Web Development - HTML, CSS & JavaScript

🚀 JavaScript Interview Questions with Answers: Part-10 91. Write a function to find the sum of an array. Using reduce()

function sumArray(arr) {
    return arr.reduce((total, num) => total + num, 0);
}

console.log(sumArray([1, 2, 3, 4]));

Output: 10 Using Loop

function sumArray(arr) {
    let sum = 0;

    for (let num of arr) {
        sum += num;
    }

    return sum;
}

92. Write a function to reverse a string. Using split(), reverse(), join()

function reverseString(str) {
    return str.split("").reverse().join("");
}

console.log(reverseString("hello"));

Output: olleh Using Loop

function reverseString(str) {
    let reversed = "";

    for (let i = str.length - 1; i >= 0; i--) {
        reversed += str[i];
    }

    return reversed;
}

93. Write a function to find the largest number in an array. Using Math.max()

function largestNumber(arr) {
    return Math.max(...arr);
}

console.log(largestNumber([2, 8, 5, 1]));

Output: 8 Using Loop

function largestNumber(arr) {
    let largest = arr[0];

    for (let num of arr) {
        if (num > largest) {
            largest = num;
        }
    }

    return largest;
}

94. Write a function to check if a string is a palindrome. A palindrome reads same forward and backward. Example:

function isPalindrome(str) {
    const reversed = str.split("").reverse().join("");
    return str === reversed;
}

console.log(isPalindrome("madam"));

Output: true Case-Insensitive Version

function isPalindrome(str) {
    str = str.toLowerCase();
    return str === str.split("").reverse().join("");
}

95. Write a function to remove duplicates from an array. Using Set

function removeDuplicates(arr) {
    return [...new Set(arr)];
}

console.log(removeDuplicates([1, 2, 2, 3, 4, 4]));

Output: [1, 2, 3, 4] Using filter()

function removeDuplicates(arr) {
    return arr.filter((item, index) =>
        arr.indexOf(item) === index
    );
}

96. Write a function to implement debounce. Debouncing delays execution until the user stops triggering events. Example:

function debounce(fn, delay) {
    let timer;

    return function(...args) {
        clearTimeout(timer);

        timer = setTimeout(() => {
            fn.apply(this, args);
        }, delay);
    };
}

Usage:

const search = debounce(() => {
    console.log("Searching...");
}, 500);

Common Uses: • Search bars • Resize events • Auto-save 97. Write a function to implement throttle. Throttling limits execution frequency. Example:

function throttle(fn, delay) {
    let lastCall = 0;

    return function(...args) {
        const now = Date.now();

        if (now - lastCall >= delay) {
            lastCall = now;
            fn.apply(this, args);
        }
    };
}

Usage:

const scrollHandler = throttle(() => {
    console.log("Scrolling...");
}, 1000);

Common Uses: • Scroll events • Mouse movement • Window resizing 98. Write a function to flatten a nested array. Using flat()

function flattenArray(arr) {
    return arr.flat(Infinity);
}

console.log(flattenArray([1, [2, [3, 4]]]));

Output: [1, 2, 3, 4] Recursive Solution

function flattenArray(arr) {
    let result = [];

    for (let item of arr) {
        if (Array.isArray(item)) {
            result = result.concat(flattenArray(item));
        } else {
            result.push(item);
        }
    }

    return result;
}

99. Write a function to implement a simple pub/sub pattern. Example:

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79. How do you optimize heavy loops or renders? Common Optimization Techniques: 1. Avoid unnecessary DOM updates 2. Use memoization 3. Use efficient loops 4. Cache repeated calculations 5. Use virtual DOM frameworks 6. Minimize reflows/repaints Example: Cache DOM selector:

const element = document.getElementById("box");
for(let i = 0; i < 1000; i++) {
    element.innerHTML = i;
}

