The primitive types

• All primitive types are referenced in lowercase. number, string, boolean, undefined, null...

• TS adds a couple of extra lowercase types solely related to its type-checking job: any, unknown, void, never...

• Arrays can be declared either by something[] or Array<something>;

Be careful: There also exists an uppercase Number type, which is a different thing from lowercase number! Types like Number, String, Boolean refer to the javascript functions that have those names.

Be careful: Both types {} and object refer to an empty object. To declare an object that can receive any property, use Record<string, any>.

Strict nulls

• Unlike some other languages, types do not implicitly include null;

• Ex: in Java, any variable can always also be null;

• In TypeScript a type is declared as nullable through a type union: type X = Something | null | undefined

• A type can be narrowed as "not null" through control flow analysis. Ex:

const x = 2 as number | null
if (x) {
    console.log(x) // x cannot be null inside this block
}

• You can tell the compiler to assume a variable is not null with the ! operator;

interface X {
    optional?: { value: number }
}
const instance: X = {}
console.log(instance.optional.value) // TS will show error
console.log(instance.optional!.value) // assume "optional" exists