JavaScript String Methods & Polyfills

What String Methods Are

String methods are built-in functions provided by JavaScript to operate on string data.

Examples:

• toUpperCase()

• slice()

• indexOf()

• trim()

• split()

Key Property

Strings in JavaScript are immutable:

• Methods do not modify the original string

• They return a new string

const str = "hello";
const result = str.toUpperCase();

console.log(str);     // "hello"
console.log(result);  // "HELLO"

Conceptual Working of String Methods

Internally, most string methods operate using:

• Iteration over characters

• Index-based access

• Conditional logic

Example: toUpperCase()

function toUpperCasePolyfill(str) {
  let result = "";

  for (let i = 0; i < str.length; i++) {
    let char = str[i];

    // Convert ASCII lowercase to uppercase
    if (char >= 'a' && char <= 'z') {
      result += String.fromCharCode(char.charCodeAt(0) - 32);
    } else {
      result += char;
    }
  }

  return result;
}

Insight

• Built-in methods are optimized in engines (V8, SpiderMonkey)

• But logic remains loop + transformation

Why Developers Write Polyfills

Definition

A polyfill is a custom implementation of a method that mimics native behavior.

Reasons

1. Browser compatibility

   • Older browsers may not support modern methods

2. Understanding internals

   • Helps in interviews and debugging

3. Control over behavior

   • Custom edge-case handling

Implementing Simple String Utilities (Polyfills)

includes()

function includesPolyfill(str, search) {
  for (let i = 0; i <= str.length - search.length; i++) {
    let match = true;

    for (let j = 0; j < search.length; j++) {
      if (str[i + j] !== search[j]) {
        match = false;
        break;
      }
    }

    if (match) return true;
  }
  return false;
}

reverse()

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

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

  return result;
}

trim()

function trimPolyfill(str) {
  let start = 0;
  let end = str.length - 1;

  while (str[start] === ' ') start++;
  while (str[end] === ' ') end--;

  let result = "";
  for (let i = start; i <= end; i++) {
    result += str[i];
  }

  return result;
}