TypeScript TypeGuard example with side effects

typeguard-example-with-side-effect.ts

type Ref = { message?: string; isValid?: boolean };

const isValid = (value: any, ref: Ref): value is string => {
  if (typeof value === "string") {
    if (value.length > 10) {
      ref.isValid = false;
      ref.message = "Value should have length less than 10.";
    } else {
      ref.isValid = true;
      ref.message = "OK";
    }
  } else {
    ref.isValid = false;
    ref.message = "Value should be string.";
  }
  return ref.isValid;
};

const main = (value: any) => {
  const ref: Ref = {};
  if (isValid(value, ref)) {
    value; // type string
    console.log(ref.message);
  } else {
    value; // type any
    console.log(ref.message);
  }
};

main("hello!!!!!!!!!!!!!");