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Here's my solution with maxDepth:

function flatten(arr, maxDepth=Infinity, currDepth = 1){
    let res = []
    for (let i = 0; i < arr.length; i++) {
        if (!Array.isArray(arr[i]) || currDepth > maxDepth) {
            res.push(arr[i])
        } else {
            res = res.concat(flatten(arr[i], maxDepth, ++currDepth))
        }
    }
    return res
}