Exploring the TypeScript Type System: Challenges with Arrays Generated and Constant Assertions

I am currently grappling with a core comprehension issue regarding TypeScript, which is highlighted in the code snippet below. I am seeking clarification on why a generated array does not function as expected and if there is a potential solution to this problem.

typescript

const mySlugs = ['post1', 'post2'] as const // working
const mySlugs = (allPosts.map(post => post._raw.sourceFileName)) as const  // not working
export type SlugType = (typeof mySlugs)[number]

The first line (const mySlugs = ['post1', 'post2'] as const;) executes correctly, however, the second line (const mySlugs = (allPosts.map(post => post._raw.sourceFileName)) as const;) does not (A 'const' assertions can only be applied to references to enum members, or string, number, boolean, array, or object literals.ts(1355)).

I am questioning if there might be a misunderstanding on my part regarding TypeScript's type system or const assertions. Any insights or recommendations would be highly valued!

arraystypescripttype-inference

Answer №1

The assertion as const is limited to simple literals, excluding general expressions. The documentation for its introduction in a specific release clarifies this restriction:

Important Note

It should be noted that const assertions are exclusively applicable to straightforward literal expressions.

This limitation is also reflected in the error message:

A 'const' assertion can solely be utilized with enums, or string, number, boolean, array, or object literals. (1355)

If you aim to enforce read-only behavior on an array generated programmatically, consider employing the ReadOnlyArray utility type:

const mySlugs = allPosts.map(post => post._raw.sourceFileName) as ReadonlyArray<string>;

Nonetheless, this approach lacks some functionalities of as const when applied to literals, resulting in an array of strings rather than a tuple containing string literal types.

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