Abstract classes in TypeScript

A base that cannot be instantiated, with members a subclass must supply.

interface Describable {
  describe(): string;
}

abstract class Check implements Describable {
  abstract readonly name: string;
  abstract passes(value: string): boolean;

  describe(): string {
    return `${this.name} check`;
  }

  report(value: string): string {
    return `${this.describe()}: ${this.passes(value) ? "pass" : "fail"}`;
  }
}

class MinLength extends Check {
  readonly name = "min length";
  constructor(private floor: number) {
    super();
  }
  passes(value: string): boolean {
    return value.length >= this.floor;
  }
}

const checks: Check[] = [new MinLength(3)];
console.log(checks.map((check) => check.report("ada")));

How it works

1. abstract members have no body and must be implemented.
2. The base can still hold shared, concrete behavior.
3. implements checks a class against an interface.

Keywords and builtins used here

• Check
• abstract
• boolean
• class
• const
• constructor
• extends
• implements
• interface
• number
• private
• readonly
• return
• string
• super
• this

The run, in numbers

Lines

29

Characters to type

610

Tokens

163

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 98 seconds.

Type this snippet

Step 3 of 4 in Classes, step 3 of 19 in Classes & patterns.

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