In every class definition written, there should be at least one method; that is the constructor method. Furthermore, typically this should be the first method defined. It doesn’t have to be, but by convention and for readability, it is usually the first thing in the class.
The constructor method is always called by JavaScript when creating a new object from a class. Therefore, you must have a constructor method. What is the constructor, you say? Well, think of the word construct. What does that mean? It means to build or construct. So, what the constructor is doing is creating the object from the class. Cool huh?
JavaScript uses the constructor to build new objects of a class type. You have created an object before – think about creating a new Array(). Those parentheses at the end of the Array is calling the constructor. Come on, that’s cool, right?
The constructor method initializes an object’s properties.
As an example, if we were creating a Person, we may want to specify properties such as:
eyeColornumberOfLegsnumberOfArmshairColorYou will learn more specifically how to do this on the next page.
As mentioned, the constructor method should be the first method in a class definition.
Furthermore, if there is data that needed within the newly created object, this should be represented as input parameters to this function, which works just like any other function. You have done all of this before. Don’t you love it when it all comes full circle!
So our example might look like:
// a class definition
class Person {
// constructor method
constructor( eyeColor, numberOfLegs, numberOfArms, hairColor ) {
// method function block
// Do stuff here to create an object
console.log(eyeColor);
}
}