How assignment compatibility is checked in Typescript - Quick Revision & Summary

Recall & Review

beginner

What does assignment compatibility mean in TypeScript?

It means you can assign a value of one type to a variable of another type if TypeScript considers the types compatible based on their structure.

intermediate

How does TypeScript check assignment compatibility between two object types?

TypeScript checks if the source object has at least the properties and types required by the target object (structural typing). Extra properties in the source are allowed.

beginner

What happens if you try to assign a value with missing required properties in TypeScript?

TypeScript will give an error because the source type does not have all the required properties of the target type, so they are not assignment compatible.

intermediate

Can TypeScript assign a function type to another function type with fewer parameters?

Yes, TypeScript allows assigning a function with fewer parameters to one expecting more, as long as the parameters it does have are compatible (called function parameter bivariance).

advanced

Does TypeScript consider private or protected members when checking assignment compatibility?

Yes, private and protected members must come from the same declaration for assignment compatibility; otherwise, the types are incompatible.

In TypeScript, when is an object type assignment allowed?

A When the target has more properties than the source

B Only when both objects have exactly the same properties

C When the source has fewer properties than the target

D When the source has all properties required by the target, plus possibly more

TypeScript uses structural typing, so the source must have at least the properties the target expects.

What happens if you assign a function with more parameters to a function type expecting fewer parameters?

A It's allowed because extra parameters are ignored

B It's not allowed because of parameter mismatch

C It's allowed only if parameters have the same names

D It's allowed only if return types differ

Assigning a function with more parameters to one expecting fewer is not allowed in TypeScript.

Are private members considered when checking assignment compatibility?

A Yes, they must come from the same declaration

B No, private members are ignored

C Only if the classes are unrelated

D Only if the members are static

Private and protected members must originate from the same declaration for compatibility.

Which typing system does TypeScript use to check assignment compatibility?

A Structural typing

B Nominal typing

C Duck typing

D Dynamic typing

TypeScript uses structural typing, meaning compatibility depends on the shape of types.

Can you assign a value of type 'number' to a variable of type 'string' in TypeScript?

A Yes, if you use type assertion

B Yes, always

C No, because number and string are incompatible

D Only if the variable is declared with 'any'

Number and string are incompatible types; direct assignment causes an error.

Explain how TypeScript checks assignment compatibility between two object types.

Think about the shape of the objects and member visibility.

Describe how function types are checked for assignment compatibility in TypeScript.

Consider how many parameters the functions have and their types.