This article describes the features and functionality of TypeScript 4.2. Using a class as an interfaceĪs we said in the previous section, a class declaration creates two things: a type representing instances of the class and a constructor function.īecause classes create types, you can use them in the same places you would be able to use interfaces.Originally published November 2018. It is also good to mention that changing static property is frowned upon, here greeter3 has 'Hey there!' instead of 'Hello, there' on standardGreeting. We show this by using new on greeterMaker, creating new instances of Greeter and invoking them as before. This type will contain all of the static members of Greeter along with the constructor that creates instances of the Greeter class. Or, more precisely, “give me the type of the symbol called Greeter,” which is the type of the constructor function.

Here we use typeof Greeter, that is “give me the type of the Greeter class itself” rather than the instance type. This variable will hold the class itself, or said another way its constructor function. Here we create a new variable called greeterMaker. We instantiate the Greeter class, and use this object. In this example, greeter1 works similarly to before. in front of instance accesses, here we prepend Grid.

In this example, we use static on the origin, as it’s a general value for all grids.Įach instance accesses this value through prepending the name of the class. We can also create static members of a class, those that are visible on the class itself rather than on the instances. Up to this point, we’ve only talked about the instance members of the class, those that show up on the object when it’s instantiated. d.ts file from your code, because users of your property can see that they can’t change it. Second, accessors with a get and no set are automatically inferred to be readonly. To prove to ourselves that our accessor is now checking the length of values, we can attempt to assign a name longer than 10 characters and verify that we get an error.Ī couple of things to note about accessors:įirst, accessors require you to set the compiler to output ECMAScript 5 or higher.ĭownleveling to ECMAScript 3 is not supported. The protected modifier acts much like the private modifier with the exception that members declared protected can also be accessed within deriving classes. When we try to assign from an Employee to Animal we get an error that these types are not compatible.Įven though Employee also has a private member called name, it’s not the one we declared in Animal. However, this is not the case for Employee. We create some instances of these classes and then try to assign them to each other to see what will happen.īecause Animal and Rhino share the private side of their shape from the same declaration of private name: string in Animal, they are compatible. We also have a new class Employee that looks identical to Animal in terms of shape. In this example, we have an Animal and a Rhino, with Rhino being a subclass of Animal. Types have separate declarations of a private property 'name'. 2322 Type 'Employee' is not assignable to type 'Animal'. Animal = employee Type 'Employee' is not assignable to type 'Animal'.

Typescript class constructor