What are the types of validator functions?

In reactive forms, the validators can be either synchronous or asynchronous functions,

1. Sync validators: These are the synchronous functions which take a control instance and immediately return either a set of validation errors or null. Also, these functions passed as second argument while instantiating the form control. The main use cases are simple checks like whether a field is empty, whether it exceeds a maximum length etc.
2. Async validators: These are the asynchronous functions which take a control instance and return a Promise or Observable that later emits a set of validation errors or null. Also, these functions passed as second argument while instantiating the form control. The main use cases are complex validations like hitting a server to check the availability of a username or email.

The representation of these validators looks like below

this.myForm = formBuilder.group({
    firstName: ['value'],
    lastName: ['value', *Some Sync validation function*],
    email: ['value', *Some validation function*, *Some asynchronous validation function*]
});