Part 1: Top(1-5) Common JavaScript Questions

JavaScript is a programming language that has come one of the most in- demand chops in the job request. It’s a protean language that’s used for web development, mobile app development, garçon- side programming, and more.

In this blog post, we will cover 5 common JavaScript interview questions that are frequently asked in specialised interviews.

These questions will cover a range of motifs, from introductory generalities similar as let and var, to more advanced motifs like closures and heritage. By understanding these questions and their answers, you’ll be more set for your coming JavaScript interview.

Question 1: What’s the difference between let, const and var in JavaScript?

In JavaScript, variables can be declared using three different keywords” var”,” let”, and” const”. Each of these keywords has its own characteristics and use cases.

The main difference between” var” and” let” is in the compass of the variables they declare. Variables declared with” var” are function scoped, meaning they’re only accessible within the function in which they were declared. Variables declared with” let” are block scoped, meaning they’re only accessible within the block in which they were declared.

The main difference between” let” and” const” is that variables declared with” let” can be reassigned, while variables declared with” const” can not be reassigned. still, if the variable is an object, we can still change its parcels.

Then’s an illustration to demonstrate the difference between” let” and” const” in terms of reassignment

let x = 1;
x = 2; // allowed

const y = 1;
y = 2; // not allowed, will throw an error

const z = {a: 1};
z.a = 2; // allowed, we can change the properties of the object

Question 2: What is closure in JavaScript and how does it work?

In JavaScript, a check is a function that has access to the variables in its parent compass, indeed after the parent function has finished executing. This allows the inner function to” flash back “ the values of the variables in its parent compass, indeed if they’re no longer in memory.

Then’s a simple illustration that demonstrates the conception of check

function outerFunction(x) {
    function innerFunction() {
        console.log(x);
    }
    return innerFunction;
}
let myClosure = outerFunction(10);
console.log(myClosure()); // Output: 10

In this illustration, the innerFunction has access to the variable x indeed though the outerFunction has completed its prosecution. The innerFunction” remembers” the value of x and is suitable to pierce it through the check.

Closures are frequently used in JavaScript to produce private variables, to produce calls and to apply modules.

Then’s an illustration that demonstrates how closures can be used to produce private variables

function createCounter() {
    let count = 0;
    return {
        increment: function() {
            count++;
        },
        getCount: function() {
            return count;
        }
    }
}
let counter = createCounter();
counter.increment();
console.log(counter.getCount()); // Output: 1

In this illustration, the count variable is a private variable that can only be entered and modified by the styles inside the check.

In summary, closures are an important generality in JavaScript that allow inner functions to retain access to variables in their parent compass indeed after the parent function has completed execution. They are considerably used in multitudinous different scripts to give a more important and flexible way of writing code.

Question 3: What is hoisting in JavaScript?

In JavaScript, hoisting is a medium where variable and function affirmations are moved to the top of their compass before the law is executed. This means that variables and functions can be used before they’re declared in the code.

For illustration, the following law will work indeed though the variable x is declared after it’s used

console.log(x); // Output: undefined
var x = 5;

This is because the variable protestation is hoisted to the top of the compass, and the initialization is left in place.

still, hoisting only applies to affirmations and not initializations. thus, the following law will throw a ReferenceError

console.log(x); // ReferenceError: x is not defined
let x = 5;

This is because the variable x is declared with the let keyword, and it’ll not be hoisted to the top of the scope.

also, function affirmations are also hoisted to the top of their compass, so the following law will work

example(); // Output: "I am a hoisted function"
function example() {
    console.log("I am a hoisted function");
}

It’s important to note that hoisting can make the law harder to read and understand, and can lead to unanticipated bugs if not used precisely. It’s recommended to always declare variables and functions at the top of their compass to avoid confusion.

In summary, hoisting is a mechanism in JavaScript where variable and function declarations are moved to the top of their scope before the code is executed. This allows variables and functions to be used before they are declared in the code, but it can make the code harder to understand and can lead to unexpected bugs if not used carefully.

Question 4: How does the “this” keyword work in JavaScript?

In JavaScript, the “this” keyword refers to the object that the function is a method of or the object that the function is called on. The value of “this” can change depending on how the function is called.

Here are a few examples to demonstrate how “this” works in different contexts:

1. In the global scope, “this” refers to the global object (window in the browser and global in Node.js)

console.log(this); // Output: window or global

2. In an object method, “this” refers to the object that the method is a property of:

let person = {
  name: "John",
  sayName: function() {
    console.log(this.name);
  }
};
person.sayName(); // Output: "John"

3. In a constructor function, “this” refers to the object being constructed:

function Person(name) {
    this.name = name;
}
let john = new Person("John");
console.log(john.name); // Output: "John"

4. In an event handler, “this” refers to the element that the event occurred on:

<button id="myButton">Click me</button>
<script>
    let button = document.getElementById("myButton");
    button.addEventListener("click", function() {
        console.log(this); // Output: the button element
    });
</script>

5. In an arrow function, “this” refers to the parent scope, or the context in which the arrow function was defined.

let person = {
  name: "John",
  sayName: () => {
    console.log(this.name);
  }
};
person.sayName(); // Output: undefined

In summary, the “this” keyword in JavaScript refers to the object that the function is a method of or the object that the function is called on. Its value can change depending on how the function is called and is an essential aspect of understanding how JavaScript works.

Question 5: Can you explain the difference between == and === in JavaScript?

In JavaScript, the double equals (==) and the triple equals (===) are both used for comparison, but they function differently.

The double equals (==) compares values for equality, but it does not check the data type of the values being compared. This means that it will perform type coercion if necessary in order to make the comparison.

console.log(1 == "1"); // Output: true
console.log(true == 1); // Output: true

On the other hand, the triple equals (===) compares both the value and the data type of the values being compared. This means that it will not perform type coercion, and the comparison will only return true if the values being compared have the same data type and value.

console.log(1 === "1"); // Output: false
console.log(true === 1); // Output: false

In general, it’s recommended to use the triple equals (===) for comparison, as it is more strict and can prevent unexpected bugs.

In short, the double equals (==) compare values for equality and performs type coercion if necessary, whereas the triple equals (===) compares both the value and the data type of the values being compared and does not perform type coercion.

If you’re interested in learning more about coding in Javascript, be sure to follow me for more code snippets and examples. I’ll be sharing valuable information and tips on how to master the language and improve your skills as a developer. So, stay tuned for more updates, and let’s continue learning together.

Thanks for reading. Happy learning 😄

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