JavaScript default parametersJavaScript functions can take arguments, but they are optional by nature. Default parameter syntax makes it easy to determine if one or more arguments have been provided and if not, initialize them.

While something like Typescript can make it easier to enforce certain practices, use is voluntary. Ultimately, you can’t force a consumer to provide one or more arguments to your function. There are ways to work around this problem, but solutions are not simple. As is often the case, for example, solutions sometimes introduce new problems. There’s good news on this front, though, and in this article, I will demonstrate the JavaScript default parameter syntax and how it solves the above-mentioned problem.

The JavaScript default parameter syntax is surprisingly simple; instead of specifying an argument in the parentheses, you initialize it. This may look a bit deceiving, and, in fact, some may think that by doing this you are absolutely setting that value. But quite the contrary: what you are saying is: “If argument X is not provided, then initialize it and set it equal to this value.” So, you’re providing a “default” value for that argument, and if that argument is provided when the function is executed, then the provided value is used.

Example # 1

See the Pen JavaScript Default Parameters – Challenge by Kevin Chisholm (@kevinchisholm) on CodePen.

In Example # 1, the addBonus() function takes one argument: “bonus.” In that function, we had to write code that checks to see if the “bonus” argument was provided. If it was, then we use the provided value. Now, this code works just fine, but there’s a problem. If we accept this solution, that means that we’ll write code that is virtually identical to it any place else in our application where the same problem needs to be solved. So, of course, it’s worth remembering here that any time we have repeated code, we know that there’s a better way to solve a problem.

Example # 2

See the Pen JavaScript Default Parameters – Solution 1 by Kevin Chisholm (@kevinchisholm) on CodePen.

Now, when you take a look at the approach in Example # 2, you’ll see an immediate improvement. We’ve leveraged default parameter syntax so that the “bonus” argument is now optional, thereby creating the biggest advantage of this approach, which is that there is no longer any repeated code. By simply initializing the “bonus” argument, we ensure that if not provided, that variable will have a value.

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