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TypeScript Intermediate: Types That Survive Real Projects
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TypeScript Intermediate: Types That Survive Real Projects

Check Yourself: Types That Transform Other Types

Five scenario questions on utility types and mapped types. If you miss more than one, reread the lessons in this section.

5 min TypeScriptObject-Oriented ProgrammingAPIs and IntegrationsType Systems Intermediate
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1. A function update takes a product and changes typed as Partial of Product, and returns a new product built with the spread of both. A caller passes the object price undefined. What happens?
It is a run-time error, because a spread cannot hold undefined. It compiles, and the price in the result is undefined at run time, even though the type says number. It compiles, and the old price is kept, because undefined is ignored by the spread. It is a compile error, because undefined is not a number.
2. You derive a public type with Omit of User and the key passwordhash, written with a lowercase h. The real property is passwordHash. What does the compiler say?
Nothing, and passwordHash is removed because the comparison ignores case. That passwordhash is not a property of User. That the type is empty, because every key was removed. Nothing. Omit does not check that the key exists, so nothing is removed and passwordHash is still allowed.
3. type Warehouse is the union of the words north and south. You write const stock as Record of Warehouse and number, with an object that has only north. What does the compiler do?
It accepts the object and adds south as undefined. It reports an error on north, because the value should be a string. It accepts the object, because Record makes keys optional. It reports that south is missing, because Record requires every member of the union as a key.
4. You see the line type Same of T as an object with the key K in keyof T and a value of T indexed by K, with a question mark after the bracket. Which utility type does it match?
Partial, because it keeps every key and its type and makes each property optional. Readonly, because it keeps every key and its type. Pick, because it selects keys from T. Required, because it keeps every key and its type.
5. Validators of T is a mapped type that gives each key of T a function taking T indexed by that key and returning a boolean. You add a phone field to the Signup type but forget its validator. What happens?
The compiler adds a default validator that always returns true. A run-time error, the first time phone is validated. The compiler reports that phone is missing from the validators object, because the mapped type wants one function per key. Nothing, because the validators object can have fewer keys than the type.

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Five scenario questions on utility types and mapped types. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/11/2026
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