Add support for "use step" functions in class instance methods
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Pull request overview
Adds SWC transform + runtime support so class instance methods can be marked with "use step" and invoked as workflow steps while preserving this via serialization.
Changes:
- Update SWC transform to detect
"use step"in instance methods, generateClassName.prototype.methodNamestep registrations, and proxy instance methods in workflow bundles. - Register serialization classes so
thisvalues can be dehydrated/hydrated across workflow ↔ step boundaries. - Add end-to-end coverage plus transform fixtures for instance-method steps (including nested steps), and adjust error fixtures.
Reviewed changes
Copilot reviewed 23 out of 23 changed files in this pull request and generated 7 comments.
Show a summary per file
| File | Description |
|---|---|
| workbench/example/workflows/99_e2e.ts | Adds Counter + instanceMethodStepWorkflow e2e workflow demonstrating instance-method steps with custom serialization. |
| packages/core/e2e/e2e.test.ts | Adds e2e test validating instance-method step execution + step list expectations. |
| packages/swc-plugin-workflow/transform/src/lib.rs | Core transform changes: allow "use step" on instance methods, register prototype step functions, and proxy them in workflow mode. |
| packages/swc-plugin-workflow/transform/tests/fixture/instance-method-step/input.js | New fixture input for instance-method step transformation. |
| packages/swc-plugin-workflow/transform/tests/fixture/instance-method-step/output-client.js | Expected client output for instance-method step fixture. |
| packages/swc-plugin-workflow/transform/tests/fixture/instance-method-step/output-step.js | Expected step-bundle output for instance-method step fixture. |
| packages/swc-plugin-workflow/transform/tests/fixture/instance-method-step/output-workflow.js | Expected workflow-bundle output for instance-method step fixture. |
| packages/swc-plugin-workflow/transform/tests/fixture/instance-method-nested-step/input.js | New fixture input for nested steps inside an instance-method step. |
| packages/swc-plugin-workflow/transform/tests/fixture/instance-method-nested-step/output-client.js | Expected client output for nested instance-method step fixture. |
| packages/swc-plugin-workflow/transform/tests/fixture/instance-method-nested-step/output-step.js | Expected step-bundle output for nested instance-method step fixture. |
| packages/swc-plugin-workflow/transform/tests/fixture/instance-method-nested-step/output-workflow.js | Expected workflow-bundle output for nested instance-method step fixture. |
| packages/swc-plugin-workflow/transform/tests/errors/instance-methods/input.js | Updates error fixture to allow "use step" on instance methods but still forbid "use workflow". |
| packages/swc-plugin-workflow/transform/tests/errors/instance-methods/output-client.js | Expected client output for updated instance-method error fixture. |
| packages/swc-plugin-workflow/transform/tests/errors/instance-methods/output-client.stderr | Updated expected stderr for client mode. |
| packages/swc-plugin-workflow/transform/tests/errors/instance-methods/output-step.js | Expected step output for updated instance-method error fixture. |
| packages/swc-plugin-workflow/transform/tests/errors/instance-methods/output-step.stderr | Updated expected stderr for step mode. |
| packages/swc-plugin-workflow/transform/tests/errors/instance-methods/output-workflow.js | Expected workflow output for updated instance-method error fixture. |
| packages/swc-plugin-workflow/transform/tests/errors/instance-methods/output-workflow.stderr | Updated expected stderr for workflow mode. |
| packages/swc-plugin-workflow/transform/tests/errors/forbidden-expressions/output-step.js | Updates expected output to treat instance method step as supported. |
| packages/swc-plugin-workflow/transform/tests/errors/forbidden-expressions/output-step.stderr | Adjusted expected stderr (now empty). |
| packages/swc-plugin-workflow/transform/tests/errors/forbidden-expressions/output-workflow.js | Updates expected output to proxy/register the instance method step. |
| packages/swc-plugin-workflow/transform/tests/errors/forbidden-expressions/output-workflow.stderr | Adjusted expected stderr (now empty). |
| .changeset/legal-parts-happen.md | Publishes a patch changeset for @workflow/swc-plugin. |
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| prop: MemberProp::Ident(IdentName::new( | ||
| method_name.into(), | ||
| DUMMY_SP, | ||
| )), |
There was a problem hiding this comment.
Instance method step registration builds ClassName.prototype.<method_name> using MemberProp::Ident, but method_name may come from PropName::Str and can contain characters that aren’t valid identifiers. This would generate invalid JS (or wrong property access). Carry the original key type through and emit computed member access (e.g. prototype["..."]) when the method key is a string.
