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Tori Crawford
Tori Crawford

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Breaking Down ES6: Default Parameters

Another week, another installment of my Breaking Down ES6 blog series! This week we will be discussing ES6’s introduction of default parameters, which in my opinion are incredibly useful. When I learned about default parameters during my bootcamp, my first thought was “why didn’t they teach us this sooner!?!” Default parameters are such a simple, yet very powerful feature.

I am actually pretty excited to write this blog post for y’all, so let’s go ahead and get started!

Before ES6

One really important thing to note about function parameters in JavaScript is that they default to undefined. In the example to follow, you will notice that when we do not pass a value for num2 our function addition() will return NaN. The reason for this is because undefined is simply just not a number and cannot be added to the value of num1.

Working Around Undefined Parameters

Before ES6 and default parameters were introduced, developers had ways to go about working around an undefined parameter. They did this by testing the parameters within the function. There were two popular ways to accomplish this.

The example above utilizes a conditional statement to check if the typeof the parameter is strictly not equal to undefined. If this is the case, the parameter will be set to the value passed to it, which in the first case is 7. Now, if it is undefined, like in the second case, the value of num2 will be set to 1.

Another way developers used to test the parameters within the function was by making use of the truthy/falsy pattern. The example below shows both cases, with and without a value being passed into num2.

Introduction of Default Parameters

Fast forward to the introduction of default parameters and we have much cleaner and easier to read code! The example provided below gives you an idea of a simple use case. Default parameters are really easy to use, all you have to do is set the parameter equal to whatever value you’d like to be the default value.

The output of the second case is 5 because num1 takes the value of 4 and num2 has the value of 1, thanks to our handy default parameter.

Omitted Values

You may be thinking at this point .. “What happens if you want to make the first parameter a default parameter? What do you do then?” Well, that is what we will be discussing in this section and it is new knowledge for me too.

When we come across a case where we would like to create a default parameter as our first parameter, or even one of the middle parameters, we need to make use of the keyword undefined. When passing in arguments to our function, undefined should be used as a placeholder.

The example above shows what it would look like to use default parameters in the first parameter slot. The example below shows what it would look like to use a default parameter as one of the middle parameters.

Final Thoughts

In this post we’ve discovered the magic of default parameters. We have learned that by using default parameters our code looks cleaner and easier for other developers to understand. Before researching the topic, I thought that you could only use the default parameter as the last parameter. Now we know that as developers we can use undefined as a placeholder in our arguments being passed to our function to accomplish this.

I hope that this post has been informative. I know it isn’t a super complex topic, but I learned something new today so I think it was worth writing about!

Happy coding.

Note: This week's featured image is from my recent trip to Ireland in March. Location: Ballycotton, Ireland.


Default parameters
Using Default Parameters in ES6
Clean Code with ES6 Default Parameters & Property Shorthands

Top comments (10)

codingsam profile image
Coding Sam • Edited

It really is a great feature.
But be carefull because, the default values only work for undefined. If those functions receive null as values for some arguments with default values, they will be kept as null.
I got caught by this so many times :)

torianne02 profile image
Tori Crawford

Thank you so much for adding this little tidbit! I didn't think to make note of it. :)

codingsam profile image
Coding Sam

A lot of developers, specially beginners can get tricked by this small detail. :)

aleksikauppila profile image
Aleksi Kauppila

Hi Victoria, thanks for posting!

I don’t think default arguments are a good feature. Using default arguments is at least not a good practice and will produce bad designs.

  • There will be multiple ways to call a function
  • Default values will have implicit meaning that have to be explained with documentation or comments
  • it will become tempting to add more arguments with a default value to a function, which will require more conditional logic leading to a lot of complexity

Any thoughts? Thanks!

torianne02 profile image
Tori Crawford

I definitely understand all of the points you are bringing up. Before jumping into each point, I just want to mention that I am a junior dev learning as I write. I began this series specifically to become more acquainted to the tools introduced by ES6 as that seems to be the popular version on most job descriptions.

Now, as to my thoughts about default parameters...I do think that implementing them in live code could definitely make things messy when trying to make sure arguments are being passed into the function properly. Having to keep in mind the specific order is not entirely easy, and yes, could require more comments and documentation.

As for your last point, I think that really falls on the developer. Being tempted to add more arguments to a function is all preference and self control.

With all that being said, I just learned about using the object spreader (introduced in ES8) for this purpose and it seems like a great tool to use!! Take a look at this article as it may interest you!

Thank you so much for adding to the discussion. :)

aleksikauppila profile image
Aleksi Kauppila

That’s cool, we’re all learning every day :) thanks for the link, i’ll check it out. 👍

rammyblog profile image
Onasanya Tunde

Great Post.

alexpaper profile image

I really needed this lesson, I found it very useful and well explained, thanks.

torianne02 profile image
Tori Crawford

Not a problem! :)

longchhun profile image

Great Post. Thank you for sharing