mheiber/0-playground-link.md

## 0-playground-link.md

      
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              0-playground-link.md
            
          
    https://www.typescriptlang.org/play/index.html#src=type%20KeyMirror%3CT%20extends%20keyof%20any%3E%20%3D%20%7B%0D%0A%09%5BK%20in%20T%5D%3A%20K%0D%0A%7D%0D%0A%0D%0A%2F%2F%20simple%20approach%0D%0A%0D%0Atype%20FileTypes1%20%3D%20'ECMAScript'%20%7C%20'TypeScript'%0D%0A%0D%0Aconst%20FILE_TYPES_1%3A%20KeyMirror%3CFileTypes1%3E%20%3D%20%7B%0D%0A%09ECMAScript%3A%20'ECMAScript'%2C%0D%0A%09TypeScript%3A%20'TypeScript'%0D%0A%7D%0D%0A%0D%0A%0D%0A%2F%2F%20fancy%20approach%0D%0A%0D%0A%0D%0Afunction%20keyMirror%3CT%20extends%20string%3E(values%3A%20T%5B%5D)%3A%20KeyMirror%3CT%3E%20%7B%0D%0A%09const%20x%20%3D%20%7B%7D%20as%20KeyMirror%3CT%3E%0D%0A%09values.forEach(key%20%3D%3E%20%7B%0D%0A%09%09x%5Bkey%5D%20%3D%20key%3B%0D%0A%09%7D)%0D%0A%09return%20x%3B%0D%0A%7D%0D%0A%0D%0Aconst%20FILE_TYPES%20%3D%20keyMirror(%5B'ECMAScript'%2C%20'TypeScript'%5D)%0D%0Atype%20FileType%20%3D%20keyof%20typeof%20FILE_TYPES%0D%0A%0D%0Aconst%20x%3A%20FileType%20%3D%20'ECMAScript'%0D%0Aconst%20y%3A%20FileType%20%3D%20'will%20type%20error'%0D%0A%0D%0A%0D%0A%0D%0A%0D%0A%0D%0A

  
## 1-enums.ts
type KeyMirror<T extends keyof any> = {
	[K in T]: K
}

// simple approach

type FileTypes1 = 'ECMAScript' | 'TypeScript'

const FILE_TYPES_1: KeyMirror<FileTypes1> = {
	ECMAScript: 'ECMAScript',
	TypeScript: 'TypeScript'
}

const a: FileTypes1 = 'ECMAScript' // type-checks
const b: FileTypes1 = 'will type error' // compile-time error


// fancy approach


function keyMirror<T extends keyof any>(values: T[]): KeyMirror<T> {
	const x = {} as KeyMirror<T>
	values.forEach(key => {
		x[key] = key;
	})
	return x;
}

const FILE_TYPES = keyMirror(['ECMAScript', 'TypeScript'])
type FileType = keyof typeof FILE_TYPES

const x: FileType = 'ECMAScript' // type-checks
const y: FileType = 'will type error' // compile-time error
	type KeyMirror<T extends keyof any> = {
	[K in T]: K
	}

	// simple approach

	type FileTypes1 = 'ECMAScript' \| 'TypeScript'

	const FILE_TYPES_1: KeyMirror<FileTypes1> = {
	ECMAScript: 'ECMAScript',
	TypeScript: 'TypeScript'
	}

	const a: FileTypes1 = 'ECMAScript' // type-checks
	const b: FileTypes1 = 'will type error' // compile-time error


	// fancy approach


	function keyMirror<T extends keyof any>(values: T[]): KeyMirror<T> {
	const x = {} as KeyMirror<T>
	values.forEach(key => {
	x[key] = key;
	})
	return x;
	}

	const FILE_TYPES = keyMirror(['ECMAScript', 'TypeScript'])
	type FileType = keyof typeof FILE_TYPES

	const x: FileType = 'ECMAScript' // type-checks
	const y: FileType = 'will type error' // compile-time error