diff --git a/external/builder/.eslintrc b/external/builder/.eslintrc
new file mode 100644
index 000000000..4a0b538f6
--- /dev/null
+++ b/external/builder/.eslintrc
@@ -0,0 +1,10 @@
+{
+  "extends": [
+    "../.eslintrc"
+  ],
+
+  "rules": {
+    // ECMAScript 6
+    "no-var": "error",
+  },
+}
diff --git a/external/builder/builder.js b/external/builder/builder.js
index 2454f3da5..a3ed4e956 100644
--- a/external/builder/builder.js
+++ b/external/builder/builder.js
@@ -1,6 +1,6 @@
 "use strict";
 
-var fs = require("fs"),
+const fs = require("fs"),
   path = require("path"),
   vm = require("vm");
 
@@ -33,18 +33,23 @@ var fs = require("fs"),
  * //#endif
  */
 function preprocess(inFilename, outFilename, defines) {
+  let lineNumber = 0;
+  function loc() {
+    return fs.realpathSync(inFilename) + ":" + lineNumber;
+  }
+
   // TODO make this really read line by line.
-  var lines = fs.readFileSync(inFilename).toString().split("\n");
-  var totalLines = lines.length;
-  var out = "";
-  var i = 0;
+  const lines = fs.readFileSync(inFilename).toString().split("\n");
+  const totalLines = lines.length;
+  let out = "";
+  let i = 0;
   function readLine() {
     if (i < totalLines) {
       return lines[i++];
     }
     return null;
   }
-  var writeLine =
+  const writeLine =
     typeof outFilename === "function"
       ? outFilename
       : function (line) {
@@ -70,10 +75,10 @@ function preprocess(inFilename, outFilename, defines) {
     }
   }
   function include(file) {
-    var realPath = fs.realpathSync(inFilename);
-    var dir = path.dirname(realPath);
+    const realPath = fs.realpathSync(inFilename);
+    const dir = path.dirname(realPath);
     try {
-      var fullpath;
+      let fullpath;
       if (file.indexOf("$ROOT/") === 0) {
         fullpath = path.join(
           __dirname,
@@ -103,28 +108,25 @@ function preprocess(inFilename, outFilename, defines) {
   }
 
   // not inside if or else (process lines)
-  var STATE_NONE = 0;
+  const STATE_NONE = 0;
   // inside if, condition false (ignore until #else or #endif)
-  var STATE_IF_FALSE = 1;
+  const STATE_IF_FALSE = 1;
   // inside else, #if was false, so #else is true (process lines until #endif)
-  var STATE_ELSE_TRUE = 2;
+  const STATE_ELSE_TRUE = 2;
   // inside if, condition true (process lines until #else or #endif)
-  var STATE_IF_TRUE = 3;
+  const STATE_IF_TRUE = 3;
   // inside else or elif, #if/#elif was true, so following #else or #elif is
   // false (ignore lines until #endif)
-  var STATE_ELSE_FALSE = 4;
+  const STATE_ELSE_FALSE = 4;
+
+  let line;
+  let state = STATE_NONE;
+  const stack = [];
+  const control = /^(?:\/\/|<!--)\s*#(if|elif|else|endif|expand|include|error)\b(?:\s+(.*?)(?:-->)?$)?/;
 
-  var line;
-  var state = STATE_NONE;
-  var stack = [];
-  var control = /^(?:\/\/|<!--)\s*#(if|elif|else|endif|expand|include|error)\b(?:\s+(.*?)(?:-->)?$)?/;
-  var lineNumber = 0;
-  var loc = function () {
-    return fs.realpathSync(inFilename) + ":" + lineNumber;
-  };
   while ((line = readLine()) !== null) {
     ++lineNumber;
-    var m = control.exec(line);
+    const m = control.exec(line);
     if (m) {
       switch (m[1]) {
         case "if":
@@ -205,27 +207,27 @@ function preprocessCSS(mode, source, destination) {
     return content.replace(
       /^\s*@import\s+url\(([^)]+)\);\s*$/gm,
       function (all, url) {
-        var file = path.join(path.dirname(baseUrl), url);
-        var imported = fs.readFileSync(file, "utf8").toString();
+        const file = path.join(path.dirname(baseUrl), url);
+        const imported = fs.readFileSync(file, "utf8").toString();
         return expandImports(imported, file);
       }
     );
   }
 
