[API reference](https://evolu.dev/docs/api-reference) › [@evolu/common](https://evolu.dev/docs/api-reference/common) › [Array](https://evolu.dev/docs/api-reference/common/Array) › concatArrays

## Call Signature

```ts
function concatArrays<T>(
  first: readonly [T, T],
  second: readonly T[],
): readonly [T, T];
```

Defined in: [packages/common/src/Array.ts:527](https://github.com/evoluhq/evolu/blob/ca15b4661c40fedb9f47434497086039cbdea38e/packages/common/src/Array.ts#L527)

Concatenates two arrays, returning a new readonly array.

Returns a non-empty array when at least one input is non-empty.

### Concatenating non-empty inputs

```ts

const nonEmpty: NonEmptyReadonlyArray<number> = [1];
const joined = concatArrays([1, 2], [3, 4]);
const fromLeft = concatArrays(nonEmpty, []);
const fromRight = concatArrays([], nonEmpty);

expect(joined).toEqual([1, 2, 3, 4]);
expectTypeOf(fromLeft).toEqualTypeOf<NonEmptyReadonlyArray<number>>();
expectTypeOf(fromRight).toEqualTypeOf<NonEmptyReadonlyArray<number>>();
```

## Call Signature

```ts
function concatArrays<T>(
  first: readonly T[],
  second: readonly [T, T],
): readonly [T, T];
```

Defined in: [packages/common/src/Array.ts:532](https://github.com/evoluhq/evolu/blob/ca15b4661c40fedb9f47434497086039cbdea38e/packages/common/src/Array.ts#L532)

Second non-empty.

## Call Signature

```ts
function concatArrays<T>(
  first: readonly T[],
  second: readonly T[],
): readonly T[];
```

Defined in: [packages/common/src/Array.ts:537](https://github.com/evoluhq/evolu/blob/ca15b4661c40fedb9f47434497086039cbdea38e/packages/common/src/Array.ts#L537)

Both possibly empty.