Limiting the combinations of types in TypeScript

I have a dilemma:

type TypeLetter = "TypeA" | "TypeB"

type TypeNumber = "Type1" | "Type2"

I am trying to restrict the combinations of values from these types. Only "TypeA" and "Type1" can be paired together, and only "TypeB" and "Type2" can be paired together.

How can I define a type called Restricted to enforce these restrictions? In other words, where these combinations are allowed:

const valid1: Restricted = {
   valLetter: "TypeA" 
   valNumber: "Type1"
}

const valid2: Restricted = {
   valLetter: "TypeB" 
   valNumber: "Type2"
}

But this combination is not allowed:

const nope: Restricted = {
   valLetter: "TypeB" 
   valNumber: "Type1"
}
javascripttypescript

Answer №1

This code snippet is specifically tailored for handling two specific types. It utilizes const to define the types in a concise manner.

type Restricted = {
    valLetter: 'TypeA',
    valNumber: 'Type1'
} | {
    valLetter: 'TypeB',
    valNumber: 'Type2'
};

const example: Restricted = {
    valLetter: "TypeB",
    valNumber: "Type1"
} // The error message indicates a type mismatch between '"Type1"' and '"Type2"'.

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