JavaScript Interview Questions & Answers

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JavaScript is widely used by web developers and is supported by all major web browsers.

If you are looking for a job as a web developer, you will most likely be asked to answer some questions about JavaScript during your interview. In this article, we have compiled a list of some of the most vital JavaScript interview questions and answers you should definitely know the answers for.

We hope this article will help you prepare for your next interview and ace it!

What is the difference between Java & JavaScript?

Java and JavaScript are both widely used, however, there are some key differences between the two. Java is a statically typed, compiled language, meaning you need to use a compiler to type check and convert your code into a bundle that can run in supported runtime environments. On the other hand, JavaScript is a dynamically typed and JIT-compiled / interpreted language, meaning you don’t need to use a compiler before shipping your code.

What are the data types supported by JavaScript?

Primitive data types:

  • Booleans: can be true or false
  • null, undefined: are types that are values in and of themselves
  • Numbers: stored as 64-bit floating-point values
  • Strings: set of Unicode characters
  • BigInt: used to precisely represent large integers 
  • Symbol: a unique and immutable primitive type that is usually used for private object property keys

Object: collections of properties that can contain any combination of data types. Anything that is not a primitive type is represented as an object in JavaScript, including functions and arrays.

What is the difference between “==” and “===” operators in JavaScript?

The “==” and “===” operators are used for comparison in JavaScript. The “===” operator will compare two values and evaluate to true if they are strictly equal, while the “==” operator will also compare two values but will perform a type conversion if they are not of the same type.

For example, if you were to compare the number 1 with the string “1”, the “==” operator would return true: first, it converts the operands to the same type, but the “===” operator would return false because they are not of the same type. It is important to note that, in some cases, using the “==” operator may give you unexpected results due to type coercion.

Is JavaScript a case-sensitive language?

JavaScript is a case-sensitive language. This means that the language recognizes differences between uppercase and lowercase letters. For example, the variables “myVar” and “MyVar” would be considered two different variables. This can be important when writing code, as even a tiny change in the case of a letter can result in an error. In general, it is good practice to use consistent casing throughout your code to help avoid such mistakes. By convention, variables are named using camelCase, classes are named with PascalCase, and constants are written as SNAKE_CASE_ALL_CAPS.

How can you create an object in JavaScript?

There are at least four ways to create objects in JavaScript: using an Object literal, constructor function, class, or with the Object.create() method.

The Object literal, class, and constructor function all create objects which inherit the root object’s prototype.

Object literal:

const obj = {
 foo: 'bar',
 bar: 42,
 baz: true
};

Constructor function:

function Constructor(foo, bar, baz) {
 this.foo = foo;
 this.bar = bar;
 this.baz = baz;
}

const obj = new Constructor(‘bar’, 42, true);

Class:

Class MyClass {
 foo = 'bar';
 bar = 42;
 baz = true;
};

const obj = new MyClass();

Object.create:

Object.create is a lower level JS primitive that can mimic the behavior of all of the above methods and create objects without prototypal parents and with special property configurations. Mostly framework authors and low-level developers use it, and it’s rarely found in application code.

const obj = Object.create(null, {
 foo: { value: 'bar' },
 bar: { value: 42 },
 baz: { value: true}
});

What is the difference between let and var in JavaScript?

In JavaScript, the keywords let and var can be used to declare variables. However, there are some essential differences between these two keywords. Variables declared with var are accessible anywhere within their containing function, including the lines before the variable declaration and possible outer block scopes in function. In contrast, variables declared with let are only accessible within the block in which they were declared. This can be useful for preventing variable collisions or for creating private variables.

In addition, variables declared with let are hoisted to the top of their containing scope in the same way as variables declared with var, but while accessing a var before it’s defined will evaluate to undefined, let will throw a ReferenceError (Temporal Dead Zone). As a result, let can be used to help avoid unexpected behavior when accessing variables before they have been initialized. For these reasons, it is generally considered best practice to use let when declaring variables in JavaScript.

What is const in JavaScript?

Const is a keyword that works very similar to let in JavaScript, but a variable created with const cannot be reassigned.

How can you create an Array in JavaScript?

