date: 2024-05-10

Basics

Abstract classes provide base functionality e.g.:

abstract class Command {
  abstract getCommandLine(): string;

  execute() {
    console.log('Executing:', this.getCommandLine());
  }
}

Abstract classes can't be instantiated directly e.g.:

abstract class Command {
  abstract getCommandLine(): string;

  execute() {
    console.log('Executing:', this.getCommandLine());
  }
}

new Command(); // Error: `getCommandLine` not provided

Extending

You must extend an abstract class and provide any abstract features as a part of the implementation e.g.:

class GitResetCommand extends Command {
  getCommandLine() {
    return 'git reset --hard';
  }
}

If you forget some abstract member it will result in an error, e.g.

// Error: missing `getCommandLine`
class GitResetCommand extends Command {}

Transcript

00:00

TypeScript allows you to create abstract classes, a concept that you might be familiar with from other programming languages. A class can be marked as abstract by using the abstract keyword, and then you get the ability to provide abstract members. For example, hey, we have an abstract method called Command line. Now the abstract members do not have any implementation, and the whole objective of an abstract class is for it to be inherited. And then for the child classes to provide an implementation for these abstract members. This means that within other portions of our abstract class,

00:33

we get to assume that these members will be provided. Here we have a dummy execute function that simply logs the command line to the console. But in a more real world scenario, it might spawn up an actual terminal process. So this abstract command class takes a particular command line from a style classes and then provides the reusable functionality to execute that command line. So the main consumers of an abstract class are classes that want to extend it. Here we create a class called GI Reset Command that extends this command. Now, as soon as we extend the base abstract class,

01:06

we get a compiler error. And if you look at the compiler error message, you can see that it is complaining that the child class does not provide the functionality required by the abstract class, which is the command line method. So we go ahead and provide the command line, and now we get the execute method for free. We can repeat the process for as many commands as you want. For example, the git fetch command. So now we can create instances of the GI Reset commands and the fetch commands and call their execute methods. Now, one final thing worth mentioning

01:39

that might be obvious is that you cannot create instances of an abstract class by itself. The obvious reason is that an abstract class has abstract that is missing features. For example, here, the command line method would be missing.

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Abstract Classes

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date: 2024-05-10

Basics

Extending

Transcript

Professional TypeScript Masterclass