Default parameters

Learn what happens with missing arguments by setting default parameter values.

Default parameters

In JavaScript you can give a parameter a default value. If the caller does not pass an argument for that parameter, the default value is used. The syntax is the parameter name followed by = and a value.

Example: function greet(name = "Guest") { console.log("Hello " + name); }. Here the default value of name is "Guest". In the call greet() no argument is given, so name takes this value and the screen shows Hello Guest.

When no argument is given

If you do not give a parameter a default and the caller passes no argument, that parameter's value becomes undefined. This is often undesirable.

When you set a default, your chosen value is used instead of undefined. So in function greet(name = "Guest"), the call greet() does not make name undefined; it makes it "Guest". The function then works safely even with a missing argument.

Exercise Greeting function

Define a function called greet. Give the name parameter a default value of "Guest". In the body, print the greeting with console.log("Hello " + name). Then call it with no arguments as greet().

Show example solution

function greet(name = "Guest") {
  console.log("Hello " + name);
}

greet();

Test yourself

Which is a valid default parameter syntax?
```
function greet(name)
```
- function greet(name := "Guest")
- function greet(name = "Guest")
- function greet(name -> "Guest")
- function greet(name: "Guest")
Show answer

function greet(name = "Guest") — Correct. The = sign assigns the default value.

Add the default value

function greet(
  ____
) {
  console.log("Merhaba " + name);
}
greet();

Show answer

function greet(
  name = "Misafir"
) {
  console.log("Merhaba " + name);
}
greet();

Match the terms

name

Parameter
"Alex"

Argument
undefined

When no argument is given
"Guest"

Default value

Next lesson: The rest parameter

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