Creating an Array in JavaScript is relatively straightforward. The Array class has a constructor that can be used to initialize an array with a set of values. For example, the following code creates an array with three elements:

const myArray = new Array(1,2,3);

Be careful though! While passing multiple numbers to new Array will work as expected, passing one single number will create an empty array of that length.

const myArray = new Array(5)
console.log(myArray)

// prints [<5 empty items>] which is just [undefined, undefined, undefined, undefined, undefined]

But as Arrays are variable length by default, if you just want to create an empty array and later push elements to it, you can omit the length.

const myArray = new Array(); myArray.push(1); myArray.push(2); myArray.push(3);

In both cases, the resulting array will be of type object and will have a length property that indicates the number of elements in the array. But usually you would do it using an array literal:

const array = [1,2,3]

What is the purpose of the this keyword in JavaScript?

The this keyword in JavaScript has a variety of uses. The this keyword can be used to refer to different objects in different situations. The value of this is determined by how a function is called (runtime binding). It can’t be set by assignment during execution, and it may be different each time the function is called. The  3 most commonly used methods for controlling this binding are Function.prototype.bind(), Function.prototype.call(), Function.prototype.apply(), and ES2015 introduced arrow functions which don’t provide their own this binding (it retains the this value of the enclosing lexical context).

What is a callback in JavaScript?

A callback is a function that is passed as an argument to another function. The callback function is invoked by the other function. Callbacks are used for inversion of control, meaning that it’s not you who decide when and how exactly your logic will be run, but by the library or framework.

What is Closure in JavaScript?

In JavaScript, you can nest functions within functions. Variables declared in the outer function are available to the inner function, even if the inner one is executed later in time. Thus the outer function’s variables are part of the inner function’s closure.

How can a JavaScript code be imported in an HTML file?

JavaScript in essence is used to add interactivity to HTML files. This can be done by enclosing the code within script tags, or providing the location of the script file as an URL passed to the script tag’s src attribute. Once the code is included in the HTML file, it will be executed when the file is loaded in a web browser.

Should you wrap the entire content of a JavaScript source file in a function block? Why or why not?

Before modules, it was considered best practice to wrap closely related functionalities in function blocks, or even the whole code too. By doing this, you could ensure that all of the variables and functions defined in the file are local to that function and don’t pollute the global namespace. However, since the introduction of modules, and modern bundlers, this practice has become obsolete.

What is memoization in JavaScript?

Memoization is an optimization technique used to speed up programs by storing the results of expensive function calls and returning the cached result when the same inputs occur again. When a memoized function is called with the same arguments, the previous result is simply looked up and returned, without needing to re-execute the entire function. While memoization is a powerful optimization tool, it is important to use it sparingly, as it can lead to increased memory usage and code that is difficult to understand.

What are classes in JavaScript?

Classes in JavaScript are templates for creating objects. A class definition can specify the kind of data that an object instance of that class type will contain, and it can also specify the methods (functions) that can be invoked on instances of that type. However, classes in JavaScript are merely a syntactic sugar over constructor functions.

In addition, a class can specify inheritance relationships, which is a syntactic sugar over prototypal inheritance.

What is the use of promises in JavaScript?

A promise in JavaScript is an object that represents the eventual result of an asynchronous operation. Promises are used in many applications to handle asynchronous events, such as server responses, and timers. Promises can be chained together to create complex sequences of events, and they can be combined with other programming constructs, such as error handling. Promises have become an essential part of many web applications, however since the introduction of async-await, it’s considered a best practice to use it instead of manual Promise chaining.

What are generator functions in JavaScript?

Generator functions are a type of function which does not follow the usual run to completion execution pattern, but can be paused and resumed. When a generator function is called, it doesn’t run the code inside the function immediately. Instead, it returns a “generator” object that can be used to control the execution of the code inside the function. The generator function can use the yield keyword to cede control to its caller which in turn resumes the execution of the generator by calling the next() method on the generator object.

Thanks for reading! We hope you found these questions and answers helpful. If you’re preparing for a JavaScript interview, be sure to brush up on your skills so you can ace the interview and land the job. Good luck!

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