chore: enable coverage

Had to downgrade jest to v24. Coverage is broken for non-js files in
v2: jestjs/jest#9490

.gitignore

@@ -2,3 +2,4 @@
 node_modules
 public/bundle.*
 public/images/
+/coverage/

jest.config.js

@@ -8,12 +8,17 @@ module.exports = {
   // The directory where Jest should output its coverage files
   coverageDirectory: "coverage",
 
+  // Only cover *.svelte files
+  collectCoverageFrom: [
+    '<rootDir>/src/**/*.svelte'
+  ],
+
   // preprocess *.ts test files with ts-jest
   preset: 'ts-jest',
 
   // look for tests in *.spec.ts files
   testMatch: [
-    '**/*.spec.ts'
+    '<rootDir>/src/**/*.spec.ts'
   ],
 
   // preprocess *.svelte files with svelte-processor (configured via svelte.config.js)

package.json

@@ -9,7 +9,7 @@
     "cross-env": "^5.2.0",
     "css-loader": "^2.1.1",
     "file-loader": "^6.0.0",
-    "jest": "^25.1.0",
+    "jest": "24",
     "mini-css-extract-plugin": "^0.6.0",
     "serve": "^11.0.0",
     "style-loader": "^0.23.1",
@@ -26,6 +26,7 @@
   "scripts": {
     "build": "cross-env NODE_ENV=production webpack",
     "dev": "webpack-dev-server --content-base public",
-    "test": "jest"
+    "test": "jest",
+    "coverage": "jest --coverage"
   }
 }

src/App.svelte

@@ -1,5 +1,5 @@
 <script>
-	import Button from './Button';
+	import Button from './Button.svelte';
 
 	export let name;
 </script>

src/Button.svelte

@@ -8,6 +8,10 @@
   function handleClick() {
     buttonText = 'Button Clicked'
   }
+
+  // These lines are not covered by tests:
+	const add = (a:number, b:number) => a+b;
+  add(1,2)
 </script>
 
 <h1>Hello {name}!</h1>