TS2420: Class incorrectly implements interface TypeScript Origin: On your device OSI layer 7 · Application Last updated 5 Nov 2026

TypeScript: error TS2420: Class 'Dog' incorrectly implements interface 'Animal'. Property 'speak' is missing in type 'Dog' but required in type 'Animal'

A class declared implements X doesn't provide every required member (or with matching types).

Exact message you may see error TS2420: Class 'Dog' incorrectly implements interface 'Animal'.Class incorrectly implements interface. Property 'speak' is missing in type 'Dog' but required in type 'Animal'TS2420 Class incorrectly implements interfacets(2420) Class 'X' incorrectly implements interface 'Y'
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Seen on: TypeScript

Meaning

Add the missing members, fix their types/visibility (public), or make interface members optional. Remember implements only checks shape — it doesn't inherit implementation. Property initialisers and getters count.

Common causes

  • Missing method/property
  • Wrong member type
  • Private/protected members vs interface
  • Static vs instance mismatch
  • Interface changed after a dependency upgrade

⚡ Quick fix

  1. Implement the missing member
  2. Match types/modifiers
  3. Make the interface member optional if appropriate

Detailed fix by platform

TypeScript

  1. class Dog implements Animal { speak(): string { return "woof"; } }

How to diagnose

  1. Missing — Which member?
  2. Types — Match?
  3. Visibility — Public?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: On your device. Errors that are often confused with or caused by this one (click to open):

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