   function removePrefixed(content, hasPrefixedFilter) {
-    var lines = content.split(/\r?\n/g);
-    var i = 0;
+    const lines = content.split(/\r?\n/g);
+    let i = 0;
     while (i < lines.length) {
-      var line = lines[i];
+      const line = lines[i];
       if (!hasPrefixedFilter(line)) {
         i++;
         continue;
       }
       if (/\{\s*$/.test(line)) {
-        var bracketLevel = 1;
-        var j = i + 1;
+        let bracketLevel = 1;
+        let j = i + 1;
         while (j < lines.length && bracketLevel > 0) {
-          var checkBracket = /([{}])\s*$/.exec(lines[j]);
+          const checkBracket = /([{}])\s*$/.exec(lines[j]);
           if (checkBracket) {
             if (checkBracket[1] === "{") {
               bracketLevel++;
@@ -263,7 +265,7 @@ function preprocessCSS(mode, source, destination) {
     throw new Error("Invalid CSS preprocessor mode");
   }
 
-  var content = fs.readFileSync(source, "utf8").toString();
+  let content = fs.readFileSync(source, "utf8").toString();
   content = expandImports(content, source);
   if (mode === "mozcentral") {
     content = removePrefixed(content, hasPrefixedMozcentral);
@@ -277,11 +279,11 @@ exports.preprocessCSS = preprocessCSS;
  * the first.
  */
 function merge(defaults, defines) {
-  var ret = {};
-  for (var key in defaults) {
+  const ret = Object.create(null);
+  for (const key in defaults) {
     ret[key] = defaults[key];
   }
-  for (key in defines) {
+  for (const key in defines) {
     ret[key] = defines[key];
   }
   return ret;
diff --git a/external/builder/preprocessor2.js b/external/builder/preprocessor2.js
index eb991a032..b918b1df2 100644
--- a/external/builder/preprocessor2.js
+++ b/external/builder/preprocessor2.js
@@ -1,13 +1,13 @@
 "use strict";
 
-var acorn = require("acorn");
-var escodegen = require("escodegen");
-var vm = require("vm");
-var fs = require("fs");
-var path = require("path");
+const acorn = require("acorn");
+const escodegen = require("escodegen");
+const vm = require("vm");
+const fs = require("fs");
+const path = require("path");
 
-var PDFJS_PREPROCESSOR_NAME = "PDFJSDev";
-var ROOT_PREFIX = "$ROOT/";
+const PDFJS_PREPROCESSOR_NAME = "PDFJSDev";
+const ROOT_PREFIX = "$ROOT/";
 const ACORN_ECMA_VERSION = 2021;
 
 function isLiteral(obj, value) {
@@ -27,21 +27,21 @@ function evalWithDefines(code, defines, loc) {
 
