Klein Maximus

let str = `[
    {"A": "Value for A", B: "Value for B", "foo": "foo"},
    {"A": "Value for A", B: "Value for B", "bar": 13},
    {"A": "Value for A", B: "Value for B", "foo": "foo", "val": true}
]`;
let result = JSON.parse(str).map(item => {
  let others = { ...item };
  delete others.A;
  delete others.B;
  return {
    A: item.A,
    B: item.B,
    ...others
  };
});