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TypeScript MCQ questions

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TypeScript tutorial Interview questions

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TypeScript practice testMultiple choice Question 1 of 10

What does the override keyword add?

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All questions with answers

What does the TypeScript compiler output?
- AMachine code
- BJavaScript
- CWebAssembly
- DBytecode for a TS VM
Show answer
Answer: B. JavaScript
TypeScript compiles (transpiles) to plain JavaScript with the types removed.
Which type accepts any value but forces you to check it before use?
- Aany
- Bunknown
- Cnever
- Dobject
Show answer
Answer: B. unknown
unknown is the type-safe counterpart of any.
What is the type of a function that always throws?
- Avoid
- Bundefined
- Cnever
- Dnull
Show answer
Answer: C. never
never means the function never returns normally.
How do you mark a property as optional?
- Aname!: string
- Bname?: string
- Coptional name: string
- Dname: string?
Show answer
Answer: B. name?: string
The ? after the name makes it optional (type string | undefined).
Which describes "a string or a number"?
- Astring & number
- Bstring | number
- Cstring, number
- D[string, number]
Show answer
Answer: B. string | number
| is a union. & is an intersection; [a, b] is a tuple.
What is the type of x?
TypeScript Illustrative
```
1const x = 'hello';
```
- Astring
- B"hello"
- Cany
- DString
Show answer
Answer: B. "hello"
const with a primitive literal infers the literal type.
Which utility type makes every property optional?
- ARequired<T>
- BReadonly<T>
- CPartial<T>
- DPick<T, K>
Show answer
Answer: C. Partial<T>
Partial<T> maps every property to optional.
Which utility type creates a type with only some keys?
- AOmit<T, K>
- BPick<T, K>
- CRecord<K, T>
- DExtract<T, U>
Show answer
Answer: B. Pick<T, K>
Pick keeps the listed keys; Omit removes them.
What does keyof { id: number; name: string } produce?
- A"id" | "name"
- Bnumber | string
- C["id", "name"]
- Dstring
Show answer
Answer: A. "id" | "name"
keyof gives a union of the property names.
What error does this produce?
TypeScript Illustrative
```
1function double(n: number) { return n * 2; }
2double('4');
```
- ANone
- BType "string" is not assignable to type "number"
- CMissing return type
- DCannot redeclare variable
Show answer
Answer: B. Type "string" is not assignable to type "number"
The parameter is typed number, so passing a string is a compile error.
Which keyword narrows a union of class instances?
- Atypeof
- Binstanceof
- Ckeyof
- Das
Show answer
Answer: B. instanceof
instanceof checks the prototype chain; typeof works for primitives.
Which syntax declares a generic function?
- Afunction id(T)(x: T)
- Bfunction id<T>(x: T): T
- Cfunction id[T](x: T)
- Dgeneric function id(x)
Show answer
Answer: B. function id<T>(x: T): T
Type parameters go in angle brackets after the name.
What does "strict": true enable?
- AOnly noImplicitAny
- BA family of strict checks including strictNullChecks
- CES strict mode only
- DFaster compilation
Show answer
Answer: B. A family of strict checks including strictNullChecks
It turns on noImplicitAny, strictNullChecks, strictFunctionTypes and more.
With strictNullChecks, can null be assigned to a string variable?
- AYes
- BNo, unless the type includes null
- COnly in classes
- DOnly with let
Show answer
Answer: B. No, unless the type includes null
null and undefined must be listed explicitly: string | null.
Which is true about interfaces?
- AThey exist at runtime
- BThey can be merged by declaring them twice
- CThey cannot extend other interfaces
- DThey can describe unions
Show answer
Answer: B. They can be merged by declaring them twice
Declaration merging combines interfaces with the same name; type aliases cannot merge.
What does a readonly property prevent?
- AReading it
- BReassigning it after initialisation
- CSerialising it
- DInheriting it
Show answer
Answer: B. Reassigning it after initialisation
readonly is a compile-time check against reassignment.
Which file configures the TypeScript compiler?
- Apackage.json
- Btsconfig.json
- Cts.config.js
- D.tsrc
Show answer
Answer: B. tsconfig.json
tsconfig.json holds compilerOptions, include and exclude.
What does Record<string, number> describe?
- AAn array of numbers
- BAn object with string keys and number values
- CA tuple
- DA Map
Show answer
Answer: B. An object with string keys and number values
Record<K, V> builds an object type from key and value types.
What is the return type of an async function that returns a number?
- Anumber
- BPromise<number>
- CAsync<number>
- Dvoid
Show answer
Answer: B. Promise<number>
Async functions always return a promise.
Which enum alternative avoids generated JavaScript?
- AA class with static fields
- BA union of string literals
- CA namespace
- DA WeakMap
Show answer
Answer: B. A union of string literals
Literal unions are type-only; enums emit an object at runtime (except const enums).
What does <T extends { id: number }> mean?
- AT must be exactly { id: number }
- BT can be any type that has a numeric id
- CT inherits from a class called id
- DT is optional
Show answer
Answer: B. T can be any type that has a numeric id
extends sets a minimum shape; T may have more properties.
From the lesson: Generics
What happens to TypeScript types when the code is compiled?
- AThey are checked at runtime
- BThey are removed, leaving plain JavaScript
- CThey become comments
- DThey are sent to the browser
Show answer
Answer: B. They are removed, leaving plain JavaScript
Types exist only at compile time.
From the lesson: Introduction to TypeScript
Every valid JavaScript program is also valid TypeScript syntax.
- ATrue
- BFalse
Show answer
Answer: A. True
TypeScript is a superset of JavaScript (though type checking may report errors).
From the lesson: Introduction to TypeScript
Who created TypeScript?
- AGoogle
- BMeta
- CMicrosoft
- DMozilla
Show answer
Answer: C. Microsoft
Microsoft released TypeScript in 2012.
From the lesson: Introduction to TypeScript
When does JavaScript usually reveal a type mistake?
- AWhile typing
- BAt compile time
- CWhen the faulty line runs
- DNever
Show answer
Answer: C. When the faulty line runs
JavaScript is dynamically typed.
From the lesson: JavaScript vs TypeScript