JavaScript Interview Question

1. Recursion vs Iterative

The concept of Recursion and Iteration is to execute a set of instructions repeatedly. The difference between them is that recursion is simply a method call in which the method being called is the same as the one making the call while iteration is when a loop is repeatedly executed until a certain condition is met. Under the hood, both recursion and iteration depends on a condition so as to know when to stop but recursion is simply a process, always applied to a function.

An example of recursion is shown below:

and here’s an example of iteration:

Key Differences between Recursion and Iteration

  • A conditional statement decides the termination of recursion while a control variable’s value decide the termination of the iteration statement (except in the case of a while loop).
  • Infinite recursion can lead to system crash whereas, infinite iteration consumes CPU cycles.
  • Recursion repeatedly invokes the mechanism, and consequently the overhead, of method calls. This can be expensive in both processor time and memory space while iteration doesn’t.
  • Recursion makes code smaller while iteration makes it longer.

These are some of the key differences between iteration and recursion. I hope this has been explanatory.

2. DOM (Document Object Model)

The Document Object Model (DOM) is a programming interface for HTML and XML(Extensible markup language) documents. It defines the logical structure of documents and the way a document is accessed and manipulated. DOM is a way to represent the webpage in the structured hierarchical way so that it will become easier for programmers and users to glide through the document. With DOM, we can easily access and manipulate tags, IDs, classes, Attributes or Elements using commands or methods provided by Document object. DOM can be thought of as Tree or Forest(more than one tree). The term structure model is sometimes used to describe the tree-like representation of a document. One important property of DOM structure models is structural isomorphism: if any two DOM implementations are used to create a representation of the same document, they will create the same structure model, with precisely the same objects and relationships.

Why called as Object Model ?

Documents are modeled using objects, and the model includes not only the structure of a document but also the behavior of a document and the objects of which it is composed of like tag elements with attributes in HTML.

Properties of DOM:

Let’s see the properties of document object that can be accessed and modified by the document object.

3. Event bubbling

Event bubbling is a method of event propagation in the HTML DOM API when an event is in an element inside another element, and both elements have registered a handle to that event. It is a process that starts with the element that triggered the event and then bubbles up to the containing elements in the hierarchy. In event bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements. Syntax: addEventListener(type, listener, useCapture)


  • type: Use to refer to the type of event.
  • listener: Function we want to call when the event of the specified type occurs.
  • userCapture: Boolean value. Boolean value indicates event phase. By Default useCapture is false. It means it is in the bubbling phase.

Example: This example shows the working of event bubbling in JavaScript.

4. Callback Function

In JavaScript, functions are objects. Can we pass objects to functions as parameters? Yes.

So, we can also pass functions as parameters to other functions and call them inside the outer functions. Sounds complicated? Let me show that in an example below:

The print( ) function takes another function as a parameter and calls it inside. This is valid in JavaScript and we call it a “callback”. So a function that is passed to another function as a parameter is a callback function. But that’s not all.

Why do we need Callback Functions?

JavaScript runs code sequentially in top-down order. However, there are some cases that code runs (or must run) after something else happens and also not sequentially. This is called asynchronous programming.

Callbacks make sure that a function is not going to run before a task is completed but will run right after the task has completed. It helps us develop asynchronous JavaScript code and keeps us safe from problems and errors.

In JavaScript, the way to create a callback function is to pass it as a parameter to another function, and then to call it back right after something has happened or some task is completed.

5. API

When you use an application on your mobile phone, the application connects to the Internet and sends data to a server. The server then retrieves that data, interprets it, performs the necessary actions and sends it back to your phone. The application then interprets that data and presents you with the information you wanted in a readable way. This is what an API is — all of this happens via API.

To explain this better, let us take a familiar example.

Imagine you’re sitting at a table in a restaurant with a menu of choices to order from. The kitchen is the part of the “system” that will prepare your order. What is missing is the critical link to communicate your order to the kitchen and deliver your food back to your table. That’s where the waiter or API comes in. The waiter is the messenger — or API — that takes your request or order and tells the kitchen — the system — what to do. Then the waiter delivers the response back to you; in this case, it is the food.

Here is a real-life API example. You may be familiar with the process of searching flights online. Just like the restaurant, you have a variety of options to choose from, including different cities, departure and return dates, and more. Let us imagine that you’re booking you are flight on an airline website. You choose a departure city and date, a return city and date, cabin class, as well as other variables. In order to book your flight, you interact with the airline’s website to access their database and see if any seats are available on those dates and what the costs might be.

6. Bind, Call and Apply

Bind(): returns a new function, allowing you to pass in an array and any number of arguments.

