Excess property checking behavior in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is excess property checking in TypeScript?

Excess property checking is a TypeScript feature that warns you when you provide extra properties in an object literal that are not expected by the target type.

intermediate

Why does TypeScript perform excess property checking only on object literals?

TypeScript checks object literals for extra properties to catch typos or mistakes early. It does not check variables or objects assigned from elsewhere to allow flexibility.

intermediate

How can you avoid excess property checking errors when passing objects with extra properties?

You can assign the object to a variable first, or use type assertions to tell TypeScript to trust your object shape and skip excess property checks.

beginner

What happens if you pass an object literal with an extra property to a function expecting a specific type?

TypeScript will give an error about the extra property because it does excess property checking on object literals to prevent mistakes.

advanced

Can excess property checking be bypassed by using index signatures in TypeScript interfaces?

Yes, if an interface has an index signature like [key: string]: any, excess property checking is relaxed because extra properties are allowed.

When does TypeScript perform excess property checking?

A On all objects, including variables and function returns

B Only on arrays

C Only on object literals assigned directly to variables or parameters

D Only on primitive types

TypeScript performs excess property checking only on object literals directly assigned to variables or passed as parameters.

What error will TypeScript give if you pass an object literal with an unexpected property to a function expecting a specific type?

A Excess property error

B Syntax error

C Runtime error

D No error

TypeScript will report an excess property error to warn about unexpected properties in object literals.

How can you avoid excess property checking errors when you want to pass extra properties?

A Use a number instead of an object

B Use a different programming language

C Remove all properties

D Assign the object to a variable before passing it

Assigning the object to a variable first disables excess property checking on that variable.

What effect does an index signature have on excess property checking?

A It disables excess property checking for extra properties

B It causes more errors

C It only allows numeric keys

D It has no effect

An index signature allows extra properties, so excess property checking is relaxed.

Which of these is a common reason for excess property checking errors?

A Passing a number instead of an object

B A typo in a property name in an object literal

C Using a variable instead of an object literal

D Using a function instead of an object

Typos in property names cause extra properties that TypeScript flags during excess property checking.

Explain what excess property checking is and why TypeScript uses it.

Think about how TypeScript helps catch typos in object properties.

Describe two ways to avoid excess property checking errors when passing objects with extra properties.

Consider how you can tell TypeScript to trust your object shape.