mobxjs/mobx · warning

Static properties are not supported ${property.key.loc?.sour

Error message

Static properties are not supported ${property.key.loc?.source}

What it means

Static class members decorated with mobx decorators cannot be converted to `makeObservable`, because makeObservable annotations only apply to instances in the constructor. The codemod warns and leaves the static decorated member untouched.

Source

Thrown at packages/mobx-undecorate/src/undecorate.ts:402

        const decorators = property.decorators
        if (!decorators || decorators.length === 0) {
            return property
        }
        if (decorators.length > 1) {
            warn("Found multiple decorators, skipping..", property.decorators[0])
            return property
        }
        const decorator = decorators[0]
        if (!j.Decorator.check(decorator)) {
            return property
        }
        let expr = decorator.expression
        if (j.Identifier.check(expr) && !decoratorsUsed.has(expr.name)) {
            warn(`Found non-mobx decorator @${expr.name}`, decorator)
            return property
        }
        if (property.static) {
            warn(`Static properties are not supported ${property.key.loc?.source}`, property)
            return property
        }

        if (options?.keepDecorators !== true) {
            property.decorators.splice(0)
        }

        // Replace decorator with @override
        if ((property as any).override) {
            const overrideDecorator = j.decorator(j.identifier("override"))
            if (options?.keepDecorators) {
                property.decorators[0] = overrideDecorator
            }
            expr = overrideDecorator.expression
        }

        effects.membersMap.push([
            property.key,

View on GitHub (pinned to 01211a698b)

Solutions

1. Move the static state to a module-level variable annotated with `observable()` from mobx instead of a static class field.
2. Remove the decorator and manage the static value with explicit mobx APIs (e.g. `observable.box`).
3. Keep the decorator and exclude this file from the codemod if static decorators are essential (not supported on modern decorators anyway).

Example fix

// before
class Store { @observable static count = 0 }
// after
const storeCount = observable.box(0)
class Store { static get count() { return storeCount.get() } }

if (/static\s+@\w+|@\w+\s+(?:public\s+)?static\s+\w+/.test(src)) {
  throw new Error('Decorated static members found; mobx-undecorate cannot convert them')
}

function isDecoratedStatic(member) {
  return member.static && member.decorators && member.decorators.length > 0
}

When it happens

Trigger: `handleProperty` finds a decorated member with `property.static === true`, e.g. `static @observable count = 0` (or `@observable static count = 0`).

Common situations: Classes using static observable state (global counters, registries) that were converted from legacy decorate-based code where statics happened to work.

Related errors

• Found multiple decorators, skipping..
• Found non-mobx decorator @${expr.name}
• Unexpected type
• Expected a decorate call with two arguments
• Expected exactly one class declaration for '${target.name}'

AI-assisted analysis of mobxjs/mobx@01211a698b (2026-08-28). Data as JSON: /api/errors/91c03cba132cbf7c. Report an issue: GitHub.