从对象中派生联合类型

问题

假设我们有一个对象

export const fruitCounts = {
	pear: 5,
	apple: 9,
	banana: 3
}

然后需要定义一个类型，该类型为此对象所有属性的联合类型：

type SingleFruitCount = { pear: number } | { apple: number } | { banana: number }

那么该怎么通过对象 fruitCounts 直接生成 SingleFruitCount 类型呢？

在线试一试

解答

首先我们先拿到 fruitCounts 的类型

type FruitCounts = typeof fruitCounts

如果我们这样定义SingleFruitCount

type SingleFruitCount = {
	[Key in keyof FruitCounts]: {}
}

其等同于：

type SingleFruitCount = {  
	pear: {};  
	apple: {};  
	banana: {};  
}

我们继续修改 SingleFruitCount 定义

type SingleFruitCount = {
	[Key in keyof FruitCounts]: {
		[K in Key]: number
	}
}

此时效果会变成：

type SingleFruitCount = {  
	pear: {  
		pear: number;  
	};  
	apple: {  
		apple: number;  
	};  
	banana: {  
		banana: number;  
	};  
}

那这个时候我们只需要把每个对象通过 key 取出来即可：

type SingleFruitCount = {
	[Key in keyof FruitCounts]: {
		[K in Key]: number
	}
}[keyof FruitCounts]

完整代码：

export const fruitCounts = {
	pear: 5,
	apple: 9,
	banana: 3
}

type FruitCounts = typeof fruitCounts

type SingleFruitCount = {
	[Key in keyof FruitCounts]: {
		[K in Key]: number
	}
}[keyof FruitCounts]