Duck typing mental model in TypeScript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is duck typing in TypeScript?

Duck typing means that if an object has all the required properties and methods, it is considered to be of that type, regardless of its actual class or inheritance.

beginner

How does TypeScript check types with duck typing?

TypeScript checks if an object has the necessary shape (properties and methods) to match a type, not if it explicitly declares that type.

beginner

Example: If an object has a 'name' string property and a 'speak()' method, can it be used where a type with those members is expected?

Yes, because the object matches the shape required by the type, TypeScript allows it even if the object does not explicitly implement that type.

intermediate

Why is duck typing useful in TypeScript?

It allows flexible and easy code reuse by focusing on what an object can do rather than its exact type or class.

intermediate

Can duck typing cause errors if an object has extra properties?

No, TypeScript allows extra properties as long as the required ones exist, but excess property checks apply in some cases like object literals.

In TypeScript, what does duck typing check for?

A If an object is explicitly declared with a type

B If an object inherits from a specific class

C If an object has the required properties and methods

D If an object has no extra properties

Duck typing checks if the object has the needed shape, ignoring inheritance or explicit declarations.

Which of these is true about duck typing in TypeScript?

A An object must explicitly implement an interface to be valid

B TypeScript does not allow extra properties in objects

C Duck typing requires inheritance

D An object with matching properties can be used even without explicit implementation

TypeScript allows objects with matching properties to be used without explicit interface implementation.

What happens if an object literal has extra properties not in the expected type?

A TypeScript always allows it

B TypeScript gives an error due to excess property checks

C TypeScript ignores extra properties silently

D The object is automatically cast to the expected type

Excess property checks cause errors when object literals have extra properties not in the target type.

Why is duck typing called that way?

A Because if it looks like a duck and quacks like a duck, it is treated as a duck

B Because it uses ducks in examples

C Because it requires inheritance from a Duck class

D Because it only works with animals

The name comes from the saying: if it looks like a duck and quacks like a duck, it is treated as a duck.

Which TypeScript feature relies on duck typing?

A Structural typing

B Nominal typing

C Class inheritance

D Type casting

TypeScript uses structural typing, which is a form of duck typing based on object shape.

Explain the duck typing mental model in TypeScript and how it affects type checking.

Think about how TypeScript decides if an object fits a type.

Describe a situation where duck typing in TypeScript can cause an error and why.

Consider when you write an object literal with more properties than expected.