Implement once(func) — ensure a function runs only once

Question

Accepted Answer

**Approach:** Flag + cached result. Preserve `this` and args on first call.

```javascript
function once(func) {
  let called = false, result;
  return function(...args) {
    if (!called) { called = true; result = func.apply(this, args); }
    return result;
  };
}

// Demo
let n = 0;
const init = once(() => ++n);
console.log(init()); // 1
console.log(init()); // 1 (cached)
console.log(n); // 1
const obj = { val: 10, get: once(function() { return this.val; }) };
console.log(obj.get()); // 10
function onceAsync(fn) {
  let p;
  return function(...args) { if (!p) p = Promise.resolve(fn.apply(this, args)); return p; };
}
(async () => {
  const fetchOnce = onceAsync(async () => { console.log('fetch'); return 42; });
  console.log(await fetchOnce()); // fetch 42
  console.log(await fetchOnce()); // 42 (no second fetch)
})();
```
Useful for singleton init, event listener setup, lazy config load.

Implement once(func) — ensure a function runs only once

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KEEP THESTREAK ALIVE.

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