JavaScript Functions as First Class Data: Do behavior

Learning Goals

• Create a JavaScript function triggered by an event

Introduction

In JavaScript, functions are what's known as first-class citizens of the language. This means they can be treated like any other variable: they can be assigned to a variable, stored in a data structure, passed as an argument to another function, or be the return value of another function. In this lab, we're going to have an event handler send some information gathered by an <input> field to a helper function.

Create a "First-Class" Function

We're going to create a web greeting! You're provided a text input field in which you should enter a time in 24 hour time. When you click the Submit button, some provided code will run which will make calls to two functions.

The greet() function

The greet function should take one argument, a String which specifies the 24-hour time in the format HH:MM.

• If the time is earlier than 12pm, return "Good Morning".
• If the time is between 12pm and 5pm, return "Good Afternoon".
• If the time is later than 5pm, return "Good Evening".

NOTE: The value returned from the <input> will be of type String. You’ll need to take the String of the 24 hour time and convert it to a number. The split() function and parseInt() function should help.

The displayMessage() function

The displayMessage function should take one argument, a String.

When the function runs it should update the text inside the #greeting node with the content of the argument.

It does not return anything.

Testing

Finally, open up your index.html file in the browser. After typing 15:00 in the input field and pressing the Submit button should, you should see "Good Afternoon" printed on the DOM.

Did it work? If so, take time to celebrate! Don't move on yet! Take some time to digest and internalize the code you've just wrote. If not, take time to debug.

Conclusion

This lab demonstrates just the tip of the iceberg when it comes to leveraging the power of JavaScript functions. There are heaps of features that JavaScript has — some are even comparable to Ruby and other programming languages. We'll be exploring more on JavaScript events in the next lesson.