☂️ 👈🏽 👩‍❤️‍👨 Pocketbook on TypeScript. Part 2. Types for every day 💶 🚹 🔬

We continue to publish a series of adapted and supplemented translations " TypeScript

. "

Other parts:

Part 1. Basics

Primitives: `string`

, `number`

and`boolean`

In the JS

commonly used 3 primitive : string

, number

and boolean

. Each of them has a corresponding type in TS

:

string

, , 'Hello World'
number

, , 42

. JS

( ), , int

float

— number
boolean

— : true

false

: String

, Number

Boolean

( ) , , , . string

, number

boolean

.

[1, 2, 3]

number[]

; (, string[]

— ..). Array<number>

, . , , (generics).

: [number]

— , (tuple).

`any`

TS

any

, :

let obj: any = { x: 0 }
//             
//  `any`   
//  `any` ,        ,  `TS`
obj.foo()
obj()
obj.bar = 100
obj = 'hello'
const n: number = obj

any

, , .

`noImplicitAny`

TS

, any

.

, , any

. noImplicitAny

any

.

const

, let

var

:

const myName: string = 'John'

, , TS

, .. :

//      - `myName`    `string`
const myName = 'John'

JS

, , . TS

(input), (output) .

, :

function greet(name: string) {
 console.log(`Hello, ${name.toUpperCase()}!`)
}

greet(42)
// Argument of type 'number' is not assignable to parameter of type 'string'.   'number'       'string'

: .

function getFavouriteNumber(): number {
 return 26
}

, TS

return

.

. , TS

, .

//   ,     `TS`  
const names = ['Alice', 'Bob', 'John']

//       
names.forEach(function (s) {
 console.log(s.toUppercase())
 // Property 'toUppercase' does not exist on type 'string'. Did you mean 'toUpperCase'?  'toUppercase'     'string'.    'toUpperCase'?
})

//          
names.forEach((s) => {
 console.log(s.toUppercase())
 // Property 'toUppercase' does not exist on type 'string'. Did you mean 'toUpperCase'?
})

s

, TS

forEach

, s

. (contextual typing).

— . . , , :

function printCoords(pt: { x: number, y: number }) {
 console.log(`  'x': ${pt.x}`)
 console.log(`  'y': ${pt.y}`)
}

printCoords({ x: 3, y: 7 })

,

;

. . any

.

?

:

function printName(obj: { first: string, last?: string }) {
 // ...
}
//     
printName({ first: 'John' })
printName({ first: 'Jane', last: 'Air' })

JS

undefined

. , undefined

:

function printName(obj: { first: string, last?: string }) {
 //  -   ,   `last`     
 console.log(obj.last.toUpperCase()) // Object is possibly 'undefined'.     'undefined'

 if (obj.last !== undefined) {
   //    
   console.log(obj.last.toUpperCase())
 }

 //  ,    `JS` -    (`?.`)
 console.log(obj.last?.toUpperCase())
}

(unions)

: , TS

, union

, , , , .

— , 2 , , . , , (members) .

, :

function printId(id: number | string) {
 console.log(` ID: ${id}`)
}

// OK
printId(101)
// OK
printId('202')
// 
printId({ myID: 22342 })
// Argument of type '{ myID: number }' is not assignable to parameter of type 'string | number'. Type '{ myID: number }' is not assignable to type 'number'.   '{ myID: number }'       'string | number'.  '{ myID: number }'      'number'

, TS

, . , string | number

, , string

:

function printId(id: number | string) {
 console.log(id.toUpperCase())
 // Property 'toUpperCase' does not exist on type 'string | number'. Property 'toUpperCase' does not exist on type 'number'.
}

(narrowing) . , TS

, string

typeof

'string'

:

function printId(id: number | string) {
 if (typeof id === 'string') {
   //    `id`   'string'
   console.log(id.toUpperCase())
 } else {
   //   `id`   'number'
   console.log(id)
 }
}

, Array.isArray

:

function welcomePeople(x: string[] | string) {
 if (Array.isArray(x)) {
   //  `x` -  'string[]'
   console.log(', ' + x.join('  '))
 } else {
   //  `x` - 'string'
   console.log(' ,   ' + x)
 }
}

. , , slice

. , :

function getFirstThree(x: number[] | string ) {
 return x.slice(0, 3)
}

(type aliases)

? :

type Point = {
 x: number
 y: number
}

//    ,     
function printCoords(pt: Point) {
 console.log(`  'x': ${pt.x}`)
 console.log(`  'y': ${pt.y}`)
}

printCoords({ x: 3, y: 7 })

, , , :

type ID = number | string

: — , "" . , , TS

, :

type UserInputSanitizedString = string

function sanitizeInput(str: string): UserInputSanitizedString {
 return sanitize(str)
}

//  "" 
let userInput = sanitizeInput(getInput())

// -     
userInput = 'new input'

— :

interface Point {
 x: number
 y: number
}

function printCoords(pt: Point) {
 console.log(`  'x': ${pt.x}`)
 console.log(`  'y': ${pt.y}`)
}

printCoords({ x: 3, y: 7 })

TS

(structurally typed type system) — TS

, printCoords

, .. .

