The Only JavaScript Article You'd Ever Need - 5/6

Oct 11, 2025

From Theory to Implementation

You’ve made it through the series and gained substantial theoretical knowledge about JavaScript. You understand complex concepts like the Event Loop, can work with Promises, and grasp advanced features. That’s impressive progress. But theoretical knowledge alone isn’t enough for professional development.

There’s a significant difference between knowing how to use a tool and understanding how to build it. Many developers can use .map(), but fewer can recreate it from scratch or explain exactly why it works the way it does.

This isn’t about academic exercise—it’s about developing the deep understanding that separates competent developers from experts. When you’re debugging complex issues or working without familiar frameworks, fundamental knowledge of the underlying implementations becomes invaluable. Today, we transition from consuming existing tools to understanding how to build them. We’re looking inside the black box to discover it’s just well-organized code.

Polyfills: Backward Compatibility Solutions

Before we build, let’s understand why you’d need to. Consider the challenge of running modern JavaScript code on older browsers. If you tried to run ES2020 code on Internet Explorer 8, the browser would fail because it doesn’t recognize modern features like Promise, fetch, or even Array.prototype.map.

You can’t control which browser users use. But you can provide fallback implementations for missing features. That’s a polyfill.

A polyfill is JavaScript code that checks if a modern feature exists in the browser. If it doesn’t, the code provides its own implementation, effectively “filling in” the gaps in older environments.

Think of it this way: A polyfill is like providing a translation guide. You can’t update an older system to understand new concepts natively, but you can teach it to recognize and handle those concepts by providing equivalent functionality using features it does understand.

The core concept is straightforward—it’s just a conditional statement.

// This is the core logic of EVERY polyfill
if (!SomeModernFeature) {
  // If the feature doesn't exist...
  // ...write our own version of it here.
  SomeModernFeature = function (/*...*/) {
    // Our custom, backward-compatible logic
  };
}

Now, let’s implement some of the fundamental features you use every day.

Building Your Own `.map()` - The Transformation Engine

What does it actually do? The .map() method iterates over an array, calls a function you provide for every single element, and returns a new array of the same length, containing the results of that function. It transforms each element into something new.

The Intuition:

We need to add our method, let’s call it myMap, to the Array.prototype so all arrays can use it.
Our function will be called on an array (this will be the array itself).
It needs to accept one argument: a callback function.
Inside, it must create a new, empty array to store the results.
It has to loop through the original array (this).
For each element, it must call the callback, passing it the element, its index, and the original array (the official map signature is callback(element, index, array)).
It takes whatever the callback returns and pushes it into our new results array.
Finally, it must return the new results array.

Let’s Build It:

// We'll wrap it in a polyfill structure.
if (!Array.prototype.myMap) {
  Array.prototype.myMap = function (callback) {
    // First, some sanity checks. What if 'this' is not an array?
    // Or what if the callback isn't a function?
    if (this == null) {
      throw new TypeError("this is null or not defined");
    }
    if (typeof callback !== "function") {
      throw new TypeError(callback + " is not a function");
    }

    // `this` refers to the array the method is called on.
    const originalArray = this;
    const newArray = []; // Our shiny new array for the results.

    // Loop through the array.
    for (let i = 0; i < originalArray.length; i++) {
      // It's good practice to check if the index actually exists.
      // This handles sparse arrays (e.g., [1, , 3]) correctly.
      if (i in originalArray) {
        // Call the callback and store its result.
        const result = callback(originalArray[i], i, originalArray);
        newArray[i] = result;
      }
    }

    return newArray;
  };
}

// LET'S TEST OUR CREATION
const numbers = [1, 4, 9, 16];
const roots = numbers.myMap((num) => Math.sqrt(num));

console.log(roots); // [1, 2, 3, 4]
// Success! You've implemented one of JavaScript's most fundamental methods.

Building Your Own `.filter()` - The Gatekeeper

What does it actually do? The .filter() method iterates over an array and runs a “predicate” function (a callback that returns true or false) on each element. It returns a new array containing only the elements for which the predicate returned a truthy value.

The Intuition: This is almost identical to map, but with one crucial difference in the loop’s logic.

Setup is the same: attach myFilter to Array.prototype.
It takes a callback (the predicate).
Create an empty newArray.
Loop through the original array.
For each element, call the callback.
The Key Step: if the callback returns true, push the original element into the newArray. If it returns false, do absolutely nothing.
Return the newArray.

Let’s Build It:

if (!Array.prototype.myFilter) {
  Array.prototype.myFilter = function (callback) {
    if (this == null) {
      throw new TypeError("this is null or not defined");
    }
    if (typeof callback !== "function") {
      throw new TypeError(callback + " is not a function");
    }

    const originalArray = this;
    const filteredArray = [];

    for (let i = 0; i < originalArray.length; i++) {
      if (i in originalArray) {
        // Call the predicate.
        const shouldKeep = callback(originalArray[i], i, originalArray);

        // If it returned true, keep the original element.
        if (shouldKeep) {
          filteredArray.push(originalArray[i]);
        }
      }
    }

    return filteredArray;
  };
}

// LET'S TEST OUR IMPLEMENTATION
const words = [
  "spray",
  "limit",
  "elite",
  "exuberant",
  "destruction",
  "present",
];
const longWords = words.myFilter((word) => word.length > 6);

console.log(longWords); // ["exuberant", "destruction", "present"]
// You've successfully implemented array filtering functionality.

Advanced Challenge: Building `Promise.all`

You’ve built synchronous tools—now let’s tackle the asynchronous functionality you learned about earlier.

