Optional and default params

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.

Exercise 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.

Show example solution

function selam(isim: string, unvan: string = "Bay"): string {
    return unvan + " " + isim;
}
console.log(selam("Ali"));

Test yourself

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
- The function ignores isim
- Compile error (missing parameter)
- unvan becomes null
Show answer

unvan is undefined — 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>.

Add the default value

function selam(isim: string, unvan = ____): string { return unvan + " " + isim; }

Show answer

function selam(isim: string, unvan = "Bay"): string { return unvan + " " + isim; }

Optional and default parameter concepts

unvan?: string

Optional parameter
unvan: string = "Bay"

Default parameter
? (parametre adından sonra)

Makes a parameter optional
= "değer"

Gives a parameter a default value

Next lesson: Rest params

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