Suggested change
| prop: MemberProp::Ident(IdentName::new( | |
| method_name.into(), | |
| DUMMY_SP, | |
| )), | |
| prop: MemberProp::Computed(ComputedPropName { | |
| span: DUMMY_SP, | |
| expr: Box::new(Expr::Lit(Lit::Str(Str { | |
| span: DUMMY_SP, | |
| value: method_name.into(), | |
| raw: None, | |
| }))), | |
| }), |
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| prop: MemberProp::Ident(IdentName::new( | ||
| method_name.into(), | ||
| DUMMY_SP, | ||
| )), |
There was a problem hiding this comment.
Workflow-mode proxy assignment uses MemberProp::Ident for method_name on ClassName.prototype, but method_name may originate from a string key. This can emit invalid JS and also won’t match the stripping logic if the class method key was a string. Use computed member access when appropriate (based on the original PropName).
Suggested change
| prop: MemberProp::Ident(IdentName::new( | |
| method_name.into(), | |
| DUMMY_SP, | |
| )), | |
| prop: MemberProp::Computed(ComputedPropName { | |
| span: DUMMY_SP, | |
| expr: Box::new(Expr::Lit(Lit::Str(Str { | |
| span: DUMMY_SP, | |
| value: method_name.into(), | |
| raw: None, | |
| }))), | |
| }), |
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| )); | ||
|
|
||
| // Visit children to process nested step functions | ||
| method.visit_mut_children_with(self); | ||
| } |
There was a problem hiding this comment.
When visiting instance method step bodies to transform nested steps, the transformer doesn’t appear to set current_parent_function_name (unlike visit_mut_fn_decl). This can cause nested step hoisting to generate self-referential aliases (e.g. const helper = helper;, which is a runtime ReferenceError). Consider setting current_parent_function_name while visiting class method children (e.g. to ClassName#methodName) so hoisted nested steps get a unique name.
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| const helper = helper; | ||
| const doubled = await helper(input); |
There was a problem hiding this comment.
This generated code is not valid at runtime: const helper = helper; will throw (TDZ/self-reference) when process() executes. The nested-step hoisting for instance method steps needs to avoid shadowing the hoisted helper name (e.g. rename the hoisted function or remove/rename the local binding).
Suggested change
| const helper = helper; | |
| const doubled = await helper(input); | |
| const nestedHelper = helper; | |
| const doubled = await nestedHelper(input); |
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| /** | ||
| * Instance method step: adds the given amount to the counter's value. | ||
| * The `this` context (the Counter instance) is serialized and passed | ||
| * to the step handler, then deserialized before the method is called. | ||
| */ | ||
| async add(amount: number): Promise<number> { | ||
| 'use step'; | ||
| return this.value + amount; | ||
| } |
There was a problem hiding this comment.
The docstring says this step "adds the given amount to the counter's value", but the implementation returns this.value + amount without updating this.value. Either update the comment to reflect that it’s a pure calculation, or update the method to mutate this.value if that’s what’s intended to be tested.
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| class_decl.class.body.retain(|member| { | ||
| if let ClassMember::Method(method) = member { | ||
| if method.is_static { | ||
| if let PropName::Ident(ident) = &method.key { | ||
| let method_name = ident.sym.to_string(); | ||
| return !methods_to_strip.contains(&method_name); | ||
| if let PropName::Ident(ident) = &method.key { | ||
| let method_name = ident.sym.to_string(); | ||
| if method.is_static { | ||
| return !static_methods_to_strip.contains(&method_name); | ||
| } else { | ||
| return !instance_methods_to_strip.contains(&method_name); | ||
| } | ||
| } |
There was a problem hiding this comment.
The workflow-mode stripping logic only checks PropName::Ident for class methods. Instance (and static) methods with string keys (e.g. async 'foo-bar'() { 'use step' }) are supported earlier, but won’t be removed here, so the original method body can remain in the workflow bundle instead of being replaced by the step proxy. Extend this retain() filter to also handle PropName::Str (and keep the stripping key representation consistent with how method_name is collected).
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| const helper = helper; | ||
| const doubled = await helper(input); |
There was a problem hiding this comment.
Callee is not a function: it has type undefined.
Suggested change
| const helper = helper; | |
| const doubled = await helper(input); | |
| const localHelper = helper; | |
| const doubled = await localHelper(input); |
This was referenced Jan 30, 2026
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Added support for
"use step"directive in class instance methods, allowing instance methods to be used as workflow steps.What changed?
ClassName.prototype.methodNamethiscontext across workflow/step boundariesHow to test?
The PR includes a new end-to-end test
instanceMethodStepWorkflowthat demonstrates the functionality:Counterclass with instance methods marked as stepsthiscontextWhy make this change?
This change enables a more natural object-oriented programming model when working with workflows. Previously, only static methods, standalone functions, and object methods could be marked as steps. Now, developers can create classes with instance methods that are steps, allowing for better encapsulation and more intuitive code organization. This is particularly useful for complex workflows that need to maintain state across multiple step invocations.