 function handlePreprocessorAction(ctx, actionName, args, loc) {
   try {
-    var arg;
+    let arg;
     switch (actionName) {
       case "test":
         arg = args[0];
         if (!arg || arg.type !== "Literal" || typeof arg.value !== "string") {
           throw new Error("No code for testing is given");
         }
-        var isTrue = !!evalWithDefines(arg.value, ctx.defines);
+        const isTrue = !!evalWithDefines(arg.value, ctx.defines);
         return { type: "Literal", value: isTrue, loc };
       case "eval":
         arg = args[0];
         if (!arg || arg.type !== "Literal" || typeof arg.value !== "string") {
           throw new Error("No code for eval is given");
         }
-        var result = evalWithDefines(arg.value, ctx.defines);
+        const result = evalWithDefines(arg.value, ctx.defines);
         if (
           typeof result === "boolean" ||
           typeof result === "string" ||
@@ -62,14 +62,14 @@ function handlePreprocessorAction(ctx, actionName, args, loc) {
         if (!arg || arg.type !== "Literal" || typeof arg.value !== "string") {
           throw new Error("Path to JSON is not provided");
         }
-        var jsonPath = arg.value;
+        let jsonPath = arg.value;
         if (jsonPath.indexOf(ROOT_PREFIX) === 0) {
           jsonPath = path.join(
             ctx.rootPath,
             jsonPath.substring(ROOT_PREFIX.length)
           );
         }
-        var jsonContent = fs.readFileSync(jsonPath).toString();
+        const jsonContent = fs.readFileSync(jsonPath).toString();
         const parsedJSON = acorn.parse("(" + jsonContent + ")", {
           ecmaVersion: ACORN_ECMA_VERSION,
         });
@@ -103,7 +103,7 @@ function postprocessNode(ctx, node) {
         ctx.map &&
         ctx.map[node.source.value]
       ) {
-        var newValue = ctx.map[node.source.value];
+        const newValue = ctx.map[node.source.value];
         node.source.value = node.source.raw = newValue;
       }
       break;
@@ -175,7 +175,7 @@ function postprocessNode(ctx, node) {
               case "string":
               case "boolean":
               case "number":
-                var equal = node.left.value === node.right.value;
+                const equal = node.left.value === node.right.value;
                 return {
                   type: "Literal",
                   value: (node.operator[0] === "=") === equal,
@@ -193,7 +193,7 @@ function postprocessNode(ctx, node) {
         node.callee.property.type === "Identifier"
       ) {
         // PDFJSDev.xxxx(arg1, arg2, ...) => transform
-        var action = node.callee.property.name;
+        const action = node.callee.property.name;
         return handlePreprocessorAction(ctx, action, node.arguments, node.loc);
       }
       // require('string')
@@ -205,12 +205,12 @@ function postprocessNode(ctx, node) {
         ctx.map &&
         ctx.map[node.arguments[0].value]
       ) {
-        var requireName = node.arguments[0];
+        const requireName = node.arguments[0];
         requireName.value = requireName.raw = ctx.map[requireName.value];
       }
       break;
     case "BlockStatement":
-      var subExpressionIndex = 0;
+      let subExpressionIndex = 0;
       while (subExpressionIndex < node.body.length) {
         switch (node.body[subExpressionIndex].type) {
           case "EmptyStatement":
@@ -219,7 +219,7 @@ function postprocessNode(ctx, node) {
             continue;
           case "BlockStatement":
             // Block statements inside a block are moved to the parent one.
-            var subChildren = node.body[subExpressionIndex].body;
+            const subChildren = node.body[subExpressionIndex].body;
             Array.prototype.splice.apply(
               node.body,
               [subExpressionIndex, 1].concat(subChildren)
@@ -240,7 +240,7 @@ function postprocessNode(ctx, node) {
       break;
     case "FunctionDeclaration":
     case "FunctionExpression":
-      var block = node.body;
+      const block = node.body;
       if (
         block.body.length > 0 &&
         block.body[block.body.length - 1].type === "ReturnStatement" &&
@@ -262,14 +262,14 @@ function fixComments(ctx, node) {
   delete node.trailingComments;
   // Removes ESLint and other service comments.
   if (node.leadingComments) {
-    var CopyrightRegExp = /\bcopyright\b/i;
-    var BlockCommentRegExp = /^\s*(globals|eslint|falls through)\b/;
-    var LineCommentRegExp = /^\s*eslint\b/;
+    const CopyrightRegExp = /\bcopyright\b/i;
+    const BlockCommentRegExp = /^\s*(globals|eslint|falls through)\b/;
+    const LineCommentRegExp = /^\s*eslint\b/;
 
-    var i = 0;
+    let i = 0;
     while (i < node.leadingComments.length) {
-      var type = node.leadingComments[i].type;
-      var value = node.leadingComments[i].value;
+      const type = node.leadingComments[i].type;
+      const value = node.leadingComments[i].value;
 
       if (ctx.saveComments === "copyright") {
         // Remove all comments, except Copyright notices and License headers.
@@ -291,8 +291,8 @@ function fixComments(ctx, node) {
 
 function traverseTree(ctx, node) {
   // generic node processing
-  for (var i in node) {
-    var child = node[i];
+  for (const i in node) {
+    const child = node[i];
     if (typeof child === "object" && child !== null && child.type) {
       const result = traverseTree(ctx, child);
       if (result !== child) {
@@ -321,18 +321,18 @@ function traverseTree(ctx, node) {
 }
 
