F (Index signature)

 title: Typescript

date: 2019-08-19 tag: Typescript

description: Typescript (Index signature)

const VERSIONS = { v1: 'V1', v2: 'V2',

}

typeof

type VERSIONSType = typeof VERSIONS;

// type VERSIONSType = {

v1: string v2: string }

Object.keys(VERSIONS).map(v => ({ value: v, title: VERSIONS[v]

})) // Throw Error // => No index signature with a parameter

indexSignature

export interface IndexSignature { [key: string]: any;

}

Object.keys(VERSIONS).map(v => ({ value: v, title: (VERSIONS as IndexSignature)[v]

}))

any key key

keyof key union

type VERSIONSKey = keyof typeof VERSIONS; (Object.keys(VERSIONS) as VERSIONSKey[]).map(v => ({

value: v, title: VERSIONS[v] }))

() => { const VERSIONS = { v1: 'V1', v2: 'V2', } interface IndexSignature { [key: string]: any; } type VERSIONSKey = keyof typeof VERSIONS; return ( Bad {Object.keys(VERSIONS).map(v => ({ value: v, title: (VERSIONS as IndexSignature)[v] }))} Good (Object.keys(VERSIONS) as VERSIONSKey[]).map(v => ({ value: v, title: VERSIONS[v] })) )

}

Typescript set map

set map

// set type Size = 'sm' | 'md' | 'lg'; // map interface Ob {

a: string b: number }

type ObKeys = keyof Ob; // 'a' | 'b' type ObVals = Ob[keyof Ob] // string | number

type SizeMap = {

[k in Size]: number

} //

type SizeMap = {

sm: number

md: number

lg: number

}

set map Record

type Record = { [P in K]: T }; type Size = 'sm' | 'md' | 'lg'; type SizeMap = Record // type SizeMap = { // sm: number // md: number // lg: number // }

map

Pick

type Pick = { [P in K]: T[P] }; type PickSizeMap = Pick; // type SizeMap = { // sm: number // md: number // }

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