# Execute a function once

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

Canonical: https://1loc.dev/function/execute-a-function-once/
Version: 1.0.0
Runtime: JavaScript · Node.js 22 / modern browsers
Verification: Usage lines executed in Node.js 22 and headless Chromium; no external services
Updated: 2026-10-28

## Implementation

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

## Usage

```javascript
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.

## 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 does not validate its arguments. Add checks where it receives untrusted input.

## Source and license

Original: https://github.com/phuocng/1loc/blob/14b481cf60dff1620ab1bb99a43b1f68068cdd9b/contents/execute-a-function-once.mdx
Author: Nguyen Huu Phuoc and original contributors
Contributors: See original repository history
Source commit: 14b481cf60dff1620ab1bb99a43b1f68068cdd9b
Copyright (c) 2020 phuoc-ng
License: MIT — https://1loc.dev/licenses/
Maintainer: Oveyon
Modifications: Forwards arguments and keeps the result separate from the function; a throwing first call can be retried. Documented as a reference with executed usage lines. Original MDX retained as inert source evidence.