Typescript 参数到来自 object 的键和值

Question

Example

const food = {
  fruit: ['apples', 'oranges'],
  meat: ['chicken', 'pig']
}

function makeFood(ingredient, category) {
 switch(true) {
   case category === 'fruit' && ingredient === 'apples' {
    // do something
   }
   case category === 'fruit' && ingredient === 'oranges' {
    // do something
   }
   case category === 'meat' && ingredient === 'chicken' {
    // do something
   }
   case category === 'meat' && ingredient === 'pig' {
    // do something
   }
 }
}

what's the best way to type category is a key of food and ingredient is a value?

Currently I'm doing

function(ingredient, keyof typeof category) {

would love to keep the relation between the two. So TS will know the ingredient type based in the category value.

Answer 1

You use typescript Generics . In this you wanted category to be generic parameter, which ingredient can then use.

You'll need to use as const (see here ) on your source object to make sure it's specific string are part of its type.

const food = {
  fruit: ['apples', 'oranges'],
  meat: ['chicken', 'pig']
} as const // important to add "as const" here

function makeFood<
  K extends keyof typeof food // Generic parameter
>(
  ingredient: typeof food[K][number], // use K to lookup types for ingredient
  category: K // provide the key to to use here as K
) {
 switch(true) {
   case category === 'fruit' && ingredient === 'apples': {
    // do something
   }
   case category === 'fruit' && ingredient === 'oranges': {
    // do something
   }
   case category === 'meat' && ingredient === 'chicken': {
    // do something
   }
   case category === 'meat' && ingredient === 'pig': {
    // do something
   }
 }
}

makeFood('apples', 'fruit')
makeFood('pig', 'fruit') // type error