Your Undo Button is Just a Stack of Pancakes

Source: Dev.to

TL;DR: I implement undo functionality using a Stack data structure because it follows the Last‑In‑First‑Out (LIFO) principle. Each state change is pushed onto the stack. When I trigger an undo, I pop the most recent action, reverting the application state in O(1) time without the overhead of re‑indexing an entire array.

Why is the undo function always a Stack?

I use a stack because it enforces a strict linear history where the only accessible element is the most recent one. This ensures that every undo call reverses the exact last action performed, preventing state corruption and keeping pointer overhead at a minimum.

When building an app that tracks state, scanning a massive array or shifting indices for each character typed is inefficient. A stack lets me drop a state snapshot on the top and retrieve it just as quickly. Pushing and popping from the top of a stack is a constant‑time operation, matching the natural way we backtrack through a sequence of events.

How does the pancake analogy explain LIFO?

LIFO (Last‑In‑First‑Out) means the last pancake I put on the plate is the first one I eat. In software, the last action I push to the history stack is the first one I pop off when the user hits undo.

If I tried to wedge a new pancake into the bottom of the pile, I’d face an O(n) operation and a broken breakfast—just like a bad architecture that shifts indices. Instead, I drop the new action on top. When undo is needed, I take the top pancake; pulling from the bottom would cause index‑shifting nightmares and a collapsed state.

Stack vs. Array: Why Efficiency Wins for History

How do I implement this in a real state management loop?

// Simple undo stack implementation
const history = [];

const trackChange = (state) => {
  history.push(state);
};

const handleUndo = () => {
  if (history.length < 2) return; // Need at least one previous state
  history.pop(); // Remove current state
  const prevState = history[history.length - 1];
  applyToUI(prevState);
};

How do I manage the memory footprint of a massive stack?

I prevent the undo stack from growing indefinitely by imposing a hard limit on its depth (e.g., 100 actions). When the limit is exceeded, I remove the oldest entry (the bottom element) or treat the stack as a circular buffer. This trade‑off discards very old history but protects the application from exhausting memory.

FAQ

Why not use a Queue for undo operations?

A queue follows FIFO (First‑In‑First‑Out) logic. Using it for undo would revert the first action performed when the app opened, not the most recent mistake—resulting in a poor user experience.

How does the ‘Redo’ button work alongside the stack?

I maintain a second stack for Redo. When I pop a state from the Undo stack, I push it onto the Redo stack. If the user makes a new change, the Redo stack is cleared. This mirrors moving pancakes between two plates.

Can I store diffs instead of full state snapshots?

Yes. For large applications, storing only the diff (the specific command or change) rather than a full snapshot reduces RAM usage while preserving O(1) stack operations. This keeps the memory footprint per “pancake” small.