// The simple one :P
const testArray1 = [1, 2, [3, 4]];
testArray1.flat(); // returns  [1, 2, 3, 4]

const testArray2 = [1, 2, [3, 4, [5, 6]]];
testArray2.flat();
// returns [1, 2, 3, 4, [5, 6]];
testArray2.flat(2); // goes two levels deep recursively as we passed the parameter
// returns [1, 2, 3, 4, 5, 6];

const testArray3 = [1, 2, , 3, 4];
testArray3.flat();
// returns [1, 2, 3, 4];