In pretty much any type of application, we are going to work with data, and of course, we need to ensure that this data is of a certain type or structure before we can act on it. This poses a number of problems. First, how do we easily define the way we want our data to be structured (often referred to as a schema), and second, how can we provide feedback in a consistent manner if the data provided isn't structured the way we want.
As with testing, if it's not easy to write or understand a schema, often, as developers, we won't do it, and instead, resort to very primitive attempts such as:
…
if (typeof username !== 'string') {
// throw or return error
}
// perform action
…First of all, please note that if you find yourself doing this, you're going to have huge problems as your application logic grows. You might have already seen examples of this—massive validation functions for performing actions with different combinations of properties.
This is hugely counterproductive...