 function preprocessPDFJSCode(ctx, code) {
-  var format = ctx.format || {
+  const format = ctx.format || {
     indent: {
       style: " ",
     },
   };
-  var parseOptions = {
+  const parseOptions = {
     ecmaVersion: ACORN_ECMA_VERSION,
     locations: true,
     sourceFile: ctx.sourceFile,
     sourceType: "module",
   };
-  var codegenOptions = {
+  const codegenOptions = {
     format,
     parse(input) {
       return acorn.parse(input, { ecmaVersion: ACORN_ECMA_VERSION });
@@ -340,7 +340,7 @@ function preprocessPDFJSCode(ctx, code) {
     sourceMap: ctx.sourceMap,
     sourceMapWithCode: ctx.sourceMap,
   };
-  var syntax = acorn.parse(code, parseOptions);
+  const syntax = acorn.parse(code, parseOptions);
   traverseTree(ctx, syntax);
   return escodegen.generate(syntax, codegenOptions);
 }
diff --git a/external/builder/test-fixtures.js b/external/builder/test-fixtures.js
index f23480abf..2cc059beb 100644
--- a/external/builder/test-fixtures.js
+++ b/external/builder/test-fixtures.js
@@ -1,13 +1,13 @@
 "use strict";
 
-var builder = require("./builder");
-var fs = require("fs");
-var path = require("path");
+const builder = require("./builder");
+const fs = require("fs");
+const path = require("path");
 
-var errors = 0;
+let errors = 0;
 
-var baseDir = path.join(__dirname, "fixtures");
-var files = fs
+const baseDir = path.join(__dirname, "fixtures");
+const files = fs
   .readdirSync(baseDir)
   .filter(function (name) {
     return /-expected\./.test(name);
@@ -16,22 +16,22 @@ var files = fs
     return path.join(baseDir, name);
   });
 files.forEach(function (expectationFilename) {
-  var inFilename = expectationFilename.replace("-expected", "");
-  var expectation = fs
+  const inFilename = expectationFilename.replace("-expected", "");
+  const expectation = fs
     .readFileSync(expectationFilename)
     .toString()
     .trim()
     .replace(/__filename/g, fs.realpathSync(inFilename));
-  var outLines = [];
+  const outLines = [];
 
-  var outFilename = function (line) {
+  const outFilename = function (line) {
     outLines.push(line);
   };
-  var defines = {
+  const defines = {
     TRUE: true,
     FALSE: false,
   };
-  var out;
+  let out;
   try {
     builder.preprocess(inFilename, outFilename, defines);
     out = outLines.join("\n").trim();
diff --git a/external/builder/test-fixtures_esprima.js b/external/builder/test-fixtures_esprima.js
index 655b965cc..35128aee5 100644
--- a/external/builder/test-fixtures_esprima.js
+++ b/external/builder/test-fixtures_esprima.js
@@ -1,13 +1,13 @@
 "use strict";
 
-var p2 = require("./preprocessor2.js");
-var fs = require("fs");
-var path = require("path");
+const p2 = require("./preprocessor2.js");
+const fs = require("fs");
+const path = require("path");
 
-var errors = 0;
+let errors = 0;
 
-var baseDir = path.join(__dirname, "fixtures_esprima");
-var files = fs
+const baseDir = path.join(__dirname, "fixtures_esprima");
+const files = fs
   .readdirSync(baseDir)
   .filter(function (name) {
     return /-expected\./.test(name);
@@ -16,29 +16,29 @@ var files = fs
     return path.join(baseDir, name);
   });
 files.forEach(function (expectationFilename) {
-  var inFilename = expectationFilename.replace("-expected", "");
-  var expectation = fs
+  const inFilename = expectationFilename.replace("-expected", "");
+  const expectation = fs
     .readFileSync(expectationFilename)
     .toString()
     .trim()
     .replace(/__filename/g, fs.realpathSync(inFilename));
-  var input = fs.readFileSync(inFilename).toString();
+  const input = fs.readFileSync(inFilename).toString();
 
-  var defines = {
+  const defines = {
     TRUE: true,
     FALSE: false,
     OBJ: { obj: { i: 1 }, j: 2 },
     TEXT: "text",
   };
-  var map = {
+  const map = {
     "import-alias": "import-name",
   };
-  var ctx = {
+  const ctx = {
     defines,
     map,
     rootPath: __dirname + "/../..",
   };
-  var out;
+  let out;
   try {
     out = p2.preprocessPDFJSCode(ctx, input);
   } catch (e) {