This Fabric.js troubleshooting guide covers the problems people hit most, in the order they usually appear.
Undo does not notice my change
Fabric.js only fires events for pointer interaction, adding and removing objects, and text editing. Changes your code makes with object.set() or canvas.bringObjectForward() fire nothing. Wrap them:
engine.transaction("Recolor", () => shape.set("fill", "teal"));My object reference stopped working after undo
Undo and redo rebuild the changed objects, so they are new instances with the same ids. Keep ids, and look objects up with engine.getObjectById(id).
My custom properties are lost after loading
With plain toJSON and loadFromJSON, Fabric.js only saves the properties it knows. Register the class with its custom properties:
engine.registerObject({ fabricClass: Sticker, properties: ["label"] });Loading fails with UNKNOWN_OBJECT_TYPE
The document contains a class your editor has not registered. Pass it in customObjects, or call registerObject before loading. The class needs a static type that matches the saved type. In plain Fabric.js the same problem shows up as klass.fromObject is not a function.
Two copies of Fabric.js in the bundle
A class registered in one copy is unknown to the other, and loads fail with UNKNOWN_OBJECT_TYPE. Make sure only one fabric is installed. With a linked package in Vite, add resolve: { dedupe: ["fabric"] }.
Loading fails with MISSING_ASSETS
error.missingAssets lists every image that did not load. Fix the URLs with assets.resolveUrl, offer a replacement with assets.replaceMissingImage, or call engine.replaceImage(oldUrl, newUrl) later.
Text looks different after reopening
The font was not loaded when the text was created, so Fabric.js measured it with a fallback. Load web fonts in assets.loadFont, and watch for FONT_UNAVAILABLE warnings.
Export fails with a tainted canvas
An image from another site was loaded without CORS, so the browser blocks PNG, JPEG and WebP export. Load it with crossOrigin: "anonymous" from a server that sends Access-Control-Allow-Origin. Run engine.preflightExport({ format }) to show the problem before the user clicks export.
Saves fail with SAVE_CONFLICT in one tab
Your adapter does not return the new revision. Return { revision } from saveDocument, or store document.revision.
The recovery prompt never appears
Pass recovery: { store: createIndexedDbRecovery() }. Copies are only written while there are unsaved changes, and private browsing may clear storage when the window closes.
React: two engines in development
StrictMode mounts effects twice. useDocumentEngine destroys the first engine, so only one stays alive. Always use the engine the hook returns.
Vue: the canvas does not render after loading
Vue wrapped the Fabric.js canvas in a reactive proxy. Keep it in shallowRef, or pass toRaw(canvas). See Vue.
Next.js: "window is not defined"
Fabric.js ran on the server. Load the editor with dynamic(() => import("./editor"), { ssr: false }). See Next.js.
If this Fabric.js troubleshooting list does not cover your problem, open an issue with the error code, the Fabric.js version and the browser.