Top 5 JavaScript Questions Asked in Coding Interviews
JavaScript is one of the most popular programming languages, widely used for web development. If you're preparing for a coding interview, it's crucial to be well-versed with common JavaScript questions. Here are the top 5 JavaScript questions frequently asked in interviews, explained in an easy-to-understand way.

1. What is hoisting in JavaScript?
Explanation:

Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope (to the top of the current script or the current function). This means that a variable can be used before it has been declared.

Example:

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console.log(x); // Output: undefined
var x = 5;
In the above example, var x is hoisted to the top, but the assignment x = 5 is not. Hence, x is undefined when we log it.

Key Points:

Only declarations are hoisted, not initializations.
let and const declarations are hoisted but are not initialized. Accessing them before initialization will result in a ReferenceError.

2. What is a closure?
Explanation:

A closure is a function that retains access to its outer (enclosing) function’s scope even after the outer function has finished executing. This allows the inner function to access variables defined in the outer function.

Example:

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function outerFunction() {
    let outerVariable = 'I am outside!';
    
    function innerFunction() {
        console.log(outerVariable); // Output: I am outside!
    }
    
    return innerFunction;
}

const myFunction = outerFunction();
myFunction();
In this example, innerFunction is a closure that captures and remembers outerVariable from its enclosing scope.

Key Points:

Closures are useful for data encapsulation and creating private variables.
They are often used in event handlers, callback functions, and functional programming.

3. Explain this keyword in JavaScript.
Explanation:

The this keyword refers to the object it belongs to. It has different values depending on where it is used:

In a method, this refers to the owner object.
Alone, this refers to the global object (in browsers, it's window).
In a function, this refers to the global object (in strict mode, it is undefined).
In an event, this refers to the element that received the event.
Example:

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const person = {
    name: 'John',
    greet: function() {
        console.log('Hello, ' + this.name); // Output: Hello, John
    }
};

person.greet();
In the example, this.name refers to the name property of the person object.

Key Points:

Arrow functions do not have their own this; they inherit this from the parent scope.
The value of this can be set explicitly using call, apply, or bind.

4. What are promises?
Explanation:

Promises are objects that represent the eventual completion (or failure) of an asynchronous operation and its resulting value. They provide a cleaner, more flexible way to handle asynchronous operations compared to callbacks.

Example:

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let myPromise = new Promise((resolve, reject) => {
    let success = true;
    
    if (success) {
        resolve('Promise fulfilled!');
    } else {
        reject('Promise rejected!');
    }
});

myPromise.then(value => {
    console.log(value); // Output: Promise fulfilled!
}).catch(error => {
    console.log(error);
});
In the example, myPromise is fulfilled and the message is logged.

Key Points:

Promises have three states: pending, fulfilled, and rejected.
The then method is used to handle fulfilled promises, while catch is used for rejected promises.
finally can be used to execute code after a promise is settled (fulfilled or rejected).

5. What is the difference between == and ===?
Explanation:

== is the loose equality operator, which compares two values for equality after converting both values to a common type. === is the strict equality operator, which compares two values for equality without type conversion.

Example:

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console.log(5 == '5');  // Output: true
console.log(5 === '5'); // Output: false
In the first comparison, 5 and '5' are converted to the same type before comparison. In the second comparison, no type conversion occurs, so the types must also be equal.

Key Points:

Use === to avoid unexpected type coercion and ensure both type and value are the same.
== performs type conversion, which can lead to subtle bugs.
By understanding and practicing these concepts, you'll be better prepared to tackle JavaScript questions in coding interviews. Happy coding!