mobxjs/mobx · warning

Generated new constructor for class ${clazz.id?.name}. But s

Error message

Generated new constructor for class ${clazz.id?.name}. But since the class does have a base class, it might be needed to revisit the arguments that are passed to `super()`

What it means

When generating a constructor for a derived class (one with a superClass), the codemod inserts `super()` and the makeObservable annotations, but it cannot know what arguments the original base constructor expects. It warns so the developer reviews the generated super call.

Source

Thrown at packages/mobx-undecorate/src/undecorate.ts:462

            // @ts-ignore
            initializeObservablesCall.expression.typeArguments = j.tsTypeParameterInstantiation([
                j.tsTypeReference(j.identifier(clazz.id!.name)),
                j.tsUnionType(
                    // @ts-ignore
                    privates.map(member => j.tsLiteralType(j.stringLiteral(member)))
                )
            ])
        }

        const needsSuper = !!clazz.superClass
        let constructorIndex = clazz.body.body.findIndex(
            member => j.ClassMethod.check(member) && member.kind === "constructor"
        )

        // create a constructor
        if (constructorIndex === -1) {
            if (needsSuper) {
                warn(
                    `Generated new constructor for class ${clazz.id?.name}. But since the class does have a base class, it might be needed to revisit the arguments that are passed to \`super()\``,
                    clazz
                )
            }

            let superClassName = j.Identifier.check(clazz.superClass)
                ? clazz.superClass.name
                : j.MemberExpression.check(clazz.superClass)
                ? j.Identifier.check(clazz.superClass.property)
                    ? clazz.superClass.property.name
                    : ""
                : ""

            // if this clazz is a react component, we now that the constructor and super call have one argument, the props
            let isReactComponent =
                hasReact && ["Component", "PureComponent"].includes(superClassName)
            let propsType = isReactComponent && clazz.superTypeParameters?.params[0]
            const propsParam = j.identifier("props")

View on GitHub (pinned to 01211a698b)

Solutions

1. Review the generated constructor and pass the correct arguments to the inserted `super(...)` call.
2. If the base class requires no arguments, ignore the warning after verifying.
3. Alternatively, write the constructor yourself before running the codemod so the codemod only appends makeObservable to it.

Example fix

// generated by codemod (warning)
constructor() { super(); makeObservable(this, {...}) }
// after manual fix
constructor(options) { super(options); makeObservable(this, {...}) }

// detect subclasses with decorated members and no constructor before codemod
const derivedNoCtor = /class\s+\w+\s+extends\s+\w+\s*\{(?![^}]*constructor)/.test(src)
if (derivedNoCtor && /@\w+/.test(src)) {
  console.warn('Derived class will get a generated constructor; verify super() args after codemod')
}

function hasOwnConstructor(clazz) {
  return clazz.body.body.some(m => m.type === 'ClassMethod' && m.kind === 'constructor')
}

When it happens

Trigger: `createConstructor` runs on a class that (a) has decorated members requiring makeObservable, (b) has no existing constructor (`constructorIndex === -1`), and (c) extends a base class (`needsSuper === true`).

Common situations: Legacy MobX 4/5 codebases where decorated subclasses relied on decorators initializing fields without a constructor; after undecoration a constructor is synthesized and base-class constructor arguments must be supplied by hand.

Related errors

• Expected a decorate call with two arguments
• Expected exactly one class declaration for '${target.name}'
• Failed to find mobx import, can't add makeObservable as depe
• Found multiple decorators, skipping..
• Found non-mobx decorator @${expr.name}

AI-assisted analysis of mobxjs/mobx@01211a698b (2026-08-28). Data as JSON: /api/errors/d1cd0b1e395ca495. Report an issue: GitHub.