TypeScript already supports treating an exact string/number as a literal, for example this function only allows two exact strings and no others:

declare function enableFeature(command: "redesign" | "newArtistPage"): void;
enableFeature("redesign");
enableFeature(`newArtistPage`);
enableFeature("newPaymentSystem");

// String literals supports all the way you can write a string in ES2020, with TypeScript 4.1 we've extended support for interpolation inside a template string literal.

type Features = "Redesign" | "newArtistPage";

// This takes the Features union above, and transforms each part of the union to add `-branch` after the string

type FeatureBranch = `${Features}-branch`;

// 4.1 supports a set of new generic-like keywords which you can use inside a template literal to manipulate strings. These are: Uppercase, Lowercase, Capitalize and Uncapitalize

type FeatureID = `${Lowercase}-id`;
type FeatureEnvVar = `${Uppercase}-ID`;

// Strings in unions are cross multiplied, so if used more than one union type then each union member is evaluated against each member from the other union.

type EnabledStates = "enabled" | "disabled";
type FeatureUIStrings = `${Features} is ${EnabledStates}`;

// This ensures that every possible combination of each union is accounted for. This type can then be used with an indexed signature to quickly make a list of keys:

type SetFeatures = {
  [K in FeatureID]: boolean
};

// Continue learning more about template literals in example:mapped-types-with-template-literals Or read the announcement blog post: https://devblogs.microsoft.com/typescript/announcing-typescript-4-1-beta/#template-literal-types