TypeScript: checking at compile time if an Array lists all property keys

[2022-07-27] dev, typescript
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In this blog post, we use TypeScript to ensure that an object stays in sync with an Array that lists its properties.

The problem  

Consider the following TypeScript code:

interface Person {
  first: string;
  last: string;

const personKeys = [
] as const;

personKeys lists the property keys of Person. Can we check at compile time if this list is correct?

The solution  

import {assert as tsafeAssert, Equals} from 'tsafe';
  keyof Person,
  (typeof personKeys)[number]

Library tsafe enables us to check if two types are equal.

The arguments of Equals<> are computed as follows:

// %inferred-type: "first" | "last"
type A = keyof Person;

// %inferred-type: readonly ["first", "last"]
type B = typeof personKeys;

// %inferred-type: "first" | "last"
type C = B[number];

To compute type C, we are using the indexed access operator T[K]: For a given type T, it computes the types of all properties whose keys are assignable to type K. The following two types are roughly equivalent. That explains the result of computing B[number].

type T = ["first", "last"];
type U = {
  0: "first",
  1: "last",

Further reading