18 Interfaces & Types questions from the TypeScript bank, written for Indian campus drives and tech interviews. Every question has a verified answer and an AI-tutor explanation on placd.
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A.interfaces merge across declarations and are object-focused; type aliases don't merge but can express unions and primitives
B.a named contract describing the shape of an object, open to declaration merging
C.a property that can be set when the object is created but not reassigned afterward
D.a property marked with ? that may be absent from the object
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2. Which term means: "a named contract describing the shape of an object, open to declaration merging"?
Junior
A.optional property
B.interface
C.type alias
D.readonly property
Answer + AI explanation with Pro
3. Which statement is correct?
Junior
A.interface — a property that can be set when the object is created but not reassigned afterward
B.interface — a named contract describing the shape of an object, open to declaration merging
C.interface — a [key: string]: T declaration describing objects with arbitrary keys mapping to a value type
D.interface — interfaces merge across declarations and are object-focused; type aliases don't merge but can express unions and primitives
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4. What is type alias?
Junior
A.a [key: string]: T declaration describing objects with arbitrary keys mapping to a value type
B.a named contract describing the shape of an object, open to declaration merging
C.a property that can be set when the object is created but not reassigned afterward
D.a name for any type — unions, primitives, tuples — that cannot be reopened or merged
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5. Which term means: "a name for any type — unions, primitives, tuples — that cannot be reopened or merged"?
Junior
A.type alias
B.readonly property
C.interface vs type alias
D.optional property
Answer + AI explanation with Pro
6. Which statement is correct?
Junior
A.type alias — a property marked with ? that may be absent from the object
B.type alias — a [key: string]: T declaration describing objects with arbitrary keys mapping to a value type
C.type alias — a named contract describing the shape of an object, open to declaration merging
D.type alias — a name for any type — unions, primitives, tuples — that cannot be reopened or merged
Answer + AI explanation with Pro
7. What is interface vs type alias?
Mid
A.a name for any type — unions, primitives, tuples — that cannot be reopened or merged
B.a named contract describing the shape of an object, open to declaration merging
C.a [key: string]: T declaration describing objects with arbitrary keys mapping to a value type
D.interfaces merge across declarations and are object-focused; type aliases don't merge but can express unions and primitives
Answer + AI explanation with Pro
8. Which term means: "interfaces merge across declarations and are object-focused; type aliases don't merge but can express unions and primitives"?
Mid
A.optional property
B.index signature
C.readonly property
D.interface vs type alias
Answer + AI explanation with Pro
9. Which statement is correct?
Mid
A.interface vs type alias — a property marked with ? that may be absent from the object
B.interface vs type alias — a named contract describing the shape of an object, open to declaration merging
C.interface vs type alias — interfaces merge across declarations and are object-focused; type aliases don't merge but can express unions and primitives
D.interface vs type alias — a property that can be set when the object is created but not reassigned afterward
Answer + AI explanation with Pro
10. What is optional property?
Junior
A.a property that can be set when the object is created but not reassigned afterward
B.a [key: string]: T declaration describing objects with arbitrary keys mapping to a value type
C.a named contract describing the shape of an object, open to declaration merging
D.a property marked with ? that may be absent from the object
Answer + AI explanation with Pro
11. Which term means: "a property marked with ? that may be absent from the object"?
Junior
A.optional property
B.interface vs type alias
C.readonly property
D.interface
Answer + AI explanation with Pro
12. Which statement is correct?
Junior
A.optional property — a property marked with ? that may be absent from the object
B.optional property — interfaces merge across declarations and are object-focused; type aliases don't merge but can express unions and primitives
C.optional property — a property that can be set when the object is created but not reassigned afterward
D.optional property — a named contract describing the shape of an object, open to declaration merging
Answer + AI explanation with Pro
13. What is readonly property?
Junior
A.a property that can be set when the object is created but not reassigned afterward
B.a named contract describing the shape of an object, open to declaration merging
C.interfaces merge across declarations and are object-focused; type aliases don't merge but can express unions and primitives
D.a property marked with ? that may be absent from the object
Answer + AI explanation with Pro
14. Which term means: "a property that can be set when the object is created but not reassigned afterward"?
Junior
A.readonly property
B.interface vs type alias
C.interface
D.optional property
Answer + AI explanation with Pro
15. Which statement is correct?
Junior
A.readonly property — interfaces merge across declarations and are object-focused; type aliases don't merge but can express unions and primitives
B.readonly property — a property that can be set when the object is created but not reassigned afterward
C.readonly property — a [key: string]: T declaration describing objects with arbitrary keys mapping to a value type
D.readonly property — a name for any type — unions, primitives, tuples — that cannot be reopened or merged
Answer + AI explanation with Pro
16. What is index signature?
Mid
A.a [key: string]: T declaration describing objects with arbitrary keys mapping to a value type
B.a name for any type — unions, primitives, tuples — that cannot be reopened or merged
C.interfaces merge across declarations and are object-focused; type aliases don't merge but can express unions and primitives
D.a property marked with ? that may be absent from the object
Answer + AI explanation with Pro
17. Which term means: "a [key: string]: T declaration describing objects with arbitrary keys mapping to a value type"?
Mid
A.optional property
B.index signature
C.interface vs type alias
D.type alias
Answer + AI explanation with Pro
18. Which statement is correct?
Mid
A.index signature — a property marked with ? that may be absent from the object
B.index signature — a [key: string]: T declaration describing objects with arbitrary keys mapping to a value type
C.index signature — a name for any type — unions, primitives, tuples — that cannot be reopened or merged
D.index signature — interfaces merge across declarations and are object-focused; type aliases don't merge but can express unions and primitives
Answer + AI explanation with Pro
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