Bindings, the JavaScript runtime, and frontend tests

How the frontend reaches services, what wails3 generate bindings writes, how errors and events look in JavaScript, and what a browser test can and cannot prove.

Generated bindings

wails3 generate bindings reads the Go packages, finds every application.NewService call, and writes one directory per Go import path below frontend/bindings (change it with -d). For a Service type in package example.com/app/notes:

• frontend/bindings/example.com/app/notes/service.ts: one function per exported method, returning a CancellablePromise. A leading context.Context parameter is left out, an error-only method resolves to void, and a pointer result is T | null.
• models.ts: a class per struct used as an argument or result (-ts -i writes interfaces instead), with fields named as encoding/json names them.
• index.ts: re-exports the services and models.
• github.com/wailsapp/wails/v3/internal/eventdata.d.ts and eventcreate.ts: types and decoders for events registered with application.RegisterEvent.

Useful flags: -ts for TypeScript, -clean (on by default) to empty the output directory first, -names to call methods by name instead of ID, and -f '<go build flags>' to load the packages with the build's tags. The generator loads packages the way go build would, so a NewService call in a file the current tags or cgo setting exclude is missed and the run reports 0 Services.

The generated code calls Call.ByID(<id>, ...), where the ID is a hash of the package path, type name, and method name. Renaming any of them, or moving the service to another package, changes the ID, so regenerate after every change to a service's methods, parameters, results, or JSON fields. Never edit generated files. Commit them only if the project already does.

Generated code imports @wailsio/runtime. The project templates depend on "latest"; pin the npm package to the same version as the Go module (for example 3.0.0-beta.29) so the runtime and the backend agree.

Calling and handling results

import { Service } from "./bindings/example.com/app/notes";

try {
  const note = await Service.SaveNote({ id: "", title, body });
} catch (err) {
  show(err instanceof Error ? err.message : String(err));
}

• A Go error rejects with a RuntimeError whose message is the Go error text. Wrong argument types reject with a TypeError. When the Go error marshals to JSON, err.cause holds that JSON. Other failures, such as an unknown method ID from stale bindings, reject with a plain Error.
• Report success only after the promise resolves, and disable a submit control while its call is pending.
• The promise has a cancel() method; it cancels the Go method's context.Context.
• A nil Go slice or map is null on the wire. Generated bindings turn it into [] or {}; a direct Call.ByName caller sees null.
• Handle a cancelled dialog ("") as "nothing happened".

Events

import { Events } from "@wailsio/runtime";

const off = Events.On("notes:changed", (event) => render(event.data.count));
// later
off();

The callback receives a WailsEvent with name, data, and sender (the window name, or empty when Go emitted it). Events.On returns a function that removes the listener; call it when the component goes away. Events.Emit(name, data) sends an event to Go, where a registered name is decoded into its Go type.

Without a bundler

The app serves the runtime as an ES module at /wails/runtime.js, so a page with no build step can use await import("/wails/runtime.js") and call Call.ByName("<import path>.<Type>.<Method>", ...args). Prefer generated bindings in a real frontend, because they type check.

Testing the frontend

Unit and browser tests (Vitest, Playwright, and similar) run without Wails. Either mock the generated service modules, or install a fake transport before the code under test runs: @wailsio/runtime exports setTransport, and every runtime call then goes to its call(objectID, method, windowName, args) instead of fetch. Binding calls use object ID 0 and carry methodID (or methodName) and args. Return resolved values and thrown errors from the fake, and cover the rejection and cancelled-dialog paths, not only success.

To exercise the real Go side without a desktop, build the app with -tags server and point the browser test at it; see building.md.

These tests check the frontend's logic against the contract. They do not exercise WebKitGTK, WebKit, WebView2, native dialogs, focus, or window lifecycle. Say so in the handoff, and launch the built app when that behavior changed.