🔧 JavaScript String Methods Reference

Parent: Strings JS Index

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Complete Methods Reference

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Q1. Check If All Unique Characters

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🧠 How to Approach This

Step 1: Use a Set. If char already in Set → duplicate.

function hasAllUnique(s) {
    const seen = new Set();
    for (const c of s) {
        if (seen.has(c)) return false;
        seen.add(c);
    }
    return true;
}

console.log(hasAllUnique("hello")); // false
console.log(hasAllUnique("world")); // true

Time: O(n) | Space: O(k) charset

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Q2. Pangram Check

function checkIfPangram(s) {
    const lower = s.toLowerCase();
    for (let i = 0; i < 26; i++) {
        if (!lower.includes(String.fromCharCode(97 + i))) return false;
    }
    return true;
}

// More elegant with Set
function checkIfPangramSet(s) {
    const letters = new Set(s.toLowerCase().replace(/[^a-z]/g, ''));
    return letters.size === 26;
}

console.log(checkIfPangram("The quick brown fox jumps over the lazy dog")); // true

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Q3. Most Frequent Character

function mostFrequent(s) {
    const freq = new Map();
    for (const c of s) freq.set(c, (freq.get(c) || 0) + 1);
    return [...freq.entries()].reduce((a, b) => b[1] > a[1] ? b : a)[0];
}

console.log(mostFrequent("aabbbcc")); // 'b'

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Q4. Remove Duplicates (Preserve Order)

function removeDuplicates(s) {
    return [...new Set(s)].join('');
}

console.log(removeDuplicates("programming")); // "progamin"

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Q5. Caesar Cipher

function caesarCipher(s, shift) {
    return s.split('').map(c => {
        if (c >= 'a' && c <= 'z') {
            return String.fromCharCode((c.charCodeAt(0) - 97 + shift) % 26 + 97);
        } else if (c >= 'A' && c <= 'Z') {
            return String.fromCharCode((c.charCodeAt(0) - 65 + shift) % 26 + 65);
        }
        return c;
    }).join('');
}

console.log(caesarCipher("Hello", 3));  // "Khoor"
console.log(caesarCipher("Khoor", 23)); // "Hello"

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Q6. Check Balanced Brackets

function isBalanced(s) {
    const stack = [];
    const map = {')':'(', ']':'[', '}':'{'};
    for (const c of s) {
        if ('([{'.includes(c)) { stack.push(c); }
        else if (map[c]) {
            if (!stack.length || stack.pop() !== map[c]) return false;
        }
    }
    return stack.length === 0;
}

console.log(isBalanced("({[]})")); // true
console.log(isBalanced("([)]"));   // false

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Q7. First Non-Repeating Character

function firstUniqChar(s) {
    const freq = new Map();
    for (const c of s) freq.set(c, (freq.get(c) || 0) + 1);
    for (let i = 0; i < s.length; i++) {
        if (freq.get(s[i]) === 1) return i;
    }
    return -1;
}

console.log(firstUniqChar("leetcode")); // 0
console.log(firstUniqChar("aabb"));     // -1

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Q8. String Compression

function compressString(s) {
    if (!s) return s;
    let result = '', count = 1;
    for (let i = 1; i <= s.length; i++) {
        if (i < s.length && s[i] === s[i-1]) { count++; continue; }
        result += s[i-1] + count;
        count = 1;
    }
    return result.length < s.length ? result : s;
}

console.log(compressString("aabcccccaaa")); // "a2b1c5a3"
console.log(compressString("abc"));         // "abc"

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Q9. Count Vowels and Consonants

function countVowCon(s) {
    let vowels = 0, consonants = 0;
    for (const c of s.toLowerCase()) {
        if ('aeiou'.includes(c))         vowels++;
        else if (c >= 'a' && c <= 'z')  consonants++;
    }
    return { vowels, consonants };
}

console.log(countVowCon("Hello World")); // {vowels:3, consonants:7}

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Q10. String Template Formatting

// Zero-pad number
const pad = (n, len) => String(n).padStart(len, '0');
console.log(pad(42, 5));   // "00042"

// Format money
const money = (n) => `$${n.toFixed(2)}`;
console.log(money(1234.5)); // "$1234.50"

// URL slug
const slug = (s) => s.toLowerCase().trim().replace(/\s+/g, '-').replace(/[^\w-]/g, '');
console.log(slug("Hello World!"));  // "hello-world"

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ES6+ String Features

// Optional chaining with strings
const obj = { name: "Alice" };
obj?.name?.toUpperCase(); // "ALICE"
obj?.missing?.toUpperCase(); // undefined (no error)

// String coercion
String(42);         // "42"
String(null);       // "null"
String(undefined);  // "undefined"
(42).toString();    // "42"
(255).toString(16); // "ff" (hex)
(8).toString(2);    // "1000" (binary)

// Number from string
parseInt("42");       // 42
parseInt("0xff", 16); // 255
parseFloat("3.14");   // 3.14
Number("42");         // 42
+"42";                // 42 (unary +)

// Nullish coalescing with string
const s = "" ?? "default"; // "" (empty string is NOT nullish)
const s2 = null ?? "default"; // "default"
const s3 = "" || "default"; // "default" (empty string IS falsy)