mobxjs/mobx · warning

Found non-mobx decorator @${expr.name}

Error message

Found non-mobx decorator @${expr.name}

What it means

The codemod only knows the mobx decorators that were used in `decorate(...)` calls it has seen. A single decorator on a member whose identifier is not in that known set is treated as a non-mobx (custom) decorator and skipped, since rewriting it would change semantics.

Source

Thrown at packages/mobx-undecorate/src/undecorate.ts:398

            membersMap: [[any, any, boolean, boolean]]
        },
        clazzPath: ASTPath<ClassDeclaration>
    ): ClassProperty | ClassMethod {
        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

View on GitHub (pinned to 01211a698b)

Solutions

1. Register the decorator by adding it to a `decorate(MyClass, { member: yourDecorator })` call if it is actually a mobx-compatible decorator, then re-run the codemod.
2. Remove or manually rewrite non-mobx decorators, converting their logic to explicit code.
3. If it is a mobx decorator applied inline (no decorate call), convert the member manually to `makeObservable`.

Example fix

// before
@log count = 1
// after
constructor() { makeObservable(this, { count: observable }) }
// plus manual logging code

const mobxDecorators = new Set(['observable','computed','action','flow','bound','override','autobind'])
const used = [...src.matchAll(/^\s*@([A-Za-z_$][\w$]*)/gm)].map(m => m[1])
const unknown = used.filter(n => !mobxDecorators.has(n))
if (unknown.length) console.warn('Non-mobx decorators present:', unknown)

function isMobxDecorator(name, decoratorsUsed) {
  return typeof name === 'string' && decoratorsUsed.has(name)
}

When it happens

Trigger: A member decorated with an identifier (e.g. `@customlog foo`) that is not among the decorators collected from decorate calls — the identifier is not in `decoratorsUsed`.

Common situations: Custom project decorators (logging, formatting), decorators from non-mobx libraries, or mobx decorators used directly on class members without a corresponding `decorate` call so they were never registered in the known set.

Related errors

• Found multiple decorators, skipping..
• Static properties are not supported ${property.key.loc?.sour
• Unexpected type
• Expected a decorate call with two arguments
• Expected exactly one class declaration for '${target.name}'

AI-assisted analysis of mobxjs/mobx@01211a698b (2026-08-28). Data as JSON: /api/errors/4ff3c60be2ac3617. Report an issue: GitHub.