String pollyfills and related interview questions

In Javascript, strings are used everywhere, but many developers do not know how they actually work behind the scenes.

When you call a method like .toUpperCase() on a string primitive, JavaScript creates a temporary wrapper object (using the String constructor) so the method can be executed. Once the operation is done, that object is immediately discarded, and the result is returned as a primitive.

What Are String Methods?

String methods are the built-in functions that are used to manipulate and update the string.

Here are some of the string methods:

toUpperCase()
toLowerCase()
slice()
substring()
includes()
split()
trim()

These methods don’t modify the original string (because strings are immutable). Instead, they return a new string.

How string method work internally?

Let's take an example toUpperCase and understand how it works.

When you call string.toUpperCase()

const str = 'Hello';

str.toUpperCase();

Javascript iterate over each character
Convert that character into uppercase
Build a new string

Think of execution like this

Input String → Process Each Character → Apply Logic → Return New String

What are polyfills?

Polyfills are a custom implementation of the built-in functions. It is mostly used to create the same functionality that built-in functions offer.

Why Developers Write Polyfills?

Provide support for new methods in older browsers
Understanding internal logic
To practice the built-in methods logic by creating them of their own

Polyfill Examples

Let's implement some polyfills for string methods

string.toLowerCase()
Here I will be writing a custom polyfill for toLowerCase.

if(!String.prototype.toLowerCase){
    String.prototype.toLowerCase = function () {
        let result = "";

        // this refer to string on which method is called
        for (let char of this) {
            const code = char.charCodeAt(0);

            // a-z → A-Z
            if (code >= 97 && code <= 122) {
                result += String.fromCharCode(code - 32);
            } else {
                result += char;
            }
        }

        return result;
    };
}

This will only work if the browser doesn't support string.toLowerCase, it will use the polyfill toLowerCase

string.includes

if(!String.prototype.includes){
    String.prototype.myIncludes = function(searchString) {
        return this.indexOf(searchString) !== -1;
    };
}

Common String Interview Questions

If you're preparing for interviews, these are must-practice:

Reverse a String function without using built-in methods
Logic: Iterate over the string and append the new string with each character in an order so that the last character comes first.
Check Palindrome
Logic: Reverse and create a new string. Check if both strings are equal or not.
Count Characters
Logic: Use a "Map" or an object to store counts as you iterate through the string.
Anagram Detection: Do two strings contain the same characters?
Logic: Sort both strings and compare them.

Why Understanding Built-in Behavior Matters

Most developers stop at using methods. But in an interview, the interviewer tests:

How things work internally
Your problem-solving ability
Your understanding of edge cases

Understanding internals helps you in writing better code, debugging faster, and performing well in interviews.

Example questions:

How does slice() differ from substring()?
Why are strings immutable?
How would you optimize a string search?

Conclusion

If you want to stand out as a developer:

Don’t just use methods — understand them, practice writing polyfills, and solve real interview problems.

That’s where real growth happens.

Follow the series to learn more about Javascript.

String Polyfills and Common Interview Questions in JavaScript

Comments

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What Are String Methods?

How string method work internally?

What are polyfills?

Why Developers Write Polyfills?

Polyfill Examples

Common String Interview Questions

Why Understanding Built-in Behavior Matters

Conclusion

Command Palette

Comments

JavaScript: Beginner to Advance 🚀

Callbacks in JavaScript: Why They Exist

More from this blog

What Are String Methods?

How string method work internally?

What are polyfills?

Why Developers Write Polyfills?

Polyfill Examples

Common String Interview Questions

Why Understanding Built-in Behavior Matters

Conclusion