Why? Repeated DOM lookups are expensive. 80. How do you handle memory leaks? Memory leaks happen when unused memory is not released. Common Causes: • Unremoved event listeners • Global variables • Timers not cleared • Detached DOM elements • Closures holding unused references Example:

const interval = setInterval(() => {
    console.log("Running");
}, 1000);

clearInterval(interval);

Best Practices: • Remove listeners • Clear timers • Avoid unnecessary global variables • Nullify unused references DevTools: Browser memory profiling tools help detect leaks. Double Tap ❤️ For Part-9

🚀 JavaScript Interview Questions with Answers — Part 8 71. What are try/catch/finally?  These are used for error handling in JavaScript. Block  try  • Purpose: Code that may cause error catch  • Purpose: Handles the error finally  • Purpose: Always executes Example: 

try {
    console.log(a);
} catch(error) {
    console.log("Error occurred");
} finally {
    console.log("Execution completed");
}

Output:  Error occurred Execution completed Important:  finally runs whether an error occurs or not. 72. What is the Error object?  The Error object contains information about runtime errors. Example: 

try {
    throw new Error("Something went wrong");
} catch(error) {
    console.log(error.message);
}

Common Error Properties:  name  • Description: Error type message  • Description: Error message stack  • Description: Stack trace Built-in Error Types:  • ReferenceError • TypeError • SyntaxError • RangeError 73. How do you create custom errors?  Using classes or extending Error. Example: 

class ValidationError extends Error {
    constructor(message) {
        super(message);
        this.name = "ValidationError";
    }
}

throw new ValidationError("Invalid input");

Benefits:  • Better debugging • More meaningful error handling 74. What are console.log, console.table, console.group?  These are debugging methods available in browser DevTools. console.log()  Prints normal output. 

console.log("Hello");

console.table()  Displays data in table format. 

console.table([
    {name: "Deepak", age: 25},
    {name: "John", age: 30}
]);

console.group()  Groups related logs. 

console.group("User Info");
console.log("Name: Deepak");
console.log("Age: 25");
console.groupEnd();

Benefit:  Cleaner debugging in large applications. 75. How do you use breakpoints and the debugger?  Breakpoints pause code execution for debugging. Using debugger: 

function test() {
    let x = 10;
    debugger;
    console.log(x);
}
test();

How It Works:  • Browser pauses at debugger • Inspect variables and execution flow Browser DevTools Features:  • Step through code • Watch variables • Inspect call stack • Monitor network requests Interview Tip:  Very important skill for frontend developers. 76. What is performance profiling in DevTools?  Performance profiling helps identify slow operations and bottlenecks. Browser DevTools Can Measure:  • Rendering performance • JavaScript execution time • Memory usage • FPS drops Common Tabs:  • Performance • Memory • Network Why It Matters:  Helps optimize web applications and improve user experience. 77. How do you avoid blocking the main thread?  Heavy tasks can freeze the UI because JavaScript is single-threaded. Solutions:  1. Use asynchronous operations 2. Break heavy tasks into chunks 3. Use Web Workers 4. Use setTimeout() 5. Optimize loops Example: 

setTimeout(() => {
    heavyTask();
}, 0);

Why?  Allows browser to handle UI updates before running heavy code. 78. What is debouncing vs throttling?  Both optimize frequent function calls. Debouncing  Runs function only after user stops triggering event. Example Use Cases:  • Search input • Resize events

function debounce(fn, delay) {
    let timer;
    return function() {
        clearTimeout(timer);
        timer = setTimeout(() => {
            fn();
        }, delay);
    };
}

Throttling  Limits function execution to fixed intervals. Example Use Cases:  • Scroll events • Mouse movement

function throttle(fn, delay) {
    let lastCall = 0;
    return function() {
        let now = Date.now();
        if (now - lastCall >= delay) {
            lastCall = now;
            fn();
        }
    };
}

Difference:  Debounce  • Waits after last event • Reduces extra calls Throttle  • Executes at intervals • Limits execution frequency

