Better type inference

Author: kognise

script/test-svg.spec.ts

@@ -6,14 +6,21 @@ import { JSDOM } from "jsdom";
 // Collect into an array so we can iterate over it again multiple times.
 const svgFiles = await Array.fromAsync(new Glob("./src/svg/*/*.svg").scan());
 
+/**
+ * Helper so TypeScript can infer `expect(...).not.toBeNull()`.
+ */
+function expectNotNull<T>(value: T | null, message: string): asserts value is T {
+  expect(value, message).not.toBeNull();
+}
+
 test.concurrent("SVG files are all 500×500", async () => {
   let numSVGs = 0;
   for await (const svgFile of svgFiles) {
     numSVGs++;
     const svgElem = new JSDOM(
       await file(svgFile).text(),
     ).window.document.querySelector("svg");
-    expect(svgElem, `${svgFile}: no svg`).not.toBeNull();
+    expectNotNull(svgElem, `${svgFile}: no svg`);
     expect(svgElem?.getAttribute("width"), `${svgFile}: wrong width`).toEqual("500");
     expect(svgElem?.getAttribute("height"), `${svgFile}: wrong height`).toEqual("500");
     expect(svgElem?.getAttribute("viewBox"), `${svgFile}: wrong viewBox`).toEqual("0 0 500 500");
@@ -68,10 +75,8 @@ test.concurrent("SVGs have no hardcoded colors", async () => {
       await file(svgFile).text(),
     ).window.document.querySelector("svg");
 
-    expect(svgElem, `${svgFile}: no svg`).not.toBeNull();
-    if (svgElem !== null) {
-      checkElement(svgElem);
-    }
+    expectNotNull(svgElem, `${svgFile}: no svg`);
+    checkElement(svgElem);
   }
 });
 
@@ -81,10 +86,10 @@ test.concurrent("SVGs are well-formed with no extraneous attributes", async () =
       await file(svgFile).text(),
     ).window.document.querySelector("svg");
 
-    expect(svgElem, `${svgFile}: no svg`).not.toBeNull();
-    expect(svgElem?.getAttribute("xmlns"), `${svgFile}: bad xmlns`)
+    expectNotNull(svgElem, `${svgFile}: no svg`);
+    expect(svgElem.getAttribute("xmlns"), `${svgFile}: bad xmlns`)
       .toBe("http://www.w3.org/2000/svg");
-    expect(svgElem?.getAttributeNames().sort(), `${svgFile}: wrong attributes`)
+    expect(svgElem.getAttributeNames().sort(), `${svgFile}: wrong attributes`)
       .toEqual(["width", "height", "viewBox", "xmlns"].sort());
   }
 });
@@ -105,9 +110,7 @@ test.concurrent("SVGs only have allowed elements", async () => {
       await file(svgFile).text(),
     ).window.document.querySelector("svg");
 
-    expect(svgElem, `${svgFile}: no svg`).not.toBeNull();
-    if (svgElem !== null) {
-      checkElement(svgElem);
-    }
+    expectNotNull(svgElem, `${svgFile}: no svg`);
+    checkElement(svgElem);
   }
 });