Skip to Content
TypeScript Advanced: Types That Hold Up in Production
Advanced
0 likes
Courses
TypeScript Advanced: Types That Hold Up in Production

Final Check: TypeScript Advanced

Fifteen scenario questions across the whole course. If you get twelve or more right, you can design types, declarations and a migration path for a codebase that other people also work in.

15 min TypeScriptTesting and DebuggingPerformance OptimizationType SystemsPackaging and Dependencies Advanced
Prev Next
Fullscreen Share
Course content

Sign in to save your progress.

Course Home
1. You see IsString of boolean or string give "no" or "yes", while a version with both sides of extends wrapped in square brackets gives "no". What explains the difference?
A bare type parameter distributes over a union, and the brackets switch distribution off so the union is checked as a whole. The brackets make the compiler read the type at run time. The brackets turn the check into a type assertion. The first form is an error, and the second is the fixed version.
2. A helper returns the value a function resolves to, whether the function returns a promise or not. What is the readable way to write it?
A function that returns any. A constraint on a function type plus Awaited of ReturnType, rather than three levels of inline infer. A conditional type with an infer at every level, written on one line. A string literal type that names the result.
3. Why is ElementOf of number, defined with infer and a never branch, a quiet problem?
It gives any, which switches off checking. The result is never, which is assignable to everything, so the mistake travels without an error. Constrain the parameter to an array instead. It is rejected by the compiler as a syntax error. It throws an error at run time.
4. What happens when you add the string "archived" to the Action union in a mapped type that renames keys with as and a template literal type?
The handler for archived is created automatically with an empty body. The derived handlers object stops compiling until a matching handler key, such as onArchived, is added. Nothing changes, because the template type is evaluated once. The compiler removes the new member.
5. Two ids are branded string types, UserId and OrderId. Which call does the compiler reject?
Only the call that passes two ids of the same kind. Passing an OrderId where a UserId is expected, and passing a plain string where either is expected. Only the call that passes a plain string. None of them, because both are strings at run time.
6. Why does "anything" as UserId weaken a branded type?
The compiler turns the string into a number. A brand cannot be applied to a string literal. The cast compiles without checking the text, so the protection only holds if casts live in the parse functions and nowhere else. The cast throws an error at run time.
7. A config object is written with as const and then checked with satisfies Record of string to a path pattern. What do you get?
The check happens again when the program starts. The paths stay literal types, so a union of the real paths can be derived, and a path that does not start with a slash is rejected. Every path becomes a plain string and the union cannot be derived. The object becomes mutable at run time.
8. A teammate writes a .d.ts and wants to be sure it matches the JavaScript. What is the practical answer?
The compiler compares them every build and reports differences. Put any in every signature so no difference can appear. Generate the declaration once and never edit it. The compiler does not compare them. Add a small test file with calls that should compile and calls that should fail, and run tsc in CI.
9. A declaration file adds a global property with declare global, and the compiler refuses it. What do you add?
The word window before the property. A triple slash comment. A second tsconfig for the global file. An export or import, for example export {}, so the file is a module.
10. You augment the Express request with an optional user property, using declare module. Which module name goes in the block, and what must the file have?
express-serve-static-core, where Request is declared, and an import or export at the top so the file is a module. Any name, because the compiler finds the right one by shape. express, with no import at the top. node:http, with an export at the top.
11. The app project references shared with composite on, and the root tsconfig has files set to an empty list and lists both projects. You change a property name in shared and run tsc -b. What do you expect?
The app still compiles, because it reads an old copy. Only shared is built, and the app is never checked. The root tsconfig reports the error, because it owns the files. shared is rebuilt, and the app fails to compile at the place that still uses the old name.
12. On a codebase where type-aware linting finds hundreds of problems, what is the adoption plan that works?
Turn on every rule as an error and fix everything in one pull request. Start with the rules as warnings, count them by rule, set a ceiling with max-warnings, then fix and promote one rule at a time. Switch off type information so that fewer rules fire. Run eslint with the fix option over the whole repository.
13. Under allowJs without checkJs, a TypeScript file imports a function from a JavaScript file with a JSDoc comment of number for a parameter. How does the compiler treat the JavaScript file?
It is read and part of the program, but not checked. The JSDoc types become checked types once checkJs is on. It is ignored completely. It is converted to TypeScript automatically. It is checked with the strictest settings.
14. Types in a project jump from about 350 to more than 11,000 after adding one short file. Which tool shows where the cost comes from?
The generateTrace flag, which writes a trace and a list of types you can search for the one with thousands of members. The skipLibCheck flag, which removes the cost. The incremental flag, which hides the cost after the first run. The noEmit flag, which makes the type smaller.
15. A tsconfig has no include and the project reads 302 of your own files, although only one is application code. What do you do first?
Turn on skipLibCheck and leave the rest as it is. Turn off strict mode. Add include for the real source folders, and give scripts or fixtures their own tsconfig if they need checking. Buy a faster build machine.

About

Fifteen scenario questions across the whole course. If you get twelve or more right, you can design types, declarations and a migration path for a codebase that other people also work in.

Duration
15 min
Last update
10/02/2026
Was this helpful?
​ 0 ​ 0

Comments (0)

Sign in to comment

No comments yet.