Write a function flattenOnce(arr) that flattens an array by exactly one level, not fully recursive. For example, flattenOnce([1, [2, 3], [4, [5, 6]]]) should return [1, 2, 3, 4, [5, 6]].
function flattenOnce(arr) {
// your code here
}
Rules:
- No use of Array.prototype.flat
- Only flatten one level deep, nested arrays inside nested arrays stay intact
- Must return a new array, not mutate the input
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