TSDoc | TSDoc

When:

undefined (default) provide suggestions as warnings to editors
true unreachable code is ignored
false raises compiler errors about unreachable code

These warnings are only about code which is provably unreachable due to the use of JavaScript syntax, for example:

function fn(n: number) {
  if (n > 5) {
    return true;
  } else {
    return false;
  }
  return true;
}

function fn(n: number) {
  if (n > 5) {
    return true;
  } else {
    return false;
  }
  return true;
}

With "allowUnreachableCode": false:

// @errors: 7027
// @allowUnreachableCode: false
function fn(n: number) {
  if (n > 5) {
    return true;
  } else {
    return false;
  }
  return true;
}

// @errors: 7027
// @allowUnreachableCode: false
function fn(n: number) {
  if (n > 5) {
    return true;
  } else {
    return false;
  }
  return true;
}

This does not affect errors on the basis of code which appears to be unreachable due to type analysis.