Execute a function once

Run a function on the first call and return that result afterwards.

JavaScriptNode.js & browsers Version 1.0.0 · MIT · .md

function once(fn) {
  let done = false, result;
  return (...args) => {
    if (!done) { result = fn(...args); done = true; }
    return result;
  };
}

Usage

let connections = 0;
const connect = once(() => ({ id: ++connections }));
connect(); // → { id: 1 }
connect(); // → { id: 1 }
connections; // → 1

Lines marked → were executed in Node.js 22 and in headless Chromium, and each result was compared with the value shown. Lines marked e.g. are illustrative.

Know the limits

• Typical use: create one API client or connection pool lazily, on first use.
• If fn throws, done stays false and the next call tries again. A rejected promise, however, is a result and is kept.
• The original forwarded no arguments and overwrote fn with its result.

The function above is the code the usage lines run. It does not validate its arguments; add checks where it receives untrusted input.

Keep exploring

• Logical xor operator — True when exactly one of two values is truthy.
• Identity function — Return the argument unchanged.
• Box handler — Chain transformations on a value, then unwrap the result.