NRM Present 1.md

Advanded Typescript Generic

Use and play with Typescript Generic like a pro.

Table of Contents:

1. Conditional Types
2. String Concatenate
3. infer Keyword
4. Type Assertion

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Conditional Types

Logical types baseed on given generic

Syntax

type NoNull<T> = T extends null ? never : T

Use Case

To build complex logical types

Usage

interface Exception {
  kind: {
    stack: number
    message: string
  }
}

type Payload<T extends Exception | string>  = {
  body: T extends Exception ? Exception["kind"] : string
}

---

String Concatenate

We can concat the given generic just with Javascript String Literal Syntax

Syntax

type FullName<F extends string, S extends string> = `${F} ${L}`

Use Case

• Type static string

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infer Keyword

A keyword to extract unknown type and assign to generic

Use Case

type Unpromisify<T> = T extends Promise<infer R> ? R : T

---

Type Assertion

Example

interface Plane {
    fly: () => void
}

interface Car {
    run: () => void
}

function canFly(entity: Plane | Car): entity is Plane {
    return !!(entity as Plane).fly
}