Optional and default params
- TypeScript course
- Beginner
- Lesson 43/89
- 4 min
Learn optional and default parameters.
Optional parameters
To make a parameter optional, place ? after its name. Optional parameters must come after required ones. When omitted, the value is undefined.
function selam(isim: string, unvan?: string): string {
return (unvan || "") + " " + isim;
}With ? a parameter becomes skippable.
Default parameters
To give a parameter a default value, write = value. If the argument is omitted or undefined, the default is used. Default parameters are automatically optional.
function selam(isim: string, unvan: string = "Bay"): string {
return unvan + " " + isim;
}
selam("Ali"); // "Bay Ali"A default value makes the parameter skippable.
Practice Function with a default parameter
Define a function named selam with isim: string and unvan: string = "Bay" parameters and return type string. The function should return unvan + " " + isim. Print the result of calling selam("Ali") with console.log.
Starter code
// selam adında bir fonksiyon tanımla
// isim zorunlu dize, unvan varsayılan "Bay" ile dize olsun
// dönüş türü string, unvan + " " + isim döndürsün
// selam("Ali") sonucunu yazdırSample solution
function selam(isim: string, unvan: string = "Bay"): string {
return unvan + " " + isim;
}
console.log(selam("Ali"));Expected output
Bay Ali
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 In <code>function selam(isim: string, unvan?: string)</code>, if you call <code>selam("Ali")</code>, what is the value of <code>unvan</code>?
- unvan is undefined Correct answer
- The function ignores isim
- Compile error (missing parameter)
- unvan becomes null
Explanation: When an optional parameter is omitted, its value is <code>undefined</code>; it does not cause a compile error because it is marked optional with <code>?</code>.
2 Add the default value
function selam(isim: string, unvan = ____): string { return unvan + " " + isim; }function selam(isim: string, unvan = "Bay"): string { return unvan + " " + isim; }Explanation: The default title must be <code>"Bay"</code>; <code>"Bayan"</code> and <code>"Hanim"</code> are wrong titles, and <code>"5"</code> is a wrong numeric content value.
3 Optional and default parameter concepts
| Term | Meaning |
|---|---|
unvan?: string |
Optional parameter |
unvan: string = "Bay" |
Default parameter |
? (parametre adından sonra) |
Makes a parameter optional |
= "değer" |
Gives a parameter a default value |