Top 25 Javascript Interview Questions for Test Automation SDET - Playwright & Cypress

Brief Summary

This video provides a comprehensive overview of common JavaScript interview questions, offering detailed explanations and practical examples. It covers topics ranging from basic concepts like objects and functions to more advanced topics such as asynchronous JavaScript, promises, and object-oriented programming principles. The video aims to equip viewers with the knowledge and confidence to tackle JavaScript interviews effectively.

• JavaScript objects can hold functions as properties, allowing for methods within objects.
• Anonymous functions are functions without a name, often used as callbacks or in functional programming.
• var, let, and const have different scoping rules: var is function-scoped, let is block-scoped, and const is block-scoped and cannot be reassigned.
• JavaScript is asynchronous, meaning it can execute code without waiting for each step to complete, which is demonstrated using setTimeout.
• Callbacks and promises are used to handle asynchronous operations, ensuring code executes in the correct order.
• Inheritance in JavaScript is achieved using extends, and super is used to call parent class constructors and methods.

Can a JavaScript object hold a function as a property? Explain with an example.

Yes, a JavaScript object can hold a function as a property. This is demonstrated by creating an object named person with properties like name and age, and then adding a function named greet as another property. The greet function can access the object's other properties using the this keyword, such as this.name. To call the function, you use person.greet(), which executes the function and can print a greeting that includes the person's name.

What are anonymous functions in JavaScript? Define their syntax and implementation.

Anonymous functions in JavaScript are functions without a name. The syntax involves using the function keyword without specifying a function name. These functions can be assigned to variables or passed as arguments to other functions. For example, an anonymous function can be assigned to a variable greet, and then greet() can be called to execute the function. Anonymous functions are often used as callbacks, where they are executed after a certain operation is completed.

What is the difference between var, const, and let? Explain with an example.

The key difference between var, let, and const lies in their scope and mutability. var is function-scoped and globally scoped, meaning it is accessible throughout the function in which it is declared, even within blocks like if statements. let is block-scoped, meaning it is only accessible within the block where it is defined. const is also block-scoped, but it is used for variables that should not be reassigned after their initial assignment. If you don't declare anything in the prefix by default it takes as a where.

Where are the push, pop, slice, shift, and unshift methods used when accessing array elements?

push adds an element to the end of an array, while pop removes the last element from the array. unshift adds an element to the beginning of the array, and shift removes the first element. indexOf is used to find the index of a specific element in the array. splice removes elements from the array based on their index. forEach iterates over each element in the array, allowing you to perform operations on each element.

Is JavaScript Asynchronous? Prove with an example.

Yes, JavaScript is asynchronous. This is demonstrated using the setTimeout function, which allows you to delay the execution of a function. When setTimeout is used, JavaScript does not wait for the timer to complete before executing the next line of code. Instead, it continues executing the rest of the script and then comes back to execute the setTimeout function when the timer expires. This behavior proves that JavaScript does not wait synchronously for each step to complete.

What are callback functions in JavaScript?

Callback functions in JavaScript are functions that are passed as arguments to other functions and are executed after a certain operation has been completed. This is particularly useful in asynchronous operations, where you need to ensure that a function is executed only after data has been fetched or a certain task has been completed. By passing a function as a callback, you can make your code more generic and reusable.

What are promises in JavaScript? Explain the difference between callback functions and promises with an example.

Promises in JavaScript are objects that represent the eventual completion (or failure) of an asynchronous operation and allow you to handle asynchronous operations in a more structured way than callbacks. Promises have three states: pending, resolved, and rejected. The main difference between callbacks and promises is that promises provide a more readable and maintainable way to handle asynchronous code, avoiding the "callback hell" that can occur with nested callbacks. Promises also allow you to chain multiple asynchronous operations together using .then() and handle errors using .catch(). Additionally, async/await syntax can be used to simplify working with promises, making asynchronous code look and behave more like synchronous code.

Create an inheritance relationship between a parent and child class. Invoke the parent constructor from the child class. Create main.js to call parent class methods from a child class object.

To create an inheritance relationship between a parent and child class in JavaScript, you use the extends keyword. The parent class contains common properties and methods, while the child class inherits these and can add its own specific properties and methods. To invoke the parent constructor from the child class, you use the super() method within the child's constructor. This ensures that the parent class is properly initialized before the child class. A main.js file can then be created to instantiate the child class and call methods from both the parent and child classes.

From the above code, explain how the super and these keywords help achieve the solution.

The super keyword is used to call the parent class's constructor and access its methods and properties from the child class. It ensures that the parent class is properly initialized before the child class. The this keyword refers to the current instance of the class and is used to access and modify the instance's properties. By attaching values to this, you make them accessible throughout the class.

What is the difference between == and ===?

The difference between == and === is that == performs type coercion, meaning it converts the operands to the same type before comparing them, while === does not perform type coercion and requires the operands to be of the same type to be considered equal. For example, 5 == "5" returns true because the string "5" is converted to the number 5 before comparison, while 5 === "5" returns false because the types are different.

What is the difference between null and undefined in JavaScript?

null is an assignment value that represents no value or no object. It is explicitly assigned to a variable to indicate that it has no value. undefined means a variable has been declared but has not been assigned a value. It is the default value of a variable that has not been initialized. null is an object, while undefined is of type undefined.

A classic programming interview question that involves using array methods (filter, map, reduce), and JavaScript objects.

This section demonstrates how to use array methods like filter, map, and reduce along with JavaScript objects to solve a common programming interview question. The problem involves creating an array of objects representing students with their names and scores, filtering out students who passed the exam (score > 36), updating the names of the passing students to uppercase, and finding the total score of all the passing students. filter is used to create a new array with only the passing students, map is used to update the names of the passing students to uppercase, and reduce is used to calculate the total score of the passing students.