Using ChatGPT to reverse engineer minified JavaScript

While browsing the Internet for inspiration, I came across an interesting-looking component.

I thought the block with the running ASCII art looked cool, but I couldn't quite figure out how it was implemented, so I started looking at the source code.

I found the code that looked like it might be doing the job, but it was minified.

const { floor: ra, abs: KE, min: QE } = Math,
    O5 = ["reactive.network REACTIVE.NETWORK", "$@B%8&WM#*oahkbdpqwmZO0QLCJUYXzcvunxrjft/|()1{}[]?-_+~<>i!lI;:,^`'. .:â–‘â–’â–“â–ˆ"],
    G7 = Date.now() % 3 ? O5[1] : O5[0],
    V5 = G7.length,
    JE = { fps: 60 };
function eT(e, t, n, r) {
    const i = t.time * 8e-5,
        s = QE(t.cols, t.rows),
        o = t.metrics.aspect * 0.2,
        l = { x: ((4 * (e.x - t.cols / 6.25)) / s) * o, y: (5 * (e.y - t.rows / 4)) / s },
        u = ra(KE(YE(l) - i) * V5 + (ra(e.x / 1) % 2) * 2) % V5;
    return G7[u];
}
const tT = () => {
    const e = j.useRef(null),
        [t, n] = j.useState({ height: null, width: null });
    return (
        j.useEffect(() => {
            function r() {
                n({ height: window.innerHeight, width: window.innerWidth });
            }
            if (typeof window < "u") return n({ height: window.innerHeight, width: window.innerWidth }), window.addEventListener("resize", r), () => window.removeEventListener("resize", r);
        }, []),
        j.useEffect(() => {
            const r = e.current;
            if (!r) return;
            const i = 12,
                s = ra(t.width / i) * 1.6,
                o = ra(t.height / i),
                l = { aspect: s / o },
                u = setInterval(() => {
                    let c = "";
                    for (let d = 0; d < o; d++) {
                        for (let f = 0; f < s; f++) c += eT({ x: f, y: d }, { cols: s, rows: o, metrics: l, time: Date.now() });
                        c += `
`;
                    }
                    r.textContent = c;
                }, 1e3 / JE.fps);
            return () => clearInterval(u);
        }, [t]),
        a.jsx("div", { style: { position: "absolute", top: 0, left: 0, width: "100%", height: "100%" }, children: a.jsx("div", { ref: e, style: { width: "100%", height: "100%", whiteSpace: "pre", overflow: "hidden" } }) })
    );
};
function nT(e) {
    return Math.cos(e.x * e.x - e.y * e.y);
}
const { floor: ia, abs: rT, min: iT } = Math,
    D5 = ["reactive.network REACTIVE.NETWORK", "$@B%8&WM#*oahkbdpqwmZO0QLCJUYXzcvunxrjft/|()1{}[]?-_+~<>i!lI;:,^`'. .:â–‘â–’â–“â–ˆ"],
    X7 = Date.now() % 3 ? D5[1] : D5[0],
    F5 = X7.length,
    sT = { fps: 60 };
function oT(e, t, n, r) {
    const i = t.time * 8e-5,
        s = iT(t.cols, t.rows),
        o = t.metrics.aspect * 0.2,
        l = { x: ((4 * (e.x - t.cols / 6.25)) / s) * o, y: (5 * (e.y - t.rows / 4)) / s },
        u = ia(rT(nT(l) - i) * F5 + (ia(e.x / 1) % 2) * 2) % F5;
    return X7[u];
}
const aT = () => {
    const e = j.useRef(null),
        [t, n] = j.useState({ height: null, width: null });
    return (
        j.useEffect(() => {
            function r() {
                n({ height: window.innerHeight, width: window.innerWidth });
            }
            if (typeof window < "u") return n({ height: window.innerHeight, width: window.innerWidth }), window.addEventListener("resize", r), () => window.removeEventListener("resize", r);
        }, []),
        j.useEffect(() => {
            const r = e.current;
            if (!r) return;
            const i = 12,
                s = ia(t.width / i) * 1.6,
                o = ia(t.height / i),
                l = { aspect: s / o },
                u = setInterval(() => {
                    let c = "";
                    for (let d = 0; d < o; d++) {
                        for (let f = 0; f < s; f++) c += oT({ x: f, y: d }, { cols: s, rows: o, metrics: l, time: Date.now() });
                        c += `
`;
                    }
                    r.textContent = c;
                }, 1e3 / sT.fps);
            return () => clearInterval(u);
        }, [t]),
        a.jsx("div", { style: { position: "absolute", top: 0, left: 0, width: "100%", height: "100%" }, children: a.jsx("div", { ref: e, style: { width: "100%", height: "100%", whiteSpace: "pre", overflow: "hidden" } }) })
    );
},

