JavaScript's Built in Objects and Classes Built in Classes

Learning objective: By the end of this lesson, learners will be able to differentiate between built-in objects and classes in JavaScript, specifically demonstrating the ability to use the Date class to create and manipulate date objects.

Built-In Classes

The JavaScript Math object contains a lot of useful methods and values that we can reference. We don’t characterize the Math object as a class though, because we cannot use it to instantiate or create new objects. This makes sense because we don’t need new math objects to use its helpful properties. We have all the functionality we need baked into Math. We can just use this object directly.

let randomNumber = Math.random()

In this example, what we want is a random number, not an object. So when would we need to instantiate new objects?

📚 To instantiate means to create a new instance of an object from a class. It’s like using a recipe (class) to bake a cake (object).

The Date class

Let’s take a look at one of JavaScript’s built-in classes now.

A great example of a common built-in class, is the JS Date class. There are many reasons for creating different instances of Date objects (for example, birthdays, anniversaries, dates and times for timecards, dates for to do lists, calendar events, etc.).

What sets the Date class apart from the Math object, is that unlike Math.random() which returns a number, utilizing the Date class returns a new date object - complete with its own methods and properties.

Let’s utilize JavaScript’s Date class to instantiate a new date object using a string:

const birthday = new Date("2023-05-31");

Our new date object is stored in the variable, birthday. And because it is an object created by the Date class, it will have its own set of methods and properties inherited from its parent.

📚 Inheritance is when an object gets properties and methods passed down from its class.

These properties are also known as instance methods. Let’s take our new birthday object for a test drive.

Instance methods

The JavaScript Date class provides a variety of instance methods that are extremely useful for handling and manipulating dates.

For reference:

const birthday = new Date("2023-05-31");

getFullYear() - Returns the year of the specified date according to local time.

let year = birthday.getFullYear();
console.log(year) // 2023

getDate() - Returns the day of the month (1-31) for the specified date.

let day = birthday.getDate();
console.log(day)  // 31

getTime() - Returns the numeric value corresponding to the time for the specified date according to universal time.

let time = birthday.getTime();
console.log(time); // 1685491200000

Why the strange number? The getTime() method returns the number of milliseconds for this date since the epoch, which is defined as the midnight at the beginning of January 1, 1970, UTC. This format, though a bit confusing, is used frequently in code because of how exact it is.

All of these helpful instance methods (and more) are attached to our new date object right out of the box.

Static (class) methods

There are also methods we can call on the Date class itself and these are called static methods.

Above, we created a Date object using a specific date entered as a parameter. Let’s say we instead wanted to create a Date object that reflects “right now”, like when someone clicks the submit button on a form. To capture that exact submission time, we can use the Date class’s .now() method.

This method is called on the Date class directly:

let now = Date.now() 
console.log(now) // 1703121829554

Now, because the values of birthday.getTime() and Date.now() are numbers of the same format, we can actually compare them to see if one date may precede the other.

if (birthday.getTime() < Date.now()) {
    console.log("Happy belated birthday!")
}