flatten()

flatten<T>(array): (T | T[])[]

Flattens array a single level deep.

DEPRECATED

Use array.flat() directly instead.

Reason:
Native equivalent method now available

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Type Parameters

T: T

The type of elements in the array.

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Parameters

array: (T | T[])[]

The array to flatten.

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Returns: (T | T[])[]

A new flattened array.

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See Also

• Array.flat() - MDN
• Browser support - Can I Use

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Since

2.0.0

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Also known as

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

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Example

const nested = [1, [2, 3], [4, [5, 6]]];

// ❌ Deprecated approach
const flattened = flatten(nested);
console.log(flattened); // [1, 2, 3, 4, [5, 6]]

// ✅ Recommended approach
const flattenedNative = nested.flat();
console.log(flattenedNative); // [1, 2, 3, 4, [5, 6]]

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How it works?

Flattens array a single level deep. Deprecated: Use array.flat() directly (ES2019).

Single Level Only

Native Equivalent

// ❌ flatten(arr)
// ✅ arr.flat()

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Use Cases

Flatten nested arrays one level 📌

Flatten one level of nesting in arrays.

#flatten#nested#arrays#level#normalize

const nested = [[1, 2], [3, 4], [5]];
nested.flat();
// => [1, 2, 3, 4, 5]

Merge grouped results

Combine results from grouped operations.

#merge#grouped#results#combine

const grouped = [users.slice(0, 10), users.slice(10, 20)];
grouped.flat();
// => all users in single array

Normalize API responses

Flatten nested response structures.

#normalize#API#responses#structure

const responses = [apiPage1.items, apiPage2.items];
responses.flat();