EC
TS1242
TypeScriptERRORNotableSyntaxHIGH confidence

'abstract' modifier can only appear on a class, method, or property declaration.

Production Risk

Build will fail; resolve before shipping.

What this means

A syntax error (TS1242): 'abstract' modifier can only appear on a class, method, or property declaration.. This diagnostic is emitted by the TypeScript compiler when 'abstract' modifier can only appear on a class, method, or property declaration..

Why it happens
  1. 1A concrete class extends an abstract class but does not implement all abstract members
How to reproduce

TypeScript compiler reports TS1242 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS1242
// 'abstract' modifier can only appear on a class, method, or property declaration.

expected output

error TS1242: 'abstract' modifier can only appear on a class, method, or property declaration.

Fix

Implement all abstract members

WHEN Extending an abstract class

Implement all abstract members
abstract class Base {
  abstract doWork(): void;
}
class Derived extends Base {
  doWork(): void { /* implementation */ }
}

Why this works

All abstract members must be implemented in concrete subclasses.

Sources
Official documentation ↗

TypeScript Compiler Diagnostics

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