Create a function that accepts a single argument

Wrap a function so it ignores every argument after the first.

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

function unary(fn) { return value => fn(value); }

Usage

['1', '2', '3'].map(Number.parseInt); // → [1, NaN, NaN]
['1', '2', '3'].map(unary(Number.parseInt)); // → [1, 2, 3]

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

• map passes (value, index, array), so parseInt received the index as its radix, as the first line shows. unary drops everything after the value.

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.