ts-utility-types-analysis

Analysing and reverse engineering Typescript's utility types:

• Partial - mapped type
• Readonly - mapped type
• Record<K,T> - mapped type
• Pick<T,K> - mapped type
• Omit<T,K> - conditional type (pick + exclude)
• Exclude<T,U> - conditional type
• Extract<T,U> - conditional type
• NonNullable - conditional type
• Parameters - conditional + infer
• ConstructorParameters - conditional + infer
• ReturnType - conditional + infer
• InstanceType - conditional + infer
• Required - mapped type
• ThisParameterType - conditional + infer
• OmitThisParameter - conditional + infer
• ThisType - ???

Source: https://github.com/microsoft/TypeScript/blob/master/lib/lib.es5.d.ts

Types as metavalues

Consider the following two levels of TypeScript code:

• Program level: At runtime, we can use values and functions.
• Type level: At compile time, we can use specific types and generic types.

The type level is a metalevel of the program level.

Level	Available at	Operands	Operations
Program	Runtime	Values	Functions
Type	Compile time	Specific types	Generic types

type ObjectLiteralType = {
  first: 1,
  second: 2,
};

type Result = keyof ObjectLiteralType; // (A)

In line A, we are taking the following steps:

• The input of our computation is the type ObjectLiteralType, an object literal type.
• We apply the operation keyof to the input. It lists the property keys of an object type.
• We give the output of keyof the name Result.

const ObjectLiteralType = {
  first: 1,
  second: 2,
};

console.log(Object.keys(ObjectLiteralType));

Content

1. mapped types

• union and intersection
• indexed type query and indexed access operator
• generics
• mapped types

2. conditional types

• conditionals
• "naked" type parameters
• never

3. conditional types with infer

• conditionals with infering

4. Bonus

• advanced custom utility type
• object type
• unknown type