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Severity: error
Category: classes
Stage: Stage 1

Description

An abstract class is an incomplete type intended only as a base for subclasses. It may declare abstract methods that have no implementation. Calling new on an abstract class directly is a type error because the resulting object would have unimplemented methods, making it unsound.

Abstract classes must be subclassed, and all abstract members must be implemented by the concrete subclass before an instance can be created.

Example

// ✗ error
abstract class Animal {
  abstract speak(): string
}

const a = new Animal()   // SJS-E016

Fix

Extend the abstract class with a concrete subclass that implements every abstract member, then instantiate the subclass:

// ✓ correct
abstract class Animal {
  abstract speak(): string

  describe(): string {
    return `I say: ${this.speak()}`
  }
}

class Dog extends Animal {
  speak(): string { return "woof" }
}

const d = new Dog()        // ✓
console.log(d.describe())  // "I say: woof"

Related codes

  • SJS-E015 — cannot narrow an access modifier on an overriding method
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