Optional parameters in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an optional parameter in TypeScript?

An optional parameter is a function parameter that may or may not be provided when the function is called. It is marked with a question mark (?) after the parameter name.

beginner

How do you declare an optional parameter in a TypeScript function?

You add a question mark (?) after the parameter name in the function definition, like <code>function greet(name?: string)</code>.

beginner

What happens if you call a function without providing an optional parameter?

The optional parameter is undefined inside the function, so you can check if it was provided or not.

intermediate

Can optional parameters be followed by required parameters in TypeScript?

No, optional parameters must come after all required parameters in the function signature.

beginner

Example: What is the output of this code?<br><pre>function sayHi(name?: string) {<br>  console.log(`Hi, ${name ?? 'friend'}!`);<br>}<br>sayHi();<br>sayHi('Anna');</pre>

The output is:<br>

Hi, friend!<br>Hi, Anna!

<br>Because when no name is given, it uses 'friend' as a default message.

How do you mark a parameter as optional in TypeScript?

A Use the keyword 'optional' before the parameter

B Add an exclamation mark (!) after the parameter name

C Use the keyword 'maybe' before the parameter

D Add a question mark (?) after the parameter name

Optional parameters are marked with a question mark (?) after the parameter name.

What is the type of an optional parameter if it is not provided when calling the function?

A undefined

B null

C empty string

D zero

If an optional parameter is not provided, its value is undefined inside the function.

Which of these function signatures is valid in TypeScript?

A function f(a?: number, b: string)

B function f(a: number, b?: string)

C function f(a?: number, b?: string, c: boolean)

D function f(a: number, b: string, c?: boolean, d: number)

Optional parameters must come after all required parameters.

What will this code print?<br>

function greet(name?: string) {<br>  console.log(name || 'Guest');<br>}<br>greet();

A undefined

B Error

C Guest

D null

Since name is undefined, the || operator returns 'Guest'.

Can you have multiple optional parameters in a TypeScript function?

A Yes, as long as they are at the end of the parameter list

B No, only one optional parameter is allowed

C Yes, but they must be separated by required parameters

D No, optional parameters are not supported

You can have multiple optional parameters, but they must come after all required parameters.

Explain how to use optional parameters in TypeScript functions and why they are useful.

Think about how to make some inputs to a function not mandatory.

Describe the rules about the order of optional and required parameters in TypeScript function definitions.

Consider what happens if you put an optional parameter before a required one.