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JavaScript interview question

Flatten Array

medium15 min

Write flatten(values): take an array that can contain other arrays and return a new array containing every non-array value, in its original left-to-right order.

Requirements

Flatten all nested arrays, regardless of their nesting depth.
Return a new array without changing the input or its nested arrays.
Preserve each non-array value, including its type and object identity.
Empty arrays add no values to the result.
The input is an array without circular references. Sparse arrays are outside this exercise.

Examples

Example 1

Input: flatten([1, [2, [3]], 4])
Output: [1, 2, 3, 4]

Example 2

Input: flatten([[], [0, false], ['hello', null]])
Output: [0, false, 'hello', null]

Example 3

Input: flatten([])
Output: []

flatten(values) accepts a nested array and returns a flat array. In TypeScript, NestedArray<T> preserves the leaf value type in the returned T[].

What the tests check

Empty input returns a new empty array
Flat input stays in order and returns a new array
Nested arrays flatten from left to right
Empty nested arrays contribute no values
Falsy values and objects are preserved
Several nesting levels are supported
Leaf identity and undefined are preserved
Frozen input is never mutated

Visible test cases (6)

Empty input returns a new empty array

Input arguments: [[]]
Expected: {"result":[],"inputAfter":[],"inputUnchanged":true,"returnedNewArray":true}

Flat input stays in order and returns a new array

Input arguments: [[3,1,2]]
Expected: {"result":[3,1,2],"inputAfter":[3,1,2],"inputUnchanged":true,"returnedNewArray":true}

Nested arrays flatten from left to right

Input arguments: [[1,[2,[3]],4]]
Expected: {"result":[1,2,3,4],"inputAfter":[1,[2,[3]],4],"inputUnchanged":true,"returnedNewArray":true}

Empty nested arrays contribute no values

Input arguments: [[[],[1,[],[2]],[]]]
Expected: {"result":[1,2],"inputAfter":[[],[1,[],[2]],[]],"inputUnchanged":true,"returnedNewArray":true}

Falsy values and objects are preserved

Input arguments: [[0,[false,"",null],[{"id":1}]]]
Expected: {"result":[0,false,"",null,{"id":1}],"inputAfter":[0,[false,"",null],[{"id":1}]],"inputUnchanged":true,"returnedNewArray":true}

Several nesting levels are supported

Input arguments: [[[[[1]]],[[2,[3,[4]]]],5]]
Expected: {"result":[1,2,3,4,5],"inputAfter":[[[[1]]],[[2,[3,[4]]]],5],"inputUnchanged":true,"returnedNewArray":true}

References

Array.isArray()

ArraysRecursionArray traversalImmutability

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