Testing Bindings

Test Images the way Devflare expects it to run

Images bindings let Workers transform, resize, and encode images without public image URLs.

Test Images by choosing the local harness that matches the product boundary instead of reaching for Cloudflare by default.

The first test should prove application control flow. Escalate to Wrangler remote binding or deployed tests only when the Cloudflare-hosted behavior is the thing under test.

Best for: image transformation/upload paths where the Worker calls the Images binding
Default harness: or
Escalate when: The assertion depends on Cloudflare-hosted product behavior rather than the app calling the binding correctly

Start with the default test loop

Keep the first test small. Name the binding, call the one method your route uses, and assert the behavior your app owns.

When Cloudflare owns the interesting behavior, mark that as a remote/deployed lane instead of building a local fake that claims too much.

Pure Images chain-shape test

import { expect, test } from 'bun:test'

import { createMockImagesBinding } from 'devflare/test'

test('returns a deterministic image response', async () => {

const images = createMockImagesBinding()

const result = await images.input(new Blob(['image']).stream()).transform({ width: 320 }).output({ format: 'image/png' })

expect(result.response().headers.get('content-type')).toBe('image/png')

})

The helper surface to remember

Use or for config-backed local worker tests.
Use / for pure unit tests.
Use or an explicit integration lane when the test needs Cloudflare credentials or a local Docker/Podman engine.

When to move beyond the default harness

Cloudflare owns hosted image storage, variants, delivery rules, billing, and final transform fidelity.
Do not let a low-fidelity mock become product documentation. Keep mocks framed as application-flow tools.
If a test would mutate paid or remote Cloudflare state, gate it separately from ordinary unit tests.

Local tests should be honest

For Images, passing locally means the Devflare contract and app flow are correct. It does not automatically prove every hosted Cloudflare behavior.

Images internals

Images compiles from to Wrangler , with local/test behavior called out explicitly.

Images example

A compact Images recipe with config and worker usage in one application path.