rewrite in javascript

node_modules/firstline/index.js

@@ -0,0 +1,30 @@
+'use strict';
+
+const fs = require('fs');
+
+module.exports = (path, usrOpts) => {
+  const opts = {
+    encoding: 'utf8',
+    lineEnding: '\n'
+  };
+  Object.assign(opts, usrOpts);
+  return new Promise((resolve, reject) => {
+    const rs = fs.createReadStream(path, {encoding: opts.encoding});
+    let acc = '';
+    let pos = 0;
+    let index;
+    rs
+      .on('data', chunk => {
+        index = chunk.indexOf(opts.lineEnding);
+        acc += chunk;
+        if (index === -1) {
+          pos += chunk.length;
+        } else {
+          pos += index;
+          rs.close();
+        }
+      })
+      .on('close', () => resolve(acc.slice(acc.charCodeAt(0) === 0xFEFF ? 1 : 0, pos)))
+      .on('error', err => reject(err));
+  });
+};