Value level vs type level

• Value level exists in JS world and type level exists in TS world
• There is one way to go from value level to type level, ie using typeof operator
• This is called type inference

Creating types from other types

1: Type tansformations

1.1 Inference basics

1.1.1 How to get the return type of a function

• Use ReturnType type helper
• typeof myFunc gives the type of the function myFunc
• ReturnType<typeof myFunc> gives the return type of the function

1.1.2 Extract Function Parameters Into A Type

• Use Parameters type helper
• Use Parameters<typeof myFunc>

1.1.3 Extract The Awaited Result of a Promise

• Use Awaited type helper
• Use Awaited<ReturnType<typeof myFunc>>

1.1.4 Create a Union Type From an Object’s Keys

• Use keyof typeof myObj

1.2 Unions and Indexing

1.2.1 Understand The Terminology Around Unions

• Union vs Discriminated unions vs enum

1.2.2 Extracting Members of a Discriminated Union

• Use Extract<Type, Union>

1.2.3 Excluding Parts of a Discriminated Union

• Use Exclude<UnionType, ExcludedMembers>

1.2.4 Resolve an Object’s Values as Literal Types

• Use as const.
• Can also be used for Arrays.

1.2.5 Create a UnionType From an Object’s Values

• First convert to object literal type using as ‘const’
• Use typeof myObj[keyof typeof myObj]

1.2.6 Create Unions out of Array Values

• First convert to Array literal type using as ‘const’
• Then use typeof myArr[number] or typeof myArr[0 | 1]

1.3 Template Literals

1.3.1 Only Allow Specified String Patterns

• Use template literals for eg. type Route = `/${string}` ;

1.3.2 Extract Union Strings Matching a Pattern

• Use Extract with template literals.

1.3.3 Create a Union of Strings with all possible permutations of two unions

• Use template literals to combine the unions.

Splitting a string into a tuple

• Use ts-toolbelt library method Split<S, D> to split a string into a tuple.

Create an Object Whose Keys Are Derived From a Union

• Use Record<UnionType, string>

Transform String Literals To Uppercase

• Use Uppercase<>

1.4: Type helpers (Generics)

• Generics are similar to functions in value world.

1.4.1 Creating a Maybe Type Helper

1.4.2 Ensure Type Safety in a Type Helper