How can I let one function use the check on a nullable field from another function?

I have below code in TypeScript. The function isDataAvailable returns true if data is not undefined. In updateData function, it has a if condition on the isDataAvailable return value but the compiler reports an error on data.i++: 'data' is possibly 'undefined'..

If the data has been checked in isDataAvailable function already, why does TypeScript still think it could be undefined? Is there a way to make it check the previous function return value?

let data: { i: number; j: number } | undefined = undefined;

const isDataAvailable = () => {
  return !!data;
};

const updateData = () => {
  if (isDataAvailable()) {
    data.i++;
  }
};

>Solution :

I think you’re looking for type predicates

They don’t deal with globals, though, they need to be arguments to the function:

type Data = { i: number; j: number }

const isDataAvailable = (data?: Data): data is Data => {
  return !!data;
};

const updateData = (data?: Data) => {
  if (isDataAvailable(data)) {
    data.i++;
  }
};

This is kind of a contrived example, because the above could be represented just by doing:

const updateData = (data?: Data) => {
  if (data) {
    data.i++;
  }
};

But hopefully type predicates is what you were looking for!