. interface

type

. , type

, interface

.

interface Animal {
 name: string
}

interface Bear extends Animal {
 honey: boolean
}

const bear = getBear()
bear.name
bear.honey

(intersection):

type Animal {
 name: string
}

type Bear = Animal & {
 honey: boolean
}

const bear = getBear()
bear.name
bear.honey

interface Window {
 title: string
}

interface Window {
 ts: TypeScriptAPI
}

const src = 'const a = 'Hello World''
window.ts.transpileModule(src, {})

type Window = {
 title: string
}

type Window = {
 ts: TypeScriptAPI
}
// :   'Window'.

: interface

, type

.

(type assertion)

, TS

.

, document.getElementById

, TS

, - HTMLElement

, , , HTMLCanvasElement

. :

const myCanvas = document.getElementById('main_canvas') as HTMLCanvasElement

( TSX-):

const myCanvas = <HTMLCanvasElement>document.getElementById('main_canvas')

TS

. , :

const x = 'hello' as number
// Conversion of type 'string' to type 'number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
//   'string'   'number'   ,     .     ,       'unknown'

. : any

( unknown

), :

const a = (expr as any) as T

(literal types)

string

number

, , .

TS

:

let changingString = 'Hello World'
changingString = 'Olá Mundo'
//  `changingString`    , 
//  TS     
changingString
 // let changingString: string

const constantString = 'Hello World'
//  `constantString`     , 
//     
constantString
 // const constantString: 'Hello World'

let x: 'hello' = 'hello'
// OK
x = 'hello'
// ...
x = 'howdy'
// Type '"howdy"' is not assignable to type '"hello"'.

, , , :

function printText(s: string, alignment: 'left' | 'right' | 'center') {
 // ...
}
printText('Hello World', 'left')
printText("G'day, mate", "centre")
// Argument of type '"centre"' is not assignable to parameter of type '"left" | "right" | "center"'.

function compare(a: string, b: string): -1 | 0 | 1 {
 return a === b ? 0 : a > b ? 1 : -1
}

, :

interface Options {
 width: number
}
function configure(x: Options | 'auto') {
 // ...
}
configure({ width: 100 })
configure('auto')
configure('automatic')
// Argument of type '"automatic"' is not assignable to parameter of type 'Options | "auto"'.

, TS

, . , :

const obj = { counter: 0 }
if (someCondition) {
 obj.counter = 1
}

TS

1

, 0

, . , TS

, obj.counter

number

, 0

.

const req = { url: 'https://example.com', method: 'GET' }
handleRequest(req.url, req.method)
// Argument of type 'string' is not assignable to parameter of type '"GET" | "POST"'.

req.method

string

, 'GET'

. req

handleRequest

, req.method

, , GUESS

, TS

, .

2 .

//  1
const req = { url: 'https://example.com', method: 'GET' as 'GET' }
//  2
handleRequest(req.url, req.method as 'GET')

as const

:

const req = { url: 'https://example.com', method: 'GET' } as const
handleRequest(req.url, req.method)

`null`

`undefined`

JS

, : null

undefined

. TS

. , , strictNullChecks

(. 1).

(non-null assertion operator)

TS

null

undefined

. !

, , .. null

undefined

:

function liveDangerously(x?: number | undefined) {
 //   
 console.log(x!.toFixed())
}

(enums)

, . .

`bigint`

BigInt

:

//  `bigint`    `BigInt`
const oneHundred: bigint = BigInt(100)

//  `bigint`    
const anotherHundred: bigint = 100n

BigInt

.

`symbol`

Symbol()

:

const firstName = Symbol('name')
const secondName = Symbol('name')

if (firstName === secondName) {
 // This condition will always return 'false' since the types 'typeof firstName' and 'typeof secondName' have no overlap.
 //  `firstName`  `lastName`    
}

TypeScript.

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Pocketbook on TypeScript. Part 2. Types for every day

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