EC
TS4115
TypeScriptERRORNotableDeclarationHIGH confidence

This parameter property must have an 'override' modifier because it overrides...

Production Risk

Build will fail; resolve before shipping.

What this means

A declaration emit error (TS4115): This parameter property must have an 'override' modifier because it overrides a member in base class 'X'.. This diagnostic is emitted by the TypeScript compiler when this parameter property must have an 'override' modifier because it overrides a member in base class 'X'..

Why it happens
  1. 1Incorrect TypeScript syntax or type usage
  2. 2Type mismatch between declared and actual value
How to reproduce

TypeScript compiler reports TS4115 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS4115
// This parameter property must have an 'override' modifier because it overrides a member in base class 'X'.

expected output

error TS4115: This parameter property must have an 'override' modifier because it overrides a member in base class 'X'.

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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