javascript

Chapter 22: The Evolution of JavaScript
Function Types
You can also specify function types, which specify the type and number of arguments expected as well
as the return type. This can be useful for functions that accept callback functions as arguments, as in this
example:
type Callback = function(int, string): void;
function doSomething(callback: Callback) {
callback(0, “Success”)
}
Here, a Callback type is defined to be a function that accepts two arguments: an int and a string .
The function shouldn ’ t return a value, so the return value is specified as void . You can specify that an
argument is optional by appending an equal sign as shown here:
type Callback = function(int, string=): void;
//second argument optional
The Callback type now optionally accepts a second argument that ’ s a string .
A function type can specify a method on a given type of object by specifying the type that this
should be, as in this example:
type Callback = function(this: Person, string): void; //method on a Person
Here, the Callback type defines a function that must be a method of a Person object and accepts a
single string argument.
The last option is to specify that there may be rest arguments to the function. This can be done using
three dots to specify any arguments or three dots followed by a type in square braces to indicate that the
arguments must be of a specific type. Here’s an example:
type SomeFunction = function(string,...): boolean
type MyFunction = function(string,...[string]): boolean
The first type, SomeFunction , specifies a function that accepts a string as its first argument and then a
variable number of arguments that can be any type. The second type, MyFunction , is the same except
that the rest arguments must be of the type string .
Parameterized Types
Parameterized types can be used when a type should have a certain structure but the data types in that
structure may change, as in this example:
type Point. < T > = {
x: T,
y: T
};
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