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flatten: flatten a nested array

Write a function flatten(array) that turns an array of any nesting depth into a flat one. Leave objects alone: only arrays get flattened.

Try to solve it without Array.prototype.flat, as interviewers usually ask.

function flatten(array) {
  // ...
}

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Tests (5)

expect(flatten([1, [2, [3, [4]], 5]])).toEqual([1, 2, 3, 4, 5]);

expect(flatten([[], [[]], 1])).toEqual([1]);

expect(flatten(["a", ["b"], { c: [1] }])).toEqual(["a", "b", { c: [1] }]);

const source = [1, [2]];
flatten(source);
expect(source).toEqual([1, [2]]);