JavaScript Optional Parameters: What You Must Know

In JavaScript, a function parameter is defined as optional if it is not necessary to provide a value for it when the function is called. This feature can be quite useful when you want to make your function more flexible and easier to use.

1function greet(name = "User") {
2  console.log("Hello, " + name);
3}
4greet(); // Outputs: Hello, User

The name parameter in the greet() function is optional. If no argument is passed when the function is called, it will default to "User". This is known as specifying an optional parameter in JavaScript.

Using undefined to Skip Optional Parameters

It's possible to skip optional parameters in JavaScript by passing undefined in their place. This way, you can provide values for later parameters without specifying earlier optional ones.

1function displayInfo(name = "John", age = 25, country = "USA") {
2  console.log(name + " is " + age + " years old and lives in " + country);
3}
4displayInfo(undefined, 30); // Outputs: John is 30 years old and lives in USA

In this example, we skipped the name parameter by passing undefined, and specified a new value for the age parameter.

Making Parameters Mandatory

JavaScript doesn't have a built-in way to make parameters mandatory. However, you can achieve this by throwing an error if no argument is provided.

1function mandatoryParam() {
2  throw new Error('Missing parameter');
3}
4function add(a = mandatoryParam(), b = mandatoryParam()) {
5  return a + b;
6}
7add(1, 2); // returns 3
8add(1); // throws an error

In this example, if either a or b is not provided when add() is called, the mandatoryParam() function will be invoked, throwing an error.

Further Learning

To learn more about JavaScript functions and parameters, check out our JavaScript course. If you're new to coding, our Introduction to Web Development course is a great place to start.

For additional resources, Mozilla's JavaScript Guide is an excellent reference. You might also find JavaScript.info helpful for understanding more complex JavaScript concepts.