Type Annotations

On variables declared using var, let and const, it is possible to optionally add a type:

const x: number = 1;

TypeScript does a good job of inferring types, especially for simple ones, so these declarations are not necessary in most cases.

On functions, it is possible to add type annotations to parameters:

function sum(a: number, b: number) {
    return a + b;
}

The following is an example using an anonymous function (also called a lambda function):

const sum = (a: number, b: number) => a + b;

These annotations can be avoided when a default value for a parameter is present:

const sum = (a = 10, b: number) => a + b;

Return type annotations can be added to functions:

const sum = (a = 10, b: number): number => a + b;

This is especially useful for more complex functions, as writing the return type before an implementation can help you think through the function.

Generally, consider annotating type signatures, but not body-local variables, and always add types to object literals.