posts/typescript_bits_indexing_into_a_mapped_type

[utterances-bot]

TypeScript - indexing into a mapped type

Indexing into a mapped type a powerful TypeScript technique to allow a collection of all different types, while retaining the type information.

https://blacksheepcode.com/posts/typescript_bits_indexing_into_a_mapped_type

[dwjohnston]

Bonus:

If you need the equivalent technique for working with tuples, here's how you could do it:

type ListOfTuples = [[NumberConstructor, number], [StringConstructor, string], [BooleanConstructor, boolean]]; 

type ConfigItem = {
  [K in 0 | 1 | 2] :{
  key: ListOfTuples[K][0],
  value: ListOfTuples[K][1]
}
}[0 | 1 | 2]


function processConfigItem(configItem: ConfigItem){

}

processConfigItem({
  key: Number,
  value:1
})


// Argument of type '{ key: NumberConstructor; value: string; }' is not assignable to parameter of type 'ConfigItem2'.
//   Types of property 'key' are incompatible.
//     Type 'NumberConstructor' is missing the following properties from type 'StringConstructor':
processConfigItem({
  key: Number,
  value:"2"
})