You’re Doing This Mistake Instead of Using Self-Defining Functions

Stop writing messy initialization code in JavaScript. Here’s how self-defining functions make your code cleaner, faster, and easier to maintain.

Introduction

We’ve all written code like this before:

A function that does heavy initialization work the first time it runs.
But on the second call, it still redoes all that work unnecessarily.
So we clutter our code with extra conditionals and “firstRun” flags.

Example mistake:

let config;
function loadConfig() {
  if (!config) {
    console.log("Fetching config...");
    config = { apiKey: "123", theme: "dark" };
  }
  return config;
}

console.log(loadConfig()); // Fetching config...
console.log(loadConfig()); // Still checks condition every time

This works, but it’s noisy and less efficient.

👉 The self-defining function pattern solves this elegantly: the function rewrites itself after the first call.

The Mistake Most Devs Make

Instead of leveraging self-defining functions, devs often use:

Boolean flags (let firstRun = true;)
Global state (if (!window.cachedResult) {...})
Unnecessary wrappers that make code harder to read

These patterns clutter logic, create hidden dependencies, and add runtime checks on every call, even after the first run.

Enter: Self-Defining Functions

A self-defining function (also called self-rewriting) changes its own definition the first time it runs.

That means:

On the first call, it does the heavy work.
On subsequent calls, it replaces itself with a new, faster function that skips initialization.

Example 1: Caching a Config

let loadConfig = function () {
  console.log("Fetching config...");
  const config = { apiKey: "123", theme: "dark" };

  // Redefine itself
  loadConfig = function () {
    console.log("Returning cached config");
    return config;
  };

  return config;
};

console.log(loadConfig()); // Fetching config...
console.log(loadConfig()); // Returning cached config

✅ Notice how the second call doesn’t waste time re-checking conditions.

Example 2: Event Handler Setup

Imagine you only need to attach an event listener once.

let initClickHandler = function () {
  console.log("Setting up click handler...");
  document.body.addEventListener("click", () => {
    console.log("Body clicked");
  });

  // Rewrite to a no-op
  initClickHandler = function () {
    console.log("Handler already set");
  };
};

initClickHandler(); // Sets up handler
initClickHandler(); // Handler already set

Much cleaner than adding if (!alreadyBound) checks everywhere.

Example 3: Lazy Function Definition

This pattern is perfect for lazy loading functions, which perform expensive work only when needed.

let heavyOperation = function () {
  console.log("Initializing heavy operation...");
  // Expensive setup (e.g., load library)
  const library = { process: (x) => x * 2 };

  // Redefine
  heavyOperation = function (x) {
    return library.process(x);
  };

  return heavyOperation.apply(this, arguments);
};

console.log(heavyOperation(5)); // Initializes + returns 10
console.log(heavyOperation(5)); // Returns 10 instantly

Why Self-Defining Functions Are Better

Cleaner code: No need for flags or global variables.
Performance boost: Removes conditional checks after the first call.
Encapsulation: State lives inside the function, not scattered outside.
Readable intent: It’s clear this function is meant to run differently after initialization.

Gotchas to Watch Out For

Team readability: Some devs may find self-modifying functions surprising if they’ve never seen the pattern. Add comments for clarity.
Testing quirks: Resetting the function between tests may require re-importing or redefining.
Overkill for trivial cases: If it’s a tiny function, don’t complicate it with this pattern.

Pro Tip: Combine With Memoization

Self-defining functions pair beautifully with memoization; you can lazy-load once and cache results for future calls.

let fetchUserData = function (id) {
  console.log("Fetching user from API...");
  const cache = {};

  fetchUserData = function (id) {
    if (cache[id]) return cache[id];
    // Simulated fetch
    return (cache[id] = { id, name: "User " + id });
  };

  return fetchUserData(id);
};

console.log(fetchUserData(1)); // Fetch + cache
console.log(fetchUserData(1)); // Cached
console.log(fetchUserData(2)); // New fetch

Conclusion

Most devs handle “first-run logic” with clunky flags or repetitive conditionals. But JavaScript gives us a cleaner solution: self-defining functions.

They let you:

Run the heavy setup only once
Rewrite themselves for faster future calls
Keep logic clean and encapsulated

✅ Key takeaway: If your function has different logic on the first call vs later calls, consider making it self-defining. It’s simpler, faster, and avoids repetitive checks.

Call to Action

Have you ever used (or seen) self-defining functions in production?
Share your experience in the comments 👇

And if you know a teammate writing messy if (firstRun) checks everywhere, share this article with them; they’ll thank you later.

Skill Stuff