String Polyfills and Common Interview Methods in JavaScript

If you are preparing for frontend or full-stack interviews, there is a 99% chance you are going to face a question about strings. JavaScript gives us a ton of built-in ways to handle text, but interviewers love to take those built-in tools away and ask: "How would you build this from scratch?"

Let's look at how JavaScript string methods actually work under the hood, what polyfills are, and how building them can make you ace your next technical round.

What Are String Methods?

In JavaScript, a string isn't just text; it acts like an object. String methods are simply pre-written functions that live inside JavaScript, attached to every string you create.

When you type "hello".toUpperCase(), you are calling a built-in method that iterates over your word and changes every character to uppercase. They are convenience tools so we don't have to write complex loops every time we want to slice, search, or modify text.

What is a Polyfill (And Why Do Developers Write Them?)

A polyfill is a piece of code that provides modern functionality on older browsers that do not natively support it.

Imagine a new JavaScript string method called .replaceAll() is released. If a user is on an ancient version of Internet Explorer, their browser will throw an error saying .replaceAll is not a function. To fix this, developers write a "polyfill"—a custom script that manually recreates the exact behavior of .replaceAll().

Why does this matter for interviews? In an interview setting, you aren't writing polyfills to support old browsers. Interviewers ask you to write them to test two things:

1. Do you truly understand how the built-in method operates conceptually?

2. Can you manipulate basic data structures (like strings and arrays) using raw logic (loops and conditions) instead of relying on magic shortcuts?

Implementing Simple String Utilities (Writing Polyfills)

Let's look at the logic behind two common string methods and write our own polyfills for them.

(Note: In JavaScript, we attach polyfills to String.prototype so every string can use them, and we use the keyword this to refer to the actual string calling the method).

1. Polyfilling .repeat()

The built-in .repeat(count) method takes a string and repeats it a specific number of times. Concept: We need a loop that runs count times, taking our original string and adding it to a new result string.

// Our custom polyfill
String.prototype.myRepeat = function(count) {
    // 'this' is the string the method is called on
    let originalString = this; 
    let finalString = "";

    for(let i = 0; i < count; i++) {
        finalString += originalString;
    }

    return finalString;
};

// Testing it
console.log("Code".myRepeat(3)); 
// Output: CodeCodeCode

2. Polyfilling .startsWith()

The .startsWith(searchString) method checks if a string begins with the exact characters of another string. Concept: We just need to check if the characters at the very beginning of our main string match the characters of the search string, one by one.

String.prototype.myStartsWith = function(searchString) {
    let originalString = this;

    // Loop exactly the length of the string we are searching for
    for(let i = 0; i < searchString.length; i++) {
        // If even one character doesn't match, return false instantly
        if(originalString[i] !== searchString[i]) {
            return false;
        }
    }
    // If the loop finishes without failing, it's a match!
    return true;
};

console.log("JavaScript".myStartsWith("Java")); // true
console.log("JavaScript".myStartsWith("Script")); // false

Common Interview String Problems

Aside from polyfills, you will often be asked to solve problems using the logic we just practiced. Here are two classics:

Problem 1: Reverse a String

The Catch: You cannot use .reverse(). The Logic: Loop through the string backwards, starting from the last character, and build a new string.

function reverseString(str) {
    let reversed = "";
    // Start at the end, stop at 0, move backwards
    for(let i = str.length - 1; i >= 0; i--) {
        reversed += str[i];
    }
    return reversed;
}

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

Problem 2: Check for a Palindrome

The Catch: A palindrome reads the same forwards and backwards (like "racecar"). The Logic: Use the reverse logic we just built. If the original string matches the reversed string, it's a palindrome.

function isPalindrome(str) {
    // Let's use two pointers instead of reversing the whole thing (Interviewers love this!)
    let left = 0;
    let right = str.length - 1;

    while(left < right) {
        if(str[left] !== str[right]) {
            return false; // Mismatch found
        }
        left++;
        right--;
    }
    return true; // All characters matched
}

console.log(isPalindrome("racecar")); // true

Why Understanding Built-In Behavior is Your Secret Weapon

It is very easy to memorize str.split('').reverse().join('') to reverse a string. But what happens when an interviewer tells you that approach takes up too much memory by creating unnecessary arrays?

When you understand the fundamental logic behind how methods like .split(), .slice(), or .includes() actually traverse a string, you stop relying on syntax memorization and start thinking like an engineer. You can optimize for speed, save memory, and easily adapt when the interviewer throws a trick question at you.

Stay tune for next Blog! Happy Coding!