Build a counter with private state

Arguments

startnumber

Optional starting value. Defaults to 0.

Returns

{ increment(): number; decrement(): number; value(): number; reset(): number } — An object exposing exactly four methods — the count itself lives in the closure and is unreachable from outside.

Examples

const c = createCounter();

c.increment(); // 1
c.increment(); // 2
c.value();     // 2
c.decrement(); // 1
c.reset();     // 0

const a = createCounter(10);
const b = createCounter(10);

a.increment(); // 11
b.value();     // 10 — a and b share nothing

What it has to do

• increment() and decrement() return the new count.
• value() returns the current count without changing it.
• reset() restores the original start value and returns it.
• The returned object's only own enumerable properties are the four methods — the count stays private.
• Separate counters keep independent state.

Notes

• This is the module pattern in miniature: the count is a let inside the factory, and the methods close over it. Hanging it on the returned object as a property would make it mutable from outside.
• If you reach for a class with a #count field, you are solving the same problem a different way — closures were the pre-class answer and still work everywhere.