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Eliahu Garcia Lozano
Eliahu Garcia Lozano

Posted on • Originally published at

Default Function Parameters

We always have to validate if the arguments passed in a function have a value or they are undefined but, do we really?


In my last post, I talked about Destructuring Assignment In A Function Parameter and why it is handy when you have optional parameters in a function and don't want to pass undefined or null.

With that been said, what if you have a function where all the parameters are required? Again, since ES2015 (aka ES6) this can be easily implemented. Let's dive in.

Use Case

If you take a Tip Calculator project as an example, you probably will need always the same parameters:

  1. Bill Amount.
  2. Number of People.
  3. Service Quality.

The function you'll need would look like this 👇

function calculateTip(totalBill, numOfPeople, serviceQuality) {
  // Your validation here
  // Your code here

console.log(calculateTip(50, 2 , 0.2))
// 5.00
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Inside the function, you'll have to do all the calculations AND all the validation; plus, most of the time I go with the same friend (2 people) and leave the standard tip (20%).

In this case, the only parameter that will be changing all the time will be the bill amount.

So, let's give numOfPeople and serviceQuality a default parameter:

function calculateTip(totalBill, numOfPeople = 2, serviceQuality = 0.2) {
  // Your code here

// 5.00
// 7.50
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Now, just when someone else is joining, or we want to leave a different tip percentage, it will be necessary to pass the rest of the arguments.


You don't really need to check if the argument in a function is undefined, you can create a default value for it.

When a different value is provided, the default value will be overwritten.

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