What is the syntax for defining a type when parameter has a default value?

Question

How do I define the type of the config parameter given that it has a default value?

function (config = {}) {};

Answer 1

function f(config: Object = {}) {}

Or, more generally:

function f(p: T = v) {}

where T is a type, and v is a value of type T.

Interestingly, the type of function f is (p?: T): void. That is, Flow understands that providing a default value makes the parameter optional. You don't need to explicitly make the parameter type optional—although it doesn't hurt.

When writing declare function statement in a .js.flow file, you can't include the default value; it will cause an error. So you must explicitly declare that the parameter is optional:

declare function f(p?: T): void;