What does it actually do? Promise.all takes an array of promises. It returns a single new promise that resolves only when all of the input promises have resolved. The resolved value is an array of the results, in the same order as the input promises. If any of the input promises reject, the main promise rejects immediately.

The Intuition (This is a level up):

Our myPromiseAll function takes an array of promises.
It has to return a new Promise. This is non-negotiable.
Inside the promise’s executor (resolve, reject), we need to manage state. We need a place to store the results and a way to count how many promises have finished.
Create a results array and a completedCount counter, initialized to 0.
Iterate through the input promises array. The index i is crucial for maintaining result order.
For each promise, attach a .then() handler to it.
When a promise resolves with a value, store that value in our results array at the correct index (results[i] = value). This is how we maintain order.
After storing the result, increment completedCount.
Check: is completedCount equal to the total number of promises? If yes, we’re done—resolve the main promise with the results array.
What about failure? We also need to handle rejection for each promise. If any promise rejects with a reason, we must immediately reject the main promise with that same reason.

Let’s Build It:

function myPromiseAll(promises) {
  // It must return a new promise.
  return new Promise((resolve, reject) => {
    const results = [];
    let completedCount = 0;
    const totalPromises = promises.length;

    // Handle the edge case of an empty array.
    if (totalPromises === 0) {
      resolve([]);
      return;
    }

    promises.forEach((promise, index) => {
      // Ensure we're dealing with an actual promise.
      Promise.resolve(promise)
        .then((value) => {
          // Store the result at the correct index.
          results[index] = value;
          completedCount++;

          // If all promises are done, resolve the main promise.
          if (completedCount === totalPromises) {
            resolve(results);
          }
        })
        // If ANY promise rejects, we immediately reject the main promise.
        .catch((reason) => {
          reject(reason);
        });
    });
  });
}

// LET'S TEST THE BEAST
const p1 = Promise.resolve(3);
const p2 = 42; // Not a promise, but our function should handle it.
const p3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, "foo");
});
const p4_reject = new Promise((resolve, reject) => {
  setTimeout(reject, 50, "I failed first");
});

myPromiseAll([p1, p2, p3])
  .then((values) => {
    console.log(values); // [3, 42, "foo"]
  })
  .catch((err) => {
    console.error("This shouldn't run:", err);
  });

myPromiseAll([p1, p3, p4_reject])
  .then((values) => {
    console.log("This won't run:", values);
  })
  .catch((err) => {
    console.error(err); // "I failed first"
  });

Essential Utility: Building `debounce` - Controlling Event Frequency

This isn’t a prototype method, but a utility function that’s essential for performance optimization.

The Problem: Some browser events fire continuously. Events like scroll, resize, or keyup on a search bar can trigger hundreds of times per second. If you attach an API call to a search input’s keyup event, you’ll send network requests for every keystroke: “s”, “se”, “sea”, “sear”, “searc”, “search”. This creates unnecessary network traffic and can overwhelm your server.

The Solution: A debounce function. It’s a higher-order function that takes your function and a delay period. It returns a new function that will only execute your original function after a specified amount of time has passed without it being called again.

Think of it this way: Debouncing is like waiting for a conversation to pause before responding. Every time there’s new input, the waiting timer resets, ensuring you only respond once the activity has settled.

The Intuition:

debounce is a function that takes a func and a delay.
It needs to hold a timer variable in its scope. This is a perfect use case for a closure.
It returns a new function.
When this new, returned function is called: a. It must first clear any existing timer. clearTimeout(timer). This resets the waiting period. b. It then sets a new setTimeout. c. The callback for this setTimeout will execute the original func after the specified delay.

Let’s Build It:

function debounce(func, delay) {
  let timer; // This variable persists because of the closure.

  // Return the new, debounced function.
  return function (...args) {
    // `this` and `args` need to be preserved for the original function.
    const context = this;

    // Clear the previous timer if it exists.
    clearTimeout(timer);

    // Set a new timer.
    timer = setTimeout(() => {
      func.apply(context, args);
    }, delay);
  };
}

// HOW TO USE IT:
const searchInput = document.getElementById("search-bar");

function makeApiCall(query) {
  console.log(`Searching for: ${query}...`);
  // Imagine fetch('/api/search?q=' + query) here
}

// Wrap our API call in a debounce function.
const debouncedApiCall = debounce(makeApiCall, 500); // 500ms delay

searchInput.addEventListener("keyup", (event) => {
  debouncedApiCall(event.target.value);
});

Now, if you type “search” quickly, the API call will only be made once, 500ms after you stop typing. You’ve effectively prevented unnecessary server requests and improved user experience.

Conclusion: Understanding Through Implementation

You’ve built .map, .filter, Promise.all, and debounce. They’re no longer mysterious black boxes—they’re understandable functions built from fundamental JavaScript concepts. You’ve examined their internal logic and understood their implementation patterns.

This represents the mindset of a true engineer: don’t just use the library, understand how you could build the library. This deep, fundamental knowledge differentiates experienced developers from those who only follow tutorials. It’s what enables you to debug complex issues, architect robust solutions, and truly master your craft.

Your next challenge: find another method you use regularly and implement it yourself. The process of working through implementation details is where deeper understanding develops.

From Theory to Implementation

Polyfills: Backward Compatibility Solutions

Building Your Own .map() - The Transformation Engine

Building Your Own .filter() - The Gatekeeper

Advanced Challenge: Building Promise.all

Essential Utility: Building debounce - Controlling Event Frequency

Conclusion: Understanding Through Implementation

Building Your Own `.map()` - The Transformation Engine

Building Your Own `.filter()` - The Gatekeeper

Advanced Challenge: Building `Promise.all`

Essential Utility: Building `debounce` - Controlling Event Frequency