backend application, or communicate with the product team to see if the feature can be excluded until a better solution can be implemented.
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