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48. What is event.target vs event.currentTarget?  Property: event.target → Meaning: Actual clicked element Property: event.currentTarget → Meaning: Element handling the event Example:

parent.addEventListener("click", function(event) {
    console.log(event.target);
    console.log(event.currentTarget);
});

Important: In event delegation, target is very useful. 49. How do you prevent default behavior? Using:

event.preventDefault();

Example: Prevent form submission.

form.addEventListener("submit", function(event) {
    event.preventDefault();
    console.log("Form prevented");
});

Common Uses: • Prevent page reload • Prevent link navigation • Custom form handling 50. How do you remove an event listener? Using: removeEventListener() Example: function handleClick() {   console.log("Clicked"); } button.addEventListener("click", handleClick); button.removeEventListener("click", handleClick); Important: The same function reference must be used while removing the listener. Wrong Example: button.removeEventListener("click", () => {}); This will not work because it creates a new function reference. Double Tap ❤️ For Part-6

Sure! Here’s the modified version with * replaced by **: 🚀 JavaScript Interview Questions with Answers — Part 5 41. What is the DOM? DOM stands for: Document Object Model It is a programming interface that represents an HTML document as a tree structure so JavaScript can access and manipulate webpage elements. Example HTML: <h1 id="title">Hello</h1> JavaScript:

const heading = document.getElementById("title");
console.log(heading);

What You Can Do With DOM: • Change text/content • Change styles • Add/remove elements • Handle events • Create interactive webpages 42. How do you select an element by id, class, or tag? Select by ID

document.getElementById("title");

Select by Class

document.getElementsByClassName("box");

Select by Tag

document.getElementsByTagName("p");

Modern Selectors querySelector() Returns first matching element.

document.querySelector(".box");

querySelectorAll() Returns all matching elements.

document.querySelectorAll(".box");

Interview Tip: querySelector() is commonly used in modern JavaScript. 43. How do you change element text or HTML? Change Text Using textContent

const heading = document.getElementById("title");
heading.textContent = "Welcome";

Change HTML Using innerHTML

heading.innerHTML = "<span>Hello</span>";

Difference: Property: textContent → Purpose: Plain text only Property: innerHTML → Purpose: HTML content Important: Avoid unsafe innerHTML with user input because of XSS security risks. 44. How do you add/remove/replace a DOM element? Create Element

const div = document.createElement("div");
div.textContent = "New Element";

Add Element

document.body.appendChild(div);

Remove Element

div.remove();

Replace Element

const newElement = document.createElement("p");
newElement.textContent = "Updated";
div.replaceWith(newElement);

45. How do you listen to click, keyup, etc.? Using addEventListener(). Click Event

const button = document.querySelector("button");
button.addEventListener("click", () => {
    console.log("Button clicked");
});

Keyup Event

const input = document.querySelector("input");
input.addEventListener("keyup", () => {
    console.log("Key released");
});

Common Events: Event: click → Purpose: Mouse click Event: keyup → Purpose: Key released Event: keydown → Purpose: Key pressed Event: submit → Purpose: Form submit Event: mouseover → Purpose: Mouse hover 46. What is event delegation? Event delegation is a technique where a parent element handles events for its child elements using event bubbling. Example:

document.getElementById("list")
.addEventListener("click", function(event) {
    if (event.target.tagName === "LI") {
        console.log(event.target.textContent);
    }
});

Benefits: • Better performance • Fewer event listeners • Works for dynamically added elements Interview Tip: Very important concept in frontend interviews. 47. What is event bubbling vs capturing? Events move through the DOM in two phases. Event Bubbling Event travels from child → parent. Event Capturing Event travels from parent → child. Example:

div.addEventListener("click", () => {
    console.log("Div clicked");
});

button.addEventListener("click", () => {
    console.log("Button clicked");
});

If button clicked: Button clicked Div clicked Enable Capturing:

div.addEventListener("click", handler, true);

Default: JavaScript uses bubbling by default.

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