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    noImplicitOverride

    Overriding has to be deliberate

    Requires the `override` keyword on any member that replaces one from the base class.

    Since
    TypeScript 4.3
    strict
    Not in strict
    In your tsconfig
    "noImplicitOverride": true

    The same snippet both times. Only the option changed.

    With it off

    "noImplicitOverride": false
    class Base {
  greet() {
    return 'hi'
  }
}

export class Child extends Base {
  greet() {
    return 'yo'
  }
}

    Compiles clean

    With it on

    "noImplicitOverride": true
    class Base {
  greet() {
    return 'hi'
  }
}

export class Child extends Base {
  greet() {
    return 'yo'
  }
}

    Emits TS4114

    Why the compiler bothers

    The failure this prevents runs in the other direction from the obvious one. The risk is not writing `override` on something that overrides nothing — it is a base class adding a method that a subclass already has, silently turning an independent member into an override nobody reviewed. With the keyword required, the compiler notices when the relationship changes underneath you, which is the only moment the information is useful.

    Takeaway

    The keyword is not documentation. It is a tripwire on the base class.

    Where to go next

    22 options