Parameters are a core part of the type checking applied by the TypeScript compiler. TypeScript offers an optional functionality by adding the ? symbol as a postfix to the parameter name we want to make optional. This allows us to leave out the second parameter in the function call. 

function greetMe(name: string, greeting?: string): string {
  console.log(greeting);
  if(!greeting) { greeting = 'Hello'; }
  return greeting + ', ' + name;
}

console.log( greetMe('Chris') );

This code will attempt to print out the greeting variable as well as produce a proper greeting. Running this code like this:

greetMe('Chris');

Will give us the following result:

undefined
Hello Chris

So, an optional parameter doesn't really get set unless you explicitly make it so. It is more of a construct so that you can get help with deciding what parameters are mandatory and which ones are optional. Let's exemplify that:

function add(mandatory: string, optional?: number) {}

You can invoke this function in the following ways:

add('some string');
add('some string', 3.14);

Both versions are allowed. Using optional parameters in your function signature forces you to place them last, like the previous example. The following example illustrates what not to do:

function add(optional?: number, mandatory: string) {}

This would create a situation where both parameters would be mandatory:

add(11); // error. mandatory parameter missing

Even the compiler would complain and say the following:

A required parameter cannot follow an optional parameter

Remember, optionals are great, but place them last.