refactor: improve the REPL (#24204)

This PR improves the Electron REPL experience. It adds a welcome message to the REPL to let users know what versions of Node.js and Electron they're running, as well as overriding the completer function in the REPL to preload and add tab autocompletion for Electron's own modules.

default_app/main.ts

@@ -1,8 +1,9 @@
-import { app, dialog } from 'electron';
+import * as electron from 'electron';
 
 import * as fs from 'fs';
 import * as path from 'path';
 import * as url from 'url';
+const { app, dialog } = electron;
 
 type DefaultAppOptions = {
   file: null | string;
@@ -145,13 +146,57 @@ function startRepl () {
     process.exit(1);
   }
 
-  // prevent quitting
+  // Prevent quitting.
   app.on('window-all-closed', () => {});
 
-  const repl = require('repl');
+  const GREEN = '32';
-  repl.start('> ').on('exit', () => {
+  const colorize = (color: string, s: string) => `\x1b[${color}m${s}\x1b[0m`;
+  const electronVersion = colorize(GREEN, `v${process.versions.electron}`);
+  const nodeVersion = colorize(GREEN, `v${process.versions.node}`);
+
+  console.info(`
+    Welcome to the Electron.js REPL \\[._.]/
+
+    You can access all Electron.js modules here as well as Node.js modules.
+    Using: Node.js ${nodeVersion} and Electron.js ${electronVersion}
+  `);
+
+  const { REPLServer } = require('repl');
+  const repl = new REPLServer({
+    prompt: '> '
+  }).on('exit', () => {
     process.exit(0);
   });
+
+  // Copied from node/lib/repl.js. For better DX, we don't want to
+  // show e.g 'contentTracing' at a higher priority than 'const', so
+  // we only trigger custom tab-completion when no common words are
+  // potentially matches.
+  const commonWords = [
+    'async', 'await', 'break', 'case', 'catch', 'const', 'continue',
+    'debugger', 'default', 'delete', 'do', 'else', 'export', 'false',
+    'finally', 'for', 'function', 'if', 'import', 'in', 'instanceof', 'let',
+    'new', 'null', 'return', 'switch', 'this', 'throw', 'true', 'try',
+    'typeof', 'var', 'void', 'while', 'with', 'yield'
+  ];
+
+  const electronBuiltins = [...Object.keys(electron), 'original-fs', 'electron'];
+
+  const defaultComplete = repl.completer;
+  repl.completer = (line: string, callback: Function) => {
+    const lastSpace = line.lastIndexOf(' ');
+    const currentSymbol = line.substring(lastSpace + 1, repl.cursor);
+
+    const filterFn = (c: string) => c.startsWith(currentSymbol);
+    const ignores = commonWords.filter(filterFn);
+    const hits = electronBuiltins.filter(filterFn);
+
+    if (!ignores.length && hits.length) {
+      callback(null, [hits, currentSymbol]);
+    } else {
+      defaultComplete.apply(repl, [line, callback]);
+    }
+  };
 }
 
 // Start the specified app if there is one specified in command line, otherwise