Usually, I would just powerthrough reading the minimized code to understand the implementation (or look for a .map file to restore the original code). However, I realized that I never tried asking ChatGPT to do it for me... So I copied all of the above code and asked ChatGPT to "explain the code". Here is the response:

The provided code is quite complex, but I'll break it down into a more understandable format, explaining its different parts and their functionalities.
Overview
The code defines several functions and components using JavaScript, specifically in the context of a React application. It deals with generating some dynamic text content, presumably ASCII art or some kind of visual effect using characters from a predefined set. Additionally, it includes utility functions for event handling, performance timing, and HTML content conversion.
Key Components and Functions
Character Set Selection:
const { floor: ra, abs: KE, min: QE } = Math;
const O5 = ["reactive.network REACTIVE.NETWORK", "$@B%8&WM#*oahkbdpqwmZO0QLCJUYXzcvunxrjft/|()1{}[]?-_+~<>i!lI;:,^`'. .:â–‘â–’â–“â–ˆ"];
const G7 = Date.now() % 3 ? O5[1] : O5[0];
const V5 = G7.length;
This part of the code imports some common Math functions (floor, abs, and min) and defines two sets of characters, stored in O5.
Based on the current time (Date.now() % 3), it selects one of these sets (G7) to use later.
The length of the selected character set is stored in V5.
Dynamic Character Selection Function:
function eT(e, t, n, r) {
    const i = t.time * 8e-5;
    const s = QE(t.cols, t.rows);
    const o = t.metrics.aspect * 0.2;
    const l = { x: ((4 * (e.x - t.cols / 6.25)) / s) * o, y: (5 * (e.y - t.rows / 4)) / s };
    const u = ra(KE(YE(l) - i) * V5 + (ra(e.x / 1) % 2) * 2) % V5;
    return G7[u];
}
The eT function generates a character based on the input position (e.x, e.y) and the current time (t.time).
It adjusts the position using some aspect ratio calculations and selects a character from G7 based on a combination of the position and time.
The purpose of YE function isn't directly clear from the code, but it seems to be a custom function used in this calculation.
React Component (
const tT = () => {
    const e = j.useRef(null);
    const [t, n] = j.useState({ height: null, width: null });
    j.useEffect(() => {
        function r() {
            n({ height: window.innerHeight, width: window.innerWidth });
        }
        if (typeof window < "u") {
            n({ height: window.innerHeight, width: window.innerWidth });
            window.addEventListener("resize", r);
            return () => window.removeEventListener("resize", r);
        }
    }, []);

    j.useEffect(() => {
        const r = e.current;
        if (!r) return;
        const i = 12;
        const s = ra(t.width / i) * 1.6;
        const o = ra(t.height / i);
        const l = { aspect: s / o };
        const u = setInterval(() => {
            let c = "";
            for (let d = 0; d < o; d++) {
                for (let f = 0; f < s; f++) c += eT({ x: f, y: d }, { cols: s, rows: o, metrics: l, time: Date.now() });
                c += `
`;
            }
            r.textContent = c;
        }, 1e3 / JE.fps);
        return () => clearInterval(u);
    }, [t]);

    return a.jsx("div", { style: { position: "absolute", top: 0, left: 0, width: "100%", height: "100%" }, children: a.jsx("div", { ref: e, style: { width: "100%", height: "100%", whiteSpace: "pre", overflow: "hidden" } }) });
};
This is a React component that sets up an interval to periodically update the content of a div element (e.current) with dynamically generated characters.
It calculates the number of columns (s) and rows (o) based on the window size and then fills the div with characters generated by the eT function.
The effect runs at a specified frames per second (JE.fps).