Call( ): The call() method invokes a function with a given ‘this’ value and arguments provided one by one. This means that we can call any function, and explicitly specify what ‘this’ should reference within the calling function.

Apply( ): Invokes the function and allows you to pass in arguments as an array.

7. “this ” Keyword

Objects are the basic building blocks in JavaScript. There’s one special object available in JavaScript, the this object. You can see the value of this at every line of JavaScript execution. The value of this is decided based on how the code is being executed.
Before getting started with this, we need to understand a little about the JavaScript runtime environment and how a JavaScript code is executed. The environment (or scope) in which the line is being executed is known as the execution context. The JavaScript runtime maintains a stack of these execution contexts, and the execution context present at the top of this stack is the one currently being executed. The object this refers to changes every time the execution context is changed.

“this” Refers to a New Instance

When a function is invoked with the new keyword, then the function is known as a constructor function and returns a new instance. In such cases, the value of this refers to a newly created instance. For example:

8. Asynchronous

In a synchronous programming model, things happen one at a time. When you call a function that performs a long-running action, it returns only when the action has finished and it can return the result. This stops your program for the time the action takes.

An asynchronous model allows multiple things to happen at the same time. When you start an action, your program continues to run. When the action finishes, the program is informed and gets access to the result (for example, the data read from disk).

We can compare synchronous and asynchronous programming using a small example: a program that fetches two resources from the network and then combines results.

In a synchronous environment, where the request function returns only after it has done its work, the easiest way to perform this task is to make the requests one after the other. This has the drawback that the second request will be started only when the first has finished. The total time taken will be at least the sum of the two response times.

The solution to this problem, in a synchronous system, is to start additional threads of control. A thread is another running program whose execution may be interleaved with other programs by the operating system — since most modern computers contain multiple processors, multiple threads may even run at the same time, on different processors. A second thread could start the second request, and then both threads wait for their results to come back, after which they resynchronize to combine their results.

In the following diagram, the thick lines represent time the program spends running normally, and the thin lines represent time spent waiting for the network. In the synchronous model, the time taken by the network is part of the timeline for a given thread of control. In the asynchronous model, starting a network action conceptually causes a split in the timeline. The program that initiated the action continues running, and the action happens alongside it, notifying the program when it is finished.

9. Event Loop

Asynchronous programs are executed piece by piece. Each piece may start some actions and schedule code to be executed when the action finishes or fails. In between these pieces, the program sits idle, waiting for the next action.

So callbacks are not directly called by the code that scheduled them. If I call setTimeout from within a function, that function will have returned by the time the callback function is called. And when the callback returns, control does not go back to the function that scheduled it.

Asynchronous behavior happens on its own empty function call stack. This is one of the reasons that, without promises, managing exceptions across asynchronous code is hard. Since each callback starts with a mostly empty stack, your catch handlers won’t be on the stack when they throw an exception.

try {
setTimeout(() => {
throw new Error(“Woosh”);
}, 20);
} catch (_) {
// This will not run
No matter how closely together events — such as timeouts or incoming requests — happen, a JavaScript environment will run only one program at a time. You can think of this as it running a big loop around your program, called the event loop. When there’s nothing to be done, that loop is stopped. But as events come in, they are added to a queue, and their code is executed one after the other. Because no two things run at the same time, slow-running code might delay the handling of other events.

This example sets a timeout but then dallies until after the timeout’s intended point of time, causing the timeout to be late.

let start =;
setTimeout(() => {
console.log(“Timeout ran at”, — start);
}, 20);
while ( < start + 50) {}
console.log(“Wasted time until”, — start);
// → Wasted time until 50
// → Timeout ran at 55
Promises always resolve or reject as a new event. Even if a promise is already resolved, waiting for it will cause your callback to run after the current script finishes, rather than right away.

console.log(“Me first!”);
// → Me first!
// → Done
In later chapters we’ll see various other types of events that run on the event loop.

10. Window Object

JavaScript Window is a global Interface (object type) which is used to control the browser window lifecycle and perform various operations on it.

A global variable window, which represents the current browser window in which the code is running, is available in our JavaScript code and can be directly accessed by using window literal.

It represents a window containing a webpage which in turn is represented by the document object. A window can be a new window, a new tab, a frame set or individual frame created with JavaScript.

In case of multi tab browser, a window object represents a single tab, but some of its properties like innerHeight, innerWidth and methods like resizeTo() will affect the whole browser window.

Whenever you open a new window or tab, a window object representing that window/tab is automatically created.

The properties of a window object are used to retrieve the information about the window that is currently open, whereas its methods are used to perform specific tasks like opening, maximizing, minimizing the window, etc.

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