For AI agents: the complete documentation index is available at /llms.txt, the full documentation bundle is available at /llms-full.txt, and this page is available as Markdown at /reference/function/unary.md.
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    unary es-toolkit

    Creates a new function that limits a function to accept only the first parameter.

    const unaryFunc = unary(func);

    Usage

    unary(func)

    Use unary when you want to limit a function to accept only one parameter. Any additional parameters passed are ignored.

    This is useful for preventing callback functions from receiving more parameters than expected in array methods like map, filter, and forEach.

    import { unary } from 'es-toolkit/function';

// Basic usage
function greet(name: string, age?: number, city?: string) {
  console.log(`Hello, ${name}!`);
  if (age) console.log(`Age: ${age}`);
  if (city) console.log(`City: ${city}`);
}

const greetOnlyName = unary(greet);
greetOnlyName('Cheolsu', 25, 'Seoul'); // Only logs 'Hello, Cheolsu!'

// Using with array methods
const numbers = ['1', '2', '3', '4', '5'];

// parseInt accepts radix as a second parameter,
// but map's callback passes (value, index, array)
console.log(numbers.map(parseInt)); // [1, NaN, NaN, NaN, NaN] (unexpected result)

// Use unary to pass only the first parameter
console.log(numbers.map(unary(parseInt))); // [1, 2, 3, 4, 5] (expected result)

// Another example: when a function accepts multiple parameters but you want to use only one
function logValue(value: any, prefix: string = 'Value:', suffix: string = '') {
  console.log(`${prefix} ${value} ${suffix}`);
}

const data = ['apple', 'banana', 'cherry'];

// When you want to log only the value without prefix and suffix
data.forEach(unary(logValue));
// Value: apple
// Value: banana
// Value: cherry

    It's also useful in function composition.

    import { unary } from 'es-toolkit/function';

// Function that accepts multiple parameters
function multiply(a: number, b: number = 1, c: number = 1) {
  return a * b * c;
}

// Limit to use only the first parameter
const multiplyOne = unary(multiply);

const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map(x => multiplyOne(x, 2, 3)); // b and c are ignored
console.log(doubled); // [1, 2, 3, 4, 5] (result of 1 * 1 * 1)

    Parameters

    funcFrequired

    The function to limit to accept only the first parameter.

    Returns

    Returns a new function that passes only the first parameter to the original function.

    (...args: any[]) => ReturnType<F>
    Source: es-toolkit

    Re-exported verbatim from es-toolkit. Implementation, edge cases, and performance behavior are owned upstream. This page mirrors the documentation at the pinned version; the linked source is authoritative.