I followed up asking to "implement equivalent code in TypeScript and make it human readable" and got the following response:

import React, { useEffect, useRef, useState } from 'react';

const selectedCharacterSet =
  "$@B%8&WM#*oahkbdpqwmZO0QLCJUYXzcvunxrjft/|()1{}[]?-_+~<>i!lI;:,^`'. .:â–‘â–’â–“â–ˆ";
const characterSetLength = selectedCharacterSet.length;

const calculateCharacter = (
  x: number,
  y: number,
  cols: number,
  rows: number,
  aspect: number,
  time: number,
): string => {
  const timeFactor = time * 0.000_08;
  const size = Math.min(cols, rows);
  const aspectRatio = aspect * 0.2;
  const position = {
    x: ((4 * (x - cols / 6.25)) / size) * aspectRatio,
    y: (5 * (y - rows / 4)) / size,
  };

  const index =
    Math.floor(
      Math.abs(
        Math.cos(position.x * position.x - position.y * position.y) -
          timeFactor,
      ) *
        characterSetLength +
        (Math.floor(x) % 2) * 2,
    ) % characterSetLength;
  return selectedCharacterSet[index];
};

type Size = {
  height: null | number;
  width: null | number;
};

const AsciiArtGenerator: React.FC = () => {
  const textRef = useRef<HTMLDivElement>(null);
  const [size, setSize] = useState<Size>({ height: null, width: null });

  useEffect(() => {
    const handleResize = () => {
      setSize({ height: window.innerHeight, width: window.innerWidth });
    };

    // Initial size setting
    handleResize();

    window.addEventListener('resize', handleResize);
    return () => {
      window.removeEventListener('resize', handleResize);
    };
  }, []);

  useEffect(() => {
    const element = textRef.current;
    if (!element || !size.width || !size.height) return undefined;

    const cols = Math.floor(size.width / 12) * 1.6;
    const rows = Math.floor(size.height / 12);
    const aspectRatio = cols / rows;

    const intervalId = setInterval(() => {
      let content = '';

      for (let y = 0; y < rows; y++) {
        for (let x = 0; x < cols; x++) {
          content += calculateCharacter(
            x,
            y,
            cols,
            rows,
            aspectRatio,
            Date.now(),
          );
        }

        content += '\n';
      }

      element.textContent = content;
    }, 1_000 / 60); // 60 fps

    return () => {
      clearInterval(intervalId);
    };
  }, [size]);

  return (
    <div
      ref={textRef}
      style={{
        height: '100%',
        overflow: 'hidden',
        whiteSpace: 'pre',
        width: '100%',
      }}
    />
  );
};

export default () => {
  return <AsciiArtGenerator />;
};

To my surprise, the response is not only good enough, but it is also very readable.

Comparing the outputs, ~it looks like LLM response overlooked a few implementation details~, but it is still a good enough implementation to learn from. Definitely a neat use case I've not considered before!

Update (2024-08-29): Initially, I thought that the LLM didn't replicate the logic accurately because the output was missing a few characters visible in the original component (e.g., ░▒▓█). However, a user on HN forum pointed out that it was likely a copy-paste error.
Upon further investigation, I discovered that the original code contains different characters than what I pasted into ChatGPT. This appears to be an encoding issue, as I was able to get the correct characters after downloading the script. After updating the code to use the correct characters, the output is now identical to the original component.
I apologize, GPT-4, for mistakenly accusing you of making mistakes.