Delay the evaluation of a function

Turn a call into a thunk: a function that runs the call only when invoked.

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

function thunkify(fn) { return (...args) => () => fn(...args); }

Usage

let calls = 0;
const render = name => (calls += 1, 'Hello, ' + name);
const later = thunkify(render)('Ana');
calls; // → 0
later(); // → 'Hello, Ana'
calls; // → 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

• The thunk runs fn every time it is called; wrap it with Execute a function once to keep the first result.
• The original named it thunkfy.

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.