Implement Array.prototype.flat

Implement Array.prototype.myFlat(depth = 1) so it behaves like the built-in flat, without calling flat or flatMap. The grader swaps both out while your code runs.

[1, [2], [3, [4]]].myFlat()           // [1, 2, 3, [4]]
[1, [2], [3, [4]]].myFlat(2)          // [1, 2, 3, 4]
[1, [2, [3, [4]]]].myFlat(Infinity)   // [1, 2, 3, 4]

Rules

• Only real arrays are flattened (Array.isArray). Strings, plain objects and array-likes are elements like any other.
• depth defaults to 1 and Infinity is valid. A depth of 0 or less copies one level without flattening anything.
• Holes ([1, , 3]) are skipped at every level the method walks, the top level included, even at depth 0. A nested array past the depth is kept as it is, holes and all.
• Return a new array and leave the input alone. Define the method with function, not an arrow, so this is the array.
• Don't recurse. The last test nests 100,000 levels deep and flattens with Infinity; a recursive walk overflows the call stack there, so that case only passes with an explicit stack.

Tests write a hole as the string "<hole>". The grader turns it into a real empty slot before calling you, and back into "<hole>" when it reads your result.