{"record":{"id":"f7ba542335cb6250","repo":"gofiber/fiber","slug":"route-handler-fn-cannot-be-nil","errorCode":null,"errorMessage":"route handler 'fn' cannot be nil","messagePattern":"route handler 'fn' cannot be nil","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"critical","filePath":"app.go","lineNumber":1201,"sourceCode":"\t}\n}\n\n// RouteChain creates a Registering instance that lets you declare a stack of\n// handlers for the same route. Handlers defined via the returned Register are\n// scoped to the provided path.\nfunc (app *App) RouteChain(path string) Register {\n\t// Create new route\n\troute := &Registering{app: app, path: path}\n\n\treturn route\n}\n\n// Route is used to define routes with a common prefix inside the supplied\n// function. It mirrors the legacy helper and reuses the Group method to create\n// a sub-router.\nfunc (app *App) Route(prefix string, fn func(router Router), name ...string) Router {\n\tif fn == nil {\n\t\tpanic(\"route handler 'fn' cannot be nil\")\n\t}\n\t// Create new group\n\tgroup := app.Group(prefix)\n\tif len(name) > 0 {\n\t\tgroup.Name(name[0])\n\t}\n\n\t// Define routes\n\tfn(group)\n\n\treturn group\n}\n\n// Error makes it compatible with the `error` interface.\nfunc (e *Error) Error() string {\n\treturn e.Message\n}\n","sourceCodeStart":1183,"sourceCodeEnd":1219,"githubUrl":"https://github.com/gofiber/fiber/blob/a105acad6c1e4576a77f01e02973f67e962bb58d/app.go#L1183-L1219","documentation":"App.Route(prefix, fn) requires a non-nil function that registers routes on the sub-router; calling it with a nil fn panics immediately. The guard prevents a silent no-op route group, which would be a confusing runtime defect.","triggerScenarios":"Passing a nil function as the second argument: app.Route(\"/api\", nil). Common when the route-registration function is conditionally assigned or fetched from a map and ends up nil.","commonSituations":"Plugin/module loader returning nil for a missing module; conditional route setup where the closure variable was never assigned; refactor leaving a dangling nil.","solutions":["Ensure the function passed to Route is a non-nil func(router fiber.Router).","If routes are optional, guard the call: if register != nil { app.Route(\"/api\", register) }.","Load modules eagerly and fail loudly if a route registrar is missing rather than passing nil."],"exampleFix":"// before\napp.Route(\"/api\", moduleLoader.Get(\"api\")) // returns nil when missing\n\n// after\nreg := moduleLoader.Get(\"api\")\nif reg == nil { log.Fatal(\"api route registrar missing\") }\napp.Route(\"/api\", reg)","handlingStrategy":"validation","validationCode":"// Guard Route against nil registrars\nfunc routeIf(app *fiber.App, prefix string, fn func(fiber.Router)) {\n    if fn == nil { return }\n    app.Route(prefix, fn)\n}","typeGuard":"func isRouteFn(v any) bool {\n    f, ok := v.(func(fiber.Router))\n    return ok && f != nil\n}","tryCatchPattern":null,"preventionTips":["Load route registrars eagerly and fail loudly if missing.","Use a helper that no-ops on nil rather than passing nil to Route.","Type-check plugin return values at load time."],"tags":["routing","nil-guard","startup"],"backgroundTag":null,"analyzedSha":"a105acad6c1e4576a77f01e02973f67e962bb58d","analyzedAt":"2026-08-11T17:33:26.942Z","contentChangedAt":null,"schemaVersion":2},"datasetVersion":"2026-09-23T08:17:48.524Z"}