How to convert a mixed type to an object type

Question

When I read an unknown variable, eg: req.body or JSON.parse() and know it is formatted in a certain way, eg:

type MyDataType = {
  key1: string,
  key2: Array<SomeOtherComplexDataType>
};

how can I convert it, so that the following works:

function fn(x: MyDataType) {}
function (req, res) {
  res.send(
    fn(req.body)
  );
}

It keeps failing telling me that: req.body is mixed. This type is incompatible with object type MyDataType.

I assume this has something to do with Dynamic Type Tests but figure out how...

Answer 1

One way I can get this to work is by iterating through the body and copying every result over, eg:

if (req.body && 
      req.body.key1 && typeof req.body.key2 === "string" && 
      req.body.key2 && Array.isArray(req.body.key2)
) { 
  const data: MyDataType = {
    key1: req.body.key1,
    key2: []
  };
  req.body.key2.forEach((value: mixed) => {
    if (value !== null && typeof value === "object" &&
        value.foo && typeof value.foo === "string" &&
        value.bar && typeof value.bar === "string") {
      data.key2.push({
        foo: value.foo,
        bar: value.bar
      });
    }
  }); 
}

I guess, technically this is correct - you are refining every single value and only putting in what you know to be true.

Is this the appropriate way of handling these kinds of cases?