shows location of error in code when using cli (revised again)

cli.js

@@ -18,6 +18,7 @@ const argv = require('yargs')
   }).argv;
 const glob = require('globby').sync;
 const read = require('read-input');
+const chalk = require('chalk');
 const ejsLint = require('./index.js');
 
 const opts = {
@@ -33,7 +34,9 @@ read(glob(argv._))
         errored = true;
         let message = `${err.message} (${err.line}:${err.column})`;
         if (file.name) message += ` in ${file.name}`;
+        message += `\n${errorContext(err, file)}`;
         console.error(message);
+
       }
     });
     if (errored) process.exit(1);
@@ -42,3 +45,24 @@ read(glob(argv._))
     console.error(err);
     process.exit(1);
   });
+
+function errorContext(err, file) {
+  const lines = file.data.split(/\r?\n/);
+  const lineText = lines[err.line - 1];
+  const before = lineText.substr(0, err.column - 1);
+  const duringText = lineText.substr(err.column - 1, 1);
+  const during = chalk.bgRed(duringText);
+  const after = lineText.substr(err.column);
+  const caret = '^';
+  const lineBreak = '\n';
+  const caretLine = addSpaces(err.column - 1) + caret;
+  return before + during + after + lineBreak +caretLine;
+}
+
+function addSpaces(n) {
+  let str = '';
+  for (let i=0; i<n; i++) {
+    str += ' ';
+  }
+  return str;
+}

package.json

@@ -23,6 +23,8 @@
     "unit": "nyc --check-coverage mocha --ui tdd --check-leaks"
   },
   "dependencies": {
+    "chalk": "^4.0.0",
+    "colors": "^1.4.0",
     "ejs": "3.0.1",
     "ejs-include-regex": "^1.0.0",
     "globby": "^11.0.0",

test/cli.js

@@ -15,10 +15,11 @@ suite('cli', () => {
   });
   test('invalid input', (done) => {
     execFile(ejslint, ['test/fixtures/invalid.ejs'], (err, stdout, stderr) => {
+      const expectedContext = `\n<% ] %>\n   ^`;
       assert.equal(err.code, 1, 'expected exit code of 1');
       assert.equal(
         stderr.trim(),
-        'Unexpected token (3:4) in test/fixtures/invalid.ejs',
+        `Unexpected token (3:4) in test/fixtures/invalid.ejs${expectedContext}`,
       );
       done();
     });