AutoComplete in VSCode

• Ctrl+space

• The arguments to a functions must be typed to avoid implicit any error.

type vs interface

Optional function arguments

• Must come after the mandatory args.

Array types

• Post[] or Array<Post>

Casting in TS

• By using 'as' keyword.
• Should not be used often.

Set generic

Map generic

Record type

Record<string, string>
// Or
{
  [id:string]: string;
}

Narrowing down Union types

Typing errors in try-catch

....
catch(e) {
  if (e instanceof Error) {
    return e.message;
  }
}

Inheriting interface properties

• Use extends.
• Can extend multiple interfaces.

Combining types to create new types

• Use & operator on interfaces.

Selectively Construct Types from Other Types

• Use Pick<Type, Keys> or Omit<Type, Keys>.

Typing Functions

Typing Async Functions

1: Type tansformations

1.1 Inference basics

1.1.1 Get the return type of a function

• Use ReturnType<typeof myFunc>

1.1.2 Extract Function Parameters Into A Type

• Use Parameters<typeof myFunc>

1.1.3 Extract The Awaited Result of a Promise

• 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