Functions

A function is a sequence of instructions that can transform some specified input into a specified output.

Describing functions

A common way to describe a function is with a function type expression.

Another way to describe a function is with an arrow function.

Defining functions with the arrow syntax is especially useful when working with functions as values. Suppose in the example above, we want to make it as easy as possible for a future developer to add support for new languages.

Return types

The type returned by a function

Parameter types

Each parameter can be typed

Construct signatures

JavaScript functions can be invoked with the new operator, which creates a special object that is referred to as an instance. TypeScript refers to these as constructors, because they usually "construct" a new object.

You can write a construct signature by adding the new keyword in front of a call signature.