Add Flow typing

 - Includes workaround for `config-chain` including intentionally broken test files in its repo: facebook/flow#869
 - Includes Hack for externals with Parcel: parcel-bundler/parcel#144

.flowconfig

@@ -0,0 +1,15 @@
+[ignore]
+# https://github.com/facebook/flow/issues/869
+.*/node_modules/config-chain/test/broken.json
+
+[include]
+
+[declarations]
+
+[libs]
+
+[lints]
+
+[options]
+
+[strict]

flow-typed/source-map.js

@@ -0,0 +1,12 @@
+declare module "source-map" {
+  declare class SourceMapConsumer {
+    sources: $ReadOnlyArray<string>,
+
+    destroy(): void;
+    sourceContentFor(url: string): ?string;
+  }
+
+  declare module.exports: {
+    SourceMapConsumer: (file: string | ArrayBuffer) => Promise<SourceMapConsumer>,
+  }
+}

js/buildTree.js

@@ -1,5 +1,14 @@
-export default function buildTree(files) {
-  const tree = {
+// @flow
+
+export type TreeNode = $ReadOnly<{|
+  name: string,
+  path: string,
+  type: 'dir' | 'file',
+  children: Map<string, TreeNode>,
+|}>;
+
+export default function buildTree(files: Array<string>): TreeNode {
+  const tree: TreeNode = {
     name: '/',
     path: '',
     type: 'dir',
@@ -16,6 +25,7 @@ export default function buildTree(files) {
         if (!next) {
           next = {
             name: piece,
+            path: `${curr.path}/${piece}`,
             type: 'dir',
             children: new Map(),
           };
@@ -35,7 +45,11 @@ export default function buildTree(files) {
   let root = tree;
   while (root && root.children.size === 1) {
     const firstKey = root.children.keys().next().value;
-    root = root.children.get(firstKey);
+    const node = firstKey && root.children.get(firstKey);
+    if (!node) {
+      break;
+    }
+    root = node;
   }
 
   return root;

js/components/App.js

@@ -1,9 +1,21 @@
+// @flow
+
+import React from 'react';
+
 import classNames from './App.css';
 import Files from './Files';
 
 import getFilesFromSourceMap from '../getFilesFromSourceMap';
 
-export default class App extends React.Component {
+type Props = {||};
+type State = {|
+  error: ?Error,
+  files: ?Map<string, ?string>,
+  id: number,
+  loading: boolean,
+|};
+
+export default class App extends React.Component<Props, State> {
   state = {
     error: null,
     files: null,
@@ -41,7 +53,7 @@ export default class App extends React.Component {
     );
   }
 
-  _onSelectFile = (evt) => {
+  _onSelectFile = (evt: {target: {files: Array<File>}}) => {
     this.setState(state => ({id: state.id + 1, loading: true}));
     const file = evt.target.files[0];
     getFilesFromSourceMap(file).then(

js/components/Code.js

@@ -1,10 +1,20 @@
+// @flow
+
+// TODO: Flow type this
+import Prism from 'prismjs';
+import React from 'react';
+
 import classNames from './Files.css';
 
-function runPrism(code) {
+function runPrism(code: string): {|__html: string|} {
   return {__html: Prism.highlight(code, Prism.languages.javascript)};
 }
 
-export default class Code extends React.PureComponent {
+type Props = {|
+  code: ?string,
+|};
+
+export default class Code extends React.PureComponent<Props> {
   render() {
     const code = this.props.code || '// File is empty';
     return (

js/components/FileTree.js

@@ -1,12 +1,27 @@
+// @flow
+
+import React from 'react';
+import nullthrows from 'nullthrows';
+
+import type {TreeNode} from '../buildTree';
 import classNames from './Files.css';
 
-export default class FileTree extends React.Component {
+type Props = {|
+  selectedDir: $ReadOnlyArray<string>,
+  selectedFile: ?string,
+  tree: TreeNode,
+
+  onSelectDir: (path: $ReadOnlyArray<string>) => void,
+  onSelectFile: (path: string) => void,
+|};
+
+export default class FileTree extends React.PureComponent<Props> {
   render() {
     const selectedPath = this.props.selectedDir;
 
     let selectedNode = this.props.tree;
     selectedPath.forEach(piece => {
-      selectedNode = selectedNode.children.get(piece);
+      selectedNode = nullthrows(selectedNode.children.get(piece));
     });
 
     const nodes = [];
@@ -50,7 +65,14 @@ export default class FileTree extends React.Component {
   }
 }
 
