Constructor parameter types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of specifying parameter types in a constructor in TypeScript?

Specifying parameter types in a constructor helps ensure that the values passed when creating an object are of the expected types, preventing errors and improving code clarity.

beginner

How do you define a constructor with typed parameters in a TypeScript class?

You declare the constructor with parameters followed by a colon and the type, for example:

constructor(name: string, age: number) { ... }

intermediate

Can constructor parameters in TypeScript be optional? How?

Yes, by adding a question mark after the parameter name, e.g.,

constructor(name?: string) { ... }

, making the parameter optional.

beginner

What happens if you pass a wrong type to a constructor parameter in TypeScript?

TypeScript will show a compile-time error, preventing the code from running until the type mismatch is fixed.

intermediate

How can you automatically create and assign class properties from constructor parameters in TypeScript?

By adding access modifiers (public, private, protected, readonly) before the constructor parameters, TypeScript creates and assigns properties automatically, e.g.,

constructor(public name: string) { }

How do you specify that a constructor parameter 'age' must be a number in TypeScript?

A constructor(age: number) {}

B constructor(age) {}

C constructor(age: string) {}

D constructor(age = number) {}

Option A correctly declares 'age' as a number type parameter.

What does adding a question mark after a constructor parameter name do?

A Throws a syntax error

B Makes the parameter required

C Makes the parameter optional

D Changes the parameter type to boolean

A question mark after a parameter name makes it optional.

Which of these will automatically create a class property from a constructor parameter?

A constructor(name: string) { this.name = name; }

B constructor(public name: string) {}

C constructor(name) {}

D constructor(private) {}

Using 'public' before the parameter automatically creates and assigns the property.

What error will TypeScript show if you pass a string to a constructor expecting a number?

A Type error: string is not assignable to number

B Syntax error

C Runtime error

D No error

TypeScript shows a type error at compile time for type mismatches.

Which syntax correctly defines a constructor with an optional string parameter 'title'?

A constructor(optional title: string) {}

B constructor(title: string?) {}

C constructor(title = string) {}

D constructor(title?: string) {}

Option D uses the correct syntax for an optional parameter.

Explain how constructor parameter types improve code safety and readability in TypeScript.

Think about what happens when you pass wrong types to constructors.

Describe how to use access modifiers in constructor parameters to create class properties automatically.

Focus on the shortcut TypeScript offers inside constructors.