diff --git a/tools/combine.js b/tools/combine.js
index a6ef32e..808f574 100644
--- a/tools/combine.js
+++ b/tools/combine.js
@@ -1,19 +1,19 @@
 // This is a mess. I know.
 
-var path = require('path');
-var fs = require('fs');
+const path = require('path');
+const fs = require('fs');
 
 // Parse command line options.
 
-var files = [];
-var includeDirs = [];
-var stripGuardsEnabled = false;
+const files = [];
+const includeDirs = [];
+let stripGuardsEnabled = false;
 
 process.argv
   .slice(2)
   .forEach((arg) => {
     let match;
-    if (match = /^-I(.*)$/.exec(arg))
+    if ((match = /^-I(.*)$/.exec(arg)))
       includeDirs.push(match[1]);
     else if (arg === '--strip-guards')
       stripGuardsEnabled = true;
@@ -21,14 +21,14 @@ process.argv
       files.push(arg);
   });
 
-var included = {};
+const included = {};
 
 function readFileWithPath(fileName, dirs) {
   if (/[/\\]/.test(fileName))
     return fs.readFileSync(fileName, 'utf8');
 
   for (let i = 0; i < dirs.length; i++) {
-    var filePath = path.resolve(dirs[i], fileName);
+    const filePath = path.resolve(dirs[i], fileName);
     try {
       return fs.readFileSync(filePath, 'utf8');
     } catch (e) {
@@ -36,24 +36,24 @@ function readFileWithPath(fileName, dirs) {
     }
   }
 
-  var err = new Error('file not found: ' + fileName);
-  err.code = 'ENOENT'
+  const err = new Error('file not found: ' + fileName);
+  err.code = 'ENOENT';
   throw err;
 }
 
 function strip_guards(filename, source) {
-  var lead_comments_re = /^(\s*\/\*((?!\*\/)[\s\S])*\*\/)*\s*/;
-  var trail_comments_re = /(\s*\/\*((?!\*\/)[\s\S])*\*\/)*\s*$/;
-  var lead_guards_re = /^#ifndef\s+(\w+)\s+#define\s+(\w+)\s+/;
-  var trail_guards_re = /#endif$/;
+  const lead_comments_re = /^(\s*\/\*((?!\*\/)[\s\S])*\*\/)*\s*/;
+  const trail_comments_re = /(\s*\/\*((?!\*\/)[\s\S])*\*\/)*\s*$/;
+  const lead_guards_re = /^#ifndef\s+(\w+)\s+#define\s+(\w+)\s+/;
+  const trail_guards_re = /#endif$/;
 
   // Strip leading and trailing comments and whitespace.
   source = source.replace(lead_comments_re, '');
   source = source.replace(trail_comments_re, '');
 
   // Find include guards.
-  var lead_guards = lead_guards_re.exec(source);
-  var trail_guards = trail_guards_re.exec(source);
+  const lead_guards = lead_guards_re.exec(source);
+  const trail_guards = trail_guards_re.exec(source);
 
   // Remove include guards, if found.
   if (lead_guards && trail_guards && lead_guards[1] == lead_guards[2]) {
@@ -69,41 +69,37 @@ function strip_guards(filename, source) {
 }
 
 function lines(filename, strip) {
-  var source = readFileWithPath(filename, ['.'].concat(includeDirs));
+  let source = readFileWithPath(filename, ['.'].concat(includeDirs));
   if (strip) source = strip_guards(filename, source);
   return source.split(/\r?\n/g);
 }
 
-function comment(s) {
-  return '';  //'/* ' + s + '*/'
-}
-
 function include(line, filename) {
-  var key = path.basename(filename).toLowerCase();
+  const key = path.basename(filename).toLowerCase();
   if (included[key])
-    return comment(line);
+    return ''; // Included earlier.
   console.error('Including: ' + key);
   included[key] = true;
   return lines(filename, true);
 }
 
 function add(filename) {
-  var key = path.basename(filename).toLowerCase();
+  const key = path.basename(filename).toLowerCase();
   console.error('Adding: ' + key);
   included[key] = true;
   return lines(filename, stripGuardsEnabled);
 }
 
-var sys_included = {};
+const sys_included = {};
 function include_sys(line, filename) {
-  var key = path.basename(filename).toLowerCase();
+  const key = path.basename(filename).toLowerCase();
   if (sys_included[key])
-    return comment(line);
+    return ''; // Included earlier.
 
   sys_included[key] = true;
 }
 
-var source = [];
+let source = [];
 
 source = source.concat('/*')
   .concat(fs.readFileSync('LICENSE', 'utf8')
@@ -118,23 +114,23 @@ source = source.concat('/*')
   .concat(' */')
   .concat('');
 
-for (var i = 0; i < files.length; i++) {
-  var filename = files[i];
+for (let i = 0; i < files.length; i++) {
+  const filename = files[i];
   source = source.concat(add(filename));
 }
 
-var patterns = [
+const patterns = [
   { re: /^\s*#include\s*"([^"]*)".*$/, fn: include },
   { re: /^\s*#include\s*<([^"]*)>.*$/, fn: include_sys }
-]
+];
 
-restart: for (var lno = 0; lno < source.length;) {
-  for (var i in patterns) {
-    var line = source[lno];
-    var pattern = patterns[i];
-    var match = pattern.re.exec(line);
+restart: for (let lno = 0; lno < source.length;) {
+  for (const i in patterns) {
+    const line = source[lno];
+    const pattern = patterns[i];
+    const match = pattern.re.exec(line);
     if (match) {
-      var repl = pattern.fn.apply(null, match);
+      const repl = pattern.fn.apply(null, match);
       if (repl != null && repl !== line) {
         source.splice.apply(source, [lno, 1].concat(repl));
         continue restart;
diff --git a/tools/run-tests.js b/tools/run-tests.js
index 3aa9652..33d69de 100644
--- a/tools/run-tests.js
+++ b/tools/run-tests.js
@@ -22,7 +22,7 @@ function run_tests(test_exes, num = 0, fail_count = 0) {
 
   child.on('exit', (code) => {
     if (code === 0) {
-      console.log('  PASS')
+      console.log('  PASS');
     } else {
       console.log('  FAIL\n' + out);
       fail_count++;
@@ -33,7 +33,7 @@ function run_tests(test_exes, num = 0, fail_count = 0) {
 }
 
 function done(test_exes, fail_count) {
-  let pass_count = test_exes.length - fail_count;
+  const pass_count = test_exes.length - fail_count;
   console.log('  DONE - %d PASSED, %d FAILED', pass_count, fail_count);
 
   process.exit(fail_count == 0 ? 0 : 1);