-class DirNode extends React.Component {
+type DirNodeProps = {|
+  name: string,
+  path: $ReadOnlyArray<string>,
+
+  onSelect: (path: $ReadOnlyArray<string>) => void,
+|}
+
+class DirNode extends React.Component<DirNodeProps> {
   render() {
     return (
       <li><a href="#" onClick={this._onSelect}>{this.props.name}/</a></li>
@@ -63,7 +85,15 @@ class DirNode extends React.Component {
   }
 }
 
-class FileNode extends React.Component {
+type FileNodeProps = {|
+  name: string,
+  node: TreeNode,
+  selectedFile: ?string,
+
+  onSelect: (path: string) => void,
+|};
+
+class FileNode extends React.Component<FileNodeProps> {
   render() {
     let name = this.props.name;
     if (this.props.selectedFile === this.props.node.path) {

js/components/Files.js

@@ -1,10 +1,23 @@
+// @flow
+
+import React from 'react';
+
 import classNames from './Files.css';
 import Code from './Code';
 import FileTree from './FileTree';
 
 import buildTree from '../buildTree';
 
-export default class Files extends React.Component {
+type Props = {|
+  files: Map<string, ?string>,
+|};
+
+type State = {|
+  selectedFile: ?string,
+  selectedDir: $ReadOnlyArray<string>,
+|};
+
+export default class Files extends React.Component<Props, State> {
   state = {
     selectedDir: [],
     selectedFile: null,
@@ -38,11 +51,11 @@ export default class Files extends React.Component {
       : '/' + this.state.selectedDir.join('/') + '/';
   }
 
-  _onSelectDir = (path) => {
+  _onSelectDir = (path: $ReadOnlyArray<string>) => {
     this.setState({selectedDir: path});
   }
 
-  _onSelectFile = (path) => {
+  _onSelectFile = (path: string) => {
     this.setState({selectedFile: path});
   }
 }

js/externals/prism.js

@@ -0,0 +1,2 @@
+// Loaded from CDN
+module.exports = Prism;

js/externals/react-dom.js

@@ -0,0 +1,2 @@
+// Loaded from CDN
+module.exports = ReactDOM;

js/externals/react.js

@@ -0,0 +1,2 @@
+// Loaded from CDN
+module.exports = React;

js/externals/source-map.js

@@ -0,0 +1,2 @@
+// Loaded from CDN
+module.exports = sourceMap;

js/getFilesFromSourceMap.js

@@ -1,4 +1,10 @@
-export default function getFilesFromSourceMap(file) {
+// @flow
+
+import sourceMap from 'source-map';
+
+export default function getFilesFromSourceMap(
+  file: File
+): Promise<Map<string, ?string>> {
   return new Promise((resolve, reject) => {
     const fileReader = new FileReader();
     fileReader.onload = () => {

js/index.js

@@ -1,6 +1,13 @@
+// @flow
+
+import React from 'react';
+import ReactDOM from 'react-dom';
+import sourceMap from 'source-map';
+import nullthrows from 'nullthrows';
+
 import App from './components/App';
 
 sourceMap.SourceMapConsumer.initialize({
   'lib/mappings.wasm': 'https://unpkg.com/source-map@0.7.3/lib/mappings.wasm',
 });
-ReactDOM.render(<App />, document.getElementById('app'));
+ReactDOM.render(<App />, nullthrows(document.getElementById('app')));

package.json

@@ -6,11 +6,23 @@
     "build": "parcel build ./index.html",
     "watch": "parcel ./index.html"
   },
+  "alias": {
+    "prismjs": "./js/externals/prism",
+    "react": "./js/externals/react",
+    "react-dom": "./js/externals/react-dom",
+    "source-map": "./js/externals/source-map"
+  },
   "devDependencies": {
     "autoprefixer": "^9.1.3",
+    "babel-preset-flow": "^6.23.0",
     "babel-preset-react-app": "^3.1.2",
+    "flow-bin": "^0.80.0",
+    "nullthrows": "^1.0.2",
     "parcel-bundler": "^1.9.7",
     "postcss-modules": "^1.3.2",
+    "prismjs": "^1.15.0",
+    "react": "^16.0.0",
+    "react-dom": "^16.4.2",
     "sanitize.css": "^7.0.1",
     "source-map": "^0.7.3"
   }