unary()

unary<Result>(func): (arg) => Result

Creates a function that accepts up to one argument, ignoring any additional arguments.

💎 Why is this a Hidden Gem?

Restrict a function to its first argument — the classic fix for ['1','2','3'].map(parseInt).

DEPRECATED

Use an inline arrow function instead.

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Type Parameters

Result: Result

The return type of the function.

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Parameters

func: (arg) => Result

The function to cap arguments for.

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Returns

The new capped function.

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See Also

Arrow functions - MDN

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Since

2.0.0

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Also known as

unary (Lodash, es-toolkit, Ramda) · ❌ (Remeda, Radashi, Effect, Modern Dash, Antfu)

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Example

// ❌ Deprecated approach
['6', '8', '10'].map(unary(parseInt));
// => [6, 8, 10]

// ✅ Recommended approach
['6', '8', '10'].map(x => parseInt(x, 10));
// => [6, 8, 10]

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How it works?

Creates a function that accepts up to one argument. Deprecated: Use an arrow function.

Native Equivalent

// ❌ arr.map(unary(parseInt))
// ✅ arr.map(x => parseInt(x))

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Use Cases

Limit to one argument 📌

Create function that only accepts one argument.

#unary#single#argument#limit

const toInt = s => parseInt(s, 10);
["1", "2", "3"].map(toInt);
// => [1, 2, 3]

Fix map callback

Prevent extra arguments from affecting result.

#fix#map#callback#parseInt

// Problem: parseInt gets index as radix
["1", "2", "3"].map(parseInt); // => [1, NaN, NaN]

// Solution: wrap to use only first arg
["1", "2", "3"].map(s => parseInt(s, 10)); // => [1, 2, 3]

Ignore extra arguments

Adapter to ignore additional parameters.

#ignore#extra#arguments#adapter

const first = (a) => a;
[1, 2, 3].map(first); // Only uses value, ignores index