What Are String Methods?

String methods are built-in functions used to process text.

Examples:

• toUpperCase()

• trim()

• split()

• includes()

Example

const name = "javascript";

console.log(name.toUpperCase());

Output

JAVASCRIPT

How String Methods Work Conceptually

A method:

1. Receives input

2. Processes characters

3. Returns new string/result

String Processing Flow

Common String Methods

Example: trim()

const text = "   hello   ";

console.log(text.trim());

Output

hello

What Is a Polyfill?

A polyfill is custom code that recreates built-in functionality.

Used when:

• Feature doesn't exist

• Older browsers lack support

• Interview practice

Why Developers Write Polyfills

Example Polyfill: Custom trim()

Built-In Version

const text = "  hello  ";

console.log(text.trim());

Simple Custom Version

function customTrim(str) {
  return str.replace(/^\\s+|\\s+$/g, "");
}

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

Polyfill Behavior Representation

Example: includes()

Built-In Method

const text = "JavaScript";

console.log(text.includes("Script"));

Output

true

Simple Custom includes()

function customIncludes(str, word) {
  return str.indexOf(word) !== -1;
}

console.log(customIncludes("JavaScript", "Script"));

Common Interview String Problems

1. Reverse a String

Example

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

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

Output

olleh

2. Check 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

3. Count Characters

function countCharacters(str) {
  return str.length;
}

console.log(countCharacters("JavaScript"));

Output

10

4. Capitalize First Letter

function capitalize(str) {
  return str[0].toUpperCase() + str.slice(1);
}

console.log(capitalize("javascript"));

Output

Javascript

Why Interviews Ask String Problems

String questions test:

• Logic building

• Loops

• Array methods

• Problem-solving

• Understanding of built-ins

Important Built-In Behaviors

Strings are immutable.

Original string does not change.

Example

const text = "hello";

text.toUpperCase();

console.log(text);

Output

hello

Correct Way

const text = "hello";

const updated = text.toUpperCase();

console.log(updated);

Output

HELLO

Simple Interview Tips

Practice Exercise

Reverse String

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

console.log(reverse("node"));

Check Substring

function containsWord(str, word) {
  return str.includes(word);
}

console.log(containsWord("JavaScript", "Script"));

Key Takeaways

Final Notes

String problems are extremely common in JavaScript interviews because they test:

• Problem-solving

• Data manipulation

• Understanding of core language behavior

A good developer should understand both:

• Using built-in methods

• Implementing logic manually