EC
TS4116
TypeScriptERRORNotableDeclarationHIGH confidence

This member must have an 'override' modifier because it overrides an abstract...

Production Risk

Build will fail; resolve before shipping.

What this means

A declaration emit error (TS4116): This member must have an 'override' modifier because it overrides an abstract method that is declared in the base class 'X'.. This diagnostic is emitted by the TypeScript compiler when this member must have an 'override' modifier because it overrides an abstract method that is declared in the base class 'X'..

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

TypeScript compiler reports TS4116 during type checking.

trigger — this will error
trigger — this will error
// Triggers TS4116
// This member must have an 'override' modifier because it overrides an abstract method that is declared in the base class 'X'.

expected output

error TS4116: This member must have an 'override' modifier because it overrides an abstract method that is declared in the 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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