Instances and References

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Table of contents

1. Instances and References
   1. Key Idea
   2. Overview
   3. Abstracting the constructor
   4. Summary
   5. Chapter Summary
2. Next Step

Key Idea

Data Classes allow us to combine data into a grouping or class and use that grouping as a data type in our programs.

Overview

There is a special keyword this that can be used from inside the constructor (or any method inside the class) that will allow us access to the member variables of the object.

Abstracting the constructor

Consider our color class

class Color{
   constructor(public red:number, public green:number,public blue:number){ }
}

What if instead of passing in values for red, green, and blue, we wanted to pass in a string (either “red”,”green”, or “blue”) to initialize our color to one of these three colors. We can go back to our original syntax and define the members explicitly, and change our constructor to take a string that is not marked with the public or private keywords since we only need it to initialize the members.

class Color{
	public red:number=0;
	public green:number=0;
	public blue:number=0;
	constructor(colorStr:string){ 
		//what goes here
	}
}

The idea is that we can use the string to determine how to set the members. We can use the this keyword to access the member variables of the current instance.

class Color{
	public red:number=0;
	public green:number=0;
	public blue:number=0;
	constructor(colorStr:string){ 
		if (colorStr==="red"){
			this.red=255;
		} else if (colorStr==="green"){
			this.green=255;
		} else if (colorStr==="blue"){
			this.blue=255;
		}
	}
}
console.log(new Color("red"));
console.log(new Color("green"));
console.log(new Color("blue"));

Here we can initialize our members indirectly by using the value of the parameter colorStr. The this keyword allows us access to our own members from within the instance. If the string is not recognized (i.e. not red, green, or blue) then the default values of (0,0,0) remain which is our intention. We would want to make a comment on our constructor that this is the behavior to help users of our class to know how to use it.

Summary

Typescript allows the use of the this keyword in order to access the members of the current instance of the class. From within the class, using the this keyword allows us access to all of the member properties (public or private) within the class instance.

Chapter Summary

Now we have the ability to create complex data types of our own using the class keyword. These data types can contain any other type of object including another class, a primitive type, or an array. There is no limitation on what the array or embedded class contain (other class objects, arrays of primitives, arrays of other class objects, etc.) We have a special method in our objects called a constructor. The constructor can be used to initialize our object, or by using the public and private keywords, it can define members of our object. Parameters without these keywords behave just like parameters to any other function, but with these keywords, that parameter also becomes a member of the object. We can access the members of our class instance using the this keyword.

Next Step

Next we’ll learn about generalized Classes: DataClasses »