Allow one function call

86.7% acceptance

Mar 2, 2026

586

Given a function fn, return a new function that is identical to the original function except that it ensures fn is called at most once. The first time the returned function is called, it should return the same result as fn. Every subsequent time it is called, it should return undefined.

Solution

TypeScript

Time O(1)

Space O(1)

LeetCode

solution.ts

type JSONValue =
  | null
  | boolean
  | number
  | string
  | JSONValue[]
  | { [key: string]: JSONValue };
type OnceFn = (...args: JSONValue[]) => JSONValue | undefined;

function once(fn: Function): OnceFn {
  let called = false;
  return function (...args: JSONValue[]): JSONValue | undefined {
  if (!called) {
    called = true;
    return fn(...args);
  }
  return undefined;
  };
}

/**
 * let fn = (a,b,c) => (a + b + c)
 * let onceFn = once(fn)
 *
 * onceFn(1,2,3); // 6
 * onceFn(2,3,6); // returns undefined without calling fn
 */