Working with strings is one of the most common tasks in JavaScript. Whether you're manipulating text, validating input, or formatting data — strings are everywhere!

In this blog, we’ll explore all important string methods, explain them with easy examples, and finally give you interview-level practice problems to boost your skills.

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🔹 1. Length of a String

To find the total number of characters in a string:

let firstName = "Vaishali";
console.log(firstName.length);

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🔹 2. Accessing Characters from a String

JavaScript allows accessing characters using charAt() or bracket notation:

console.log(firstName.charAt(2)); // i
console.log(firstName[2]);        // i
console.log(firstName.charCodeAt(2)); // ASCII Code

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🔹 3. Check Presence of a Character

Use includes, indexOf, and lastIndexOf:

console.log(firstName.includes("r")); // false
console.log(firstName.indexOf("i"));  // 2
console.log(firstName.lastIndexOf("i")); // 7

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🔹 4. Compare Two Strings

localeCompare() compares two strings lexicographically:

let anotherName = "Vishal";
console.log(firstName.localeCompare(anotherName)); // -1

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🔹 5. Replace Substring in a String

const str = "Vishal is Best Frontend Developer. Vishal is Best Developer.";

console.log(str.replace("Vishal", "Sujit"));  
console.log(str.replaceAll("Vishal", "Sujit"));

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🔹 6. Extract Substring

console.log(str.substring(6, 30));
console.log(str.slice(-10, -1));

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🔹 7. Split and Join Strings

console.log(str.split("")); // Split into characters

const subString = str.split(" ");
console.log(subString.join(" "));

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🔹 8. Check String Start & End

console.log(str.startsWith("Vishal"));    // true
console.log(str.endsWith("Developer"));   // true

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🔹 9. Trim & Case Conversion

const trimStr = str.trim();
const trimStrStart = str.trimStart();
const trimStrEnd = str.trimEnd();

console.log(trimStr, trimStr.length);
console.log(str.toLowerCase());
console.log(str.toUpperCase());

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🔹 10. Convert Number or Object to String

const num = 123;
console.log(num.toString());

const obj = { name: "Vishal", course: "DSA with Vishal" };
console.log(JSON.stringify(obj));

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🔹 11. Concatenate Strings

const lastName = "Rajput";

console.log(firstName + lastName);
console.log(`${firstName} ${lastName} is a Best Developer`);
console.log(firstName.concat(lastName, " is a", " Best"));

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🧠 Practice Questions (Interview Level)

These are common string problems asked in companies like TCS, Infosys, Amazon, Google, Wipro, etc.

1. Find the Index of the First Occurrence in a String

2. Reverse String

3. Valid Anagram

4. Longest Common Prefix

5. Merge Strings Alternately

6. Length of Last Word

7. Valid Palindrome

8. String Compression

9. Reverse Words in a String

10. Reverse Vowels of a String

11. Rotate String

✅ 1. Find the Index of the First Occurrence in a String

👉 Return the first index where needle appears in haystack.

✔ Solution

function strStr(haystack, needle) {
    return haystack.indexOf(needle);
}

console.log(strStr("hello", "ll")); // 2
console.log(strStr("abc", "d"));    // -1

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✅ 2. Reverse String

👉 Reverse the characters in a string.

✔ Solution

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

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

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✅ 3. Valid Anagram

👉 Two strings are anagrams if both contain the same characters in the same frequency.

✔ Solution

function isAnagram(s, t) {
    if (s.length !== t.length) return false;

    return s.split("").sort().join("") === t.split("").sort().join("");
}

console.log(isAnagram("anagram", "nagaram")); // true
console.log(isAnagram("rat", "car")); // false

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✅ 4. Longest Common Prefix

👉 Find the longest prefix shared by all strings.

✔ Solution

function longestCommonPrefix(strs) {
    strs.sort();
    let first = strs[0];
    let last = strs[strs.length - 1];
    let i = 0;

    while (i < first.length && first[i] === last[i]) {
        i++;
    }
    return first.slice(0, i);
}

console.log(longestCommonPrefix(["flower", "flow", "flight"])); // "fl"

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✅ 5. Merge Strings Alternately

👉 Merge characters one-by-one from two strings.

✔ Solution

function mergeAlternately(a, b) {
    let result = "";
    let i = 0;

    while (i < a.length || i < b.length) {
        if (i < a.length) result += a[i];
        if (i < b.length) result += b[i];
        i++;
    }
    return result;
}

console.log(mergeAlternately("abc", "pqr")); // "apbqcr"

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✅ 6. Length of Last Word

👉 Return the length of the last word in a string.

✔ Solution

function lengthOfLastWord(s) {
    return s.trim().split(" ").pop().length;
}

console.log(lengthOfLastWord("Hello World")); // 5

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✅ 7. Valid Palindrome

👉 Check if string is same when reversed (ignore symbols & spaces).

✔ Solution

function isPalindrome(s) {
    s = s.toLowerCase().replace(/[^a-z0-9]/g, '');
    return s === s.split("").reverse().join("");
}

console.log(isPalindrome("A man, a plan, a canal: Panama")); // true

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✅ 8. String Compression

👉 Compress repeated characters like:
aabbccc → a2b2c3

✔ Solution

function compress(chars) {
    let i = 0, ans = "";

    while (i < chars.length) {
        let char = chars[i];
        let count = 0;

        while (i < chars.length && chars[i] === char) {
            count++;
            i++;
        }

        ans += char + (count > 1 ? count : "");
    }
    return ans;
}

console.log(compress("aabbccc")); // a2b2c3

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✅ 9. Reverse Words in a String

👉 Example:
" hello world " → "world hello"

✔ Solution

function reverseWords(s) {
    return s.trim().split(/\s+/).reverse().join(" ");
}

console.log(reverseWords("  hello world  ")); // "world hello"

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✅ 10. Reverse Vowels of a String

👉 Only reverse vowels (a, e, i, o, u).

✔ Solution

function reverseVowels(s) {
    let vowels = "aeiouAEIOU";
    let arr = s.split("");
    let left = 0, right = arr.length - 1;

    while (left < right) {
        if (!vowels.includes(arr[left])) left++;
        else if (!vowels.includes(arr[right])) right--;
        else {
            [arr[left], arr[right]] = [arr[right], arr[left]];
            left++;
            right--;
        }
    }
    return arr.join("");
}

console.log(reverseVowels("hello")); // "holle"

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✅ 11. Rotate String

👉 Check if string can be rotated to become another string.

Example:
"abcde" rotated → "cdeab"

✔ Solution

function rotateString(s, goal) {
    return s.length === goal.length && (s + s).includes(goal);
}

console.log(rotateString("abcde", "cdeab")); // true

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🎯 Final Thoughts

JavaScript provides a rich set of string manipulation methods that help developers handle text efficiently. Mastering these methods not only sharpens your coding skills but also prepares you for interviews and real-world development tasks.