Add tests for FilterCanvas.ts

src/__tests__/filtercanvas.test.ts

@@ -0,0 +1,50 @@
+import { getCanvasElement, loadImage } from "./../filtercanvas"
+
+describe("FilterCanvas", () => {
+    beforeAll(() => (document.body.innerHTML = `<canvas id="canvas"></canvas>`))
+
+    it("should find a canvas element", () => {
+        expect(getCanvasElement("#canvas")).toBeInstanceOf(HTMLCanvasElement)
+    })
+
+    it("should throw on an invalid selector", () => {
+        expect(() => getCanvasElement("#invalid")).toThrow(TypeError)
+    })
+})
+
+describe("loadImage", () => {
+    const originalImageFn = Object.getOwnPropertyDescriptor(
+        Image.prototype,
+        "src",
+    ) as PropertyDescriptor
+
+    beforeAll(() => {
+        const LOAD_SUCCESS = "image-success.png"
+
+        Object.defineProperty(Image.prototype, "src", {
+            set(src) {
+                if (src === LOAD_SUCCESS) {
+                    setTimeout(() => this.dispatchEvent(new Event("load")))
+                } else {
+                    setTimeout(() => this.dispatchEvent(new Event("error")))
+                }
+            },
+        })
+    })
+
+    afterAll(() => {
+        Object.defineProperty(Image.prototype, "src", originalImageFn)
+    })
+
+    it("should load if imgUrl is valid", async () => {
+        await expect(loadImage("image-success.png")).resolves.toBeInstanceOf(
+            HTMLImageElement,
+        )
+    })
+
+    it("should error if imgUrl is invalid", async () => {
+        await expect(loadImage("image-error.png")).rejects.toThrow(
+            "image could not be loaded",
+        )
+    })
+})