Implement a function once(fn) that accepts a callback fn and restricts its invocation to at most once.

The function should:

• Return a new function that calls fn only on the first invocation.
• On subsequent calls, return the result from the first call without invoking fn again.

Examples

const initialize = once(() => "initialized");

initialize(); // "initialized"
initialize(); // "initialized" (same result, fn not called again)