tbd - notes taken while learning Typescript

typescript_notes.ts

/*
- Static typing
- Code completion
- Refactoring
- Shorthand notations

transpilation
    typescript -> javascript
*/

// creating a configuration file for the compiler
// tsc --init

//#region variables

let age: number = 20
console.log(age)

//#region types

// javascript:
//  number, string, boolean, null, undefined, object
// typescript:
//  any, unknown, never, enum, tuple

let sales: number = 123_456_789 //can use underscores for large numbers
let course: string = 'Typescript'
let is_published: boolean = true
let level //any - avoid. not best practice

// arrays
let numbers: number[] = [1, 2, 3]

// tuple
let user: [number, string] = [1, 'Jake']

// enums - list of related constants
const small = 1
const medium = 2
const large = 3

// values here are auto assigned:
enum Size { Small, Medium, Large }
// more optimized javascript code:
const enum Size2 { Small, Medium, Large }
// can explicity set values
enum shirtSize { Small = 2, Medium = 5, Large = 10 }
let mySize: Size = Size.Medium
console.log(mySize)

// objects
let employee: {
    readonly id: number
    name: string
    retire: (date: Date) => void
} = {
    id: 1,
    name: 'Jake',
    retire: (date: Date) => {
        console.log(date)
    }
}
employee.name = 'Joe'

//#endregion

//#endregion

// annotate what the function is going to return
function calculateTax(income: number): number {
    if (income < 50_000)
        return income * 1.2
    // else would be undefined here
    // return 0
}

// optional parameters
function calculateTaxS(income: number, taxYear = 2022): number {
    if (taxYear < 2022) {
        return income * 1.2
    }
    return income * 1.3
}

// type alias
type Employee = {
    readonly id: number
    name: string
    retire: (date: Date) => void
}
let aEmployee: Employee = {
    id: 1,
    name: 'Jake',
    retire: (date: Date) => {
        console.log(date)
    }
}

// union
function kgToLbs(weight: number | string): number {
    // Narrowing
    if (typeof weight === 'number') {
        return weight * 2.2
    }
    else {
        return parseInt(weight) * 2.2
    }
}
kgToLbs(10)
kgToLbs('10kg')

// intersection
let weight: number & string
type Draggable = {
    drag: () => void
}
type Resizable = {
    resize: () => void
}
type UIWidget = Draggable & Resizable
let textBox: UIWidget = {
    drag: () => { },
    resize: () => { }
}

// literal types - Literal (exact, specific)
let quantity: 50 | 100 = 100

type Quantity = 50 | 100
let zeQuantity: Quantity = 100

type Metric = 'cm' | 'inch'

// nullable types
function greet(name: string | null) {
    if (name) {
        console.log(name.toUpperCase())
    }
    else {
        console.log('Hola!')
    }
}
greet(null)

// null checks
type Customer = {
    birthday: Date
}
function getCustomer(id: number): Customer | null {
    return id === 0 ? null : { birthday: new Date() }
}
let customer = getCustomer(0)
// optional property access operator
console.log(customer?.birthday)

// optional element access operator
// customers?.[0]
// optional call
let log: any = null
log?.('a')