JavaScript: write a tiny score rule you can test

Name the state before writing the function

State is the information your game remembers between actions. This miniature game remembers which of three seeds have been collected. A function handles a request and returns a result the caller can inspect. You can test those rules before adding artwork or buttons.

JavaScript uses let for bindings you may reassign and const for bindings you will not reassign. A const array can still have its contents changed. That distinction matters: keeping the same array binding does not make the contents immutable.

Keep the change in one place

Run this self-contained example in a fresh JavaScript session. Each seed has an index from zero to two. The function rejects an invalid index or an already collected seed before changing the array. It derives the score from the array, so there is no separate score variable to drift out of sync.

const seeds = [false, false, false];

function collectSeed(index) {
  if (!Number.isInteger(index) || index < 0 || index >= seeds.length) {
    return { accepted: false, reason: "invalid seed" };
  }
  if (seeds[index]) {
    return { accepted: false, reason: "already collected" };
  }
  seeds[index] = true;
  const score = seeds.filter(Boolean).length;
  return { accepted: true, score, won: score === seeds.length };
}

console.log(collectSeed(1)); // score 1, won false
console.log(collectSeed(1)); // rejected: duplicate
console.log(collectSeed(0)); // score 2, won false
console.log(collectSeed(2)); // score 3, won true
console.log(collectSeed(99)); // rejected: invalid

Separate the rule from its presentation

The result object reports what happened. A future button handler could use an accepted result to update text or play a sound. The rule itself does not need to know where that label is or which audio file you chose.

Before adding more features, try to break the assumptions. What happens with -1, 1.5, or the text '1'? The Number.isInteger guard rejects them. This is a design choice: a different system might convert input deliberately, but accidental conversion makes debugging harder. Reloading the practice file starts a new game; this example does not save progress.

Common mistakes to watch for.

• Array indices begin at zero in this example.
• Rerunning a const declaration in the same console scope can cause a redeclaration error; use a fresh session or file run.
• A printed correct result is not proof that a later UI or save system is correct.