JavaScript Interview Questions and Answers (2023)

JavaScript also referred to as JS, is a scripting language primarily used for updating and changing CSS and HTML. Developed initially to create dynamic web pages by Brendan Eich in 1995, any device that runs a JavaScript Engine can now use JavaScript.

With its dynamic usage, proficiency in JavaScript is becoming one of the major requirements of companies for their IT solutions. It is among the top-ranking programming languages one can learn to progress in their career and land well-paying jobs.

Let us better understand how to crack interviews testing your JavaScript skills by knowing some frequently-asked JavaScript interview questions and answers.

Basic JavaScript Interview Questions

Interviewers may pose these JavaScript interview questions to test your elementary knowledge.

1. List the different data types of JavaScript.

JavaScript variables can be categorized as primitive types and non-primitive types.

The primitive JavaScript data types are:

String: A string is represented with a series of characters and written in quotes. We can represent a string either using a single or double quote.
Number: The number data type represents numbers. We can write Number data types with or without decimals.
Biglnt: This data type is used for storing numbers above the limitation of the Number data type. The Biglnt data type is represented by adding an ‘n’ to the integer literal. It can also represent large integers.
Boolean: This data type represents a logical entity that has only two values—true and false. This data type is employed mainly for conditional testing.
Undefined: In cases where a specific variable is not assigned but declared, it carries the undefined value. The type of such variables is also undefined.
Null: This data type represents either an invalid or a non-existing value.
Symbol: This data type is used for storing an anonymous or unique value. It is a new data type introduced in the current ES6 JavaScript version.

While primitive data types are used only for storing single values, non-primitive data types can be used to store complex and multiple values. The non-primitive JavaScript data type is:

Object: This data type is employed for storing a collection of data.

2. State the difference between the operators “==” and “===”.

“==” and “===” are comparison operators. The “==” operator is used to compare values, and the “===” operator is used to compare values and types.

3. What is implicit type coercion?

In JavaScript, the implicit type of coercion is the automatic conversion of values from one data type to another. This happens when different types of operations are present in an expression. The different coercion types are:

String Coercion: The string coercion can be employed by using the operator “+”. When adding a number to a string, the number type gets transformed into a string type. When this operation is used for adding up numbers, the result is a number.
Boolean Coercion: Boolean coercion is employed for ternary operators, loop checks, logical operators, and if statements.
Logical operators: Unlike other programming languages, the logical operators in JavaScript do not give the result as true or false. They return one of the operands. In the OR operator, if the first value is true, then it is returned. Otherwise, the second value gets returned all the time. If all values are true in the AND operator, the second value is returned. If the first value is false, that is returned. Similarly, when the second value is false, it is returned.
Equality Coercion: The equality coercion takes place while using the ‘==’ operator. This operator is used for comparing values and not types. It converts the operands to a similar type for comparing them.

4. What is the call, apply and bind methods?

Call: This is a predefined method that leads to method function by stating the owner object. It also permits an object to use another object's method. Further, it also accepts arguments.

Apply: This method is like the call method. The only difference is that it takes the arguments as an array instead of the call method, which takes the arguments separately.

Bind: The blind method returns a new function. In the new function, the value of the ‘this’ function is bound with the owner object (provided as a parameter).

5. What is the scope and scope chain?

In JavaScript, scope directs the accessibility of functions and variables at different parts of the code. Scope helps the viewer understand which variables and functions are accessible and which are not at any given part of the code. There are primarily three kinds of scope in JavaScript:

Global scope: All the functions and variables have global scope and can be accessed from every place inside the code.
Function scope: All the functions and variables have only local scope and can be accessed from within the function.
Block scope: This scope is connected to variables with let and const. All the variables and functions declared inside a block scope can be accessed only inside the box.

If the JavaScript engine fails to find the variable in the local scope, it attempts to find it in the outer scope. Further, if the variable is not found even in the outer scope, the engine looks for it in the global scope. If the variable goes unidentified even in the global scope, a reference error comes up. This is known as the scope chain.

6. Do we identify JavaScript as a dynamically typed language or a statically typed language?

JavaScript falls in the category of dynamically typed language. In a dynamically typed language, the variable type is checked during run time, and in a statically typed language, it is checked during compile time.

Besides these elementary JavaScript interview questions, let us study some of the more advanced interview questions on JavaScript, which will test your detailed knowledge of the subject.

Advanced JavaScript Interview Questions

Some more nuanced JavaScript interview questions are:

1. Explain the spread operator and rest parameter.

The spread operator and rest parameter got launched as a part of the ES6 version.

Spread operator: This operator is used for spreading an array and object literals. There is an additional use of this operator when we expect one or more arguments in any function call.
Rest parameter: It gives us an enhanced way to handle any function’s parameters. This parameter is employed for creating functions capable of taking a variable number of arguments. It is possible to convert multiple arguments to an array with the help of this parameter. We can also extract only a part of any argument with the rest parameter. To use the rest parameter, one must put three dots before any parameter.

The main difference between the spread operator and the rest parameter is that while the rest parameter takes multiple variables to turn into an array, the spread operator takes an array to spread it. Furthermore, while the spread operator is employed in the case of function calls, the rest parameter is employed in the function declaration.

2. Explain the role of promises in JavaScript.

We use promises for handling asynchronous operations. However, before them, callbacks were employed for the same purpose. But due to their limitations, operating multiple callbacks for handling asynchronous data could result in unmanageable code. There are four different states of a promise object:

Pending state: The initial state of promise represents that a promise is pending, neither fulfilled nor rejected.
Fulfilled state: This state of promise represents that it has already been fulfilled, with the async operation complete.
Rejected state: This promise represents that it has been rejected with the async operation failed.
Settled state: This state of promise represents that it has either been fulfilled or rejected.

A promise constructor that allows a callback instruction of reject and resolve is used for creating these promises.

3. What is Weak Set in JavaScript?

The Set represents a collective of ordered and unique elements. Similar to Set, Weak Set is a collection of different ordered and unique elements with some exceptions like:

It consists of only objects and nothing else.
The objects inside Weak Set have a poor reference, which means they will be garbage collected if they do not have any reference.
There are only three methods of Weak Set, namely add, has, and delete.

4. What is Weak Map in JavaScript?

We use maps for storing the key-value pairs. These pairs are other primitive or non-primitive types. The only factors that differentiate Weak Map from other maps are:

All values and keys in the Weak Map must be an object.
If no reference is provided for the object, it will be garbage collected.

With these foundational JS interview questions, you can form a clearer picture of what to expect during this crucial round. JavaScript is a common language that helps make online pages more interactive; develop mobile applications, web servers, and applications; and in-game development. For this reason, most companies use JavaScript to create interactive web pages for engaging their customers and attracting attention. Thus, learning JavaScript lays down the path to having a promising future in programming.

Besides familiarizing yourself with common JavaScript interview questions, you can look up Top 10 Node JS Project topics to try right away to gain insight into JS and its application. You can also check out Board Infinity’s latest Full-Stack Development Program to hone your skills and get a cutting-edge advantage in the industry to give your professional career a head start.