Skip to Content
TypeScript Intermediate: Types That Survive Real Projects
Intermediate
0 likes
Courses
TypeScript Intermediate: Types That Survive Real Projects

Final Check: TypeScript Intermediate

Fifteen scenario questions across the whole course. If you get twelve or more right, you can use generics, utility types and third-party typings in a shared codebase.

15 min TypeScriptObject-Oriented ProgrammingAPIs and IntegrationsType Systems Intermediate
Prev Next
Fullscreen Share
Course content

Sign in to save your progress.

Course Home
1. A function is declared with a type parameter T, takes an array of T and returns T or undefined. You call it with an array of strings and then call toFixed on the result after checking it is not undefined. What happens?
The compiler rejects toFixed, because T was worked out as string. The compiler accepts it, because T is any unless you write it. It compiles but fails at run time. The compiler asks you to write the type argument by hand.
2. A cache is made by a function with a type parameter V that holds a Map from string to V. It has a get method that returns V or undefined. Why is undefined part of the return type?
To make the cache slower and safer. A lookup for a missing key gives back undefined, so the type has to say so and callers must check. Because V might be undefined by default. Because TypeScript adds undefined to every return type.
3. A generic type Box has a type parameter T with a default of string. You write const note: Box with no angle brackets. What type does it have?
It is an error, because the argument is required. Box of string, because the default is used when no type argument is given. Box of unknown. Box of any.
4. Which description of Pick of Product with the keys id and name is correct?
A type with every property of Product except id and name. A type with only the id and name properties of Product, and a key that does not exist on Product is an error. A type where id and name are optional and the rest are required. A type that removes id and name from the object at run time.
5. You write a mapped type with the key K in keyof T, then as a template string that starts with get and capitalizes the key name, and the value is a function that returns T indexed by K. What does it build for a type with a name property?
An object type with a get property only. An object type with a getName function that returns the type of name. An object type with the same keys as T. A type error, because keys cannot be renamed.
6. A library has no types of its own, and npm has no types package for it. You want the compiler to stop complaining quickly. What does a declare module line with only the library name do?
It types the library from its source code. It makes the library read-only. It makes everything from the library any. It silences the error but does not type the library. It downloads the types from the internet.
7. Where should an @types package go in package.json, and why?
It does not matter, because both are installed the same way. In devDependencies, because only the compiler reads the types and the running program never loads them. In dependencies, because the program imports them. In peerDependencies, so that users install them.
8. An update helper types its changes parameter as Partial of Product. A caller passes an empty object. What does the compiler do?
It rejects it, because at least one property is required. It rejects it, because the object has no id. It fills in the missing properties from Product. It accepts it, because every property of Partial is optional, so you must check at run time if at least one change is required.
9. Why is a cast like as Settings on the result of JSON.parse a problem, even if it compiles?
It slows the program down at run time. It changes the JSON before using it. It makes the compiler report errors in other files. It tells the compiler to trust you without checking anything, so wrong data keeps the right-looking type.
10. A function parses text into settings and returns a Result with an ok flag. On success it builds a fresh object from the checked fields. What is the benefit over returning the parsed object?
Extra fields in the input are dropped, and every field that remains was checked. The compiler stops checking the settings. The function becomes faster. JSON parsing no longer needs a try block.
11. Two classes hold a balance, one with the private keyword and one with a hash prefix. A script uses bracket notation on the word balance for both. What does it get?
Both give the value, because bracket notation ignores modifiers. Both give undefined, because brackets are not allowed on classes. A compile error for both. The private one gives the value, and the hash one gives undefined.
12. A types-only file is imported with the inline type keyword inside braces, and verbatimModuleSyntax is on. The imported module prints a message when it loads. What happens when the program runs?
Nothing prints, because the name is marked as a type. The message prints, because the output keeps an empty import statement that still loads the module. The message prints twice. The compiler refuses the line.
13. An object with a method property set to the word POST is declared as a plain constant and passed to a function that wants the literal union GET or POST. The compiler refuses. Why?
In the plain constant, method is widened to string, and not every string is GET or POST. A type annotation or as const fixes it. POST is not a valid HTTP method. The constant needs to be declared with var. The function should accept any string.
14. A function returns an object with an optional data property and an optional error property, and callers keep writing outcome.data with an exclamation mark. What is the best change?
Use a union of a success shape that has data and a failure shape that has error, with a tag such as success true or false. Make both properties required. Replace the object with any. Keep the shape and add a comment that data may be missing.
15. A barrel file contains only export type lines that re-export names from another module, and the other module prints a message when it loads. What happens when a program imports the barrel?
The other module loads anyway, because the barrel points at it. The compiler reports an error, because a barrel must export values. The compiled barrel is an empty module, so nothing behind it loads and no message prints. The message prints, but only the first time.

About

Fifteen scenario questions across the whole course. If you get twelve or more right, you can use generics, utility types and third-party typings in a shared codebase.

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

Comments (0)

Sign in to comment

No comments yet.