diff --git a/eslint/babel-eslint-parser/.eslintrc.js b/eslint/babel-eslint-parser/.eslintrc.js
index 64dc12e693..169516cb3f 100644
--- a/eslint/babel-eslint-parser/.eslintrc.js
+++ b/eslint/babel-eslint-parser/.eslintrc.js
@@ -5,12 +5,22 @@ module.exports = {
     "prettier"
   ],
   rules: {
-    "no-var": 0,
-    "max-len": 0,
+    "max-len": "off",
+    "strict": "error",
     "prettier/prettier": "error",
   },
   env: {
     node: true,
-    mocha: true
-  }
+  },
+  parserOptions: {
+    sourceType: "script",
+  },
+  overrides: [
+    {
+      files: ["test/**/*"],
+      env: {
+        mocha: true
+      }
+    }
+  ]
 };
diff --git a/eslint/babel-eslint-parser/.npmrc b/eslint/babel-eslint-parser/.npmrc
new file mode 100644
index 0000000000..c1ca392fea
--- /dev/null
+++ b/eslint/babel-eslint-parser/.npmrc
@@ -0,0 +1 @@
+package-lock = false
diff --git a/eslint/babel-eslint-parser/lib/babylon-to-espree/attachComments.js b/eslint/babel-eslint-parser/lib/babylon-to-espree/attachComments.js
index 8c608a45ad..6e19875384 100644
--- a/eslint/babel-eslint-parser/lib/babylon-to-espree/attachComments.js
+++ b/eslint/babel-eslint-parser/lib/babylon-to-espree/attachComments.js
@@ -3,8 +3,8 @@
 // comment fixes
 module.exports = function(ast, comments, tokens) {
   if (comments.length) {
-    var firstComment = comments[0];
-    var lastComment = comments[comments.length - 1];
+    const firstComment = comments[0];
+    const lastComment = comments[comments.length - 1];
     // fixup program start
     if (!tokens.length) {
       // if no tokens, the program starts at the end of the last comment
@@ -17,7 +17,7 @@ module.exports = function(ast, comments, tokens) {
       }
     } else if (firstComment.start < tokens[0].start) {
       // if there are comments before the first token, the program starts at the first token
-      var token = tokens[0];
+      const token = tokens[0];
       // ast.start = token.start;
       // ast.loc.start.line = token.loc.start.line;
       // ast.loc.start.column = token.loc.start.column;
@@ -25,18 +25,18 @@ module.exports = function(ast, comments, tokens) {
       // estraverse do not put leading comments on first node when the comment
       // appear before the first token
       if (ast.body.length) {
-        var node = ast.body[0];
+        const node = ast.body[0];
         node.leadingComments = [];
-        var firstTokenStart = token.start;
-        var len = comments.length;
-        for (var i = 0; i < len && comments[i].start < firstTokenStart; i++) {
+        const firstTokenStart = token.start;
+        const len = comments.length;
+        for (let i = 0; i < len && comments[i].start < firstTokenStart; i++) {
           node.leadingComments.push(comments[i]);
         }
       }
     }
     // fixup program end
     if (tokens.length) {
-      var lastToken = tokens[tokens.length - 1];
+      const lastToken = tokens[tokens.length - 1];
       if (lastComment.end > lastToken.end) {
         // If there is a comment after the last token, the program ends at the
         // last token and not the comment
diff --git a/eslint/babel-eslint-parser/lib/babylon-to-espree/convertComments.js b/eslint/babel-eslint-parser/lib/babylon-to-espree/convertComments.js
index 17d7117372..57b0e44d4d 100644
--- a/eslint/babel-eslint-parser/lib/babylon-to-espree/convertComments.js
+++ b/eslint/babel-eslint-parser/lib/babylon-to-espree/convertComments.js
@@ -1,8 +1,8 @@
 "use strict";
 
 module.exports = function(comments) {
-  for (var i = 0; i < comments.length; i++) {
-    var comment = comments[i];
+  for (let i = 0; i < comments.length; i++) {
+    const comment = comments[i];
     if (comment.type === "CommentBlock") {
       comment.type = "Block";
     } else if (comment.type === "CommentLine") {
diff --git a/eslint/babel-eslint-parser/lib/babylon-to-espree/index.js b/eslint/babel-eslint-parser/lib/babylon-to-espree/index.js
index 6d6e12bfc0..c0fdf323a8 100644
--- a/eslint/babel-eslint-parser/lib/babylon-to-espree/index.js
+++ b/eslint/babel-eslint-parser/lib/babylon-to-espree/index.js
@@ -1,9 +1,9 @@
 "use strict";
 
-var attachComments = require("./attachComments");
-var convertComments = require("./convertComments");
-var toTokens = require("./toTokens");
-var toAST = require("./toAST");
+const attachComments = require("./attachComments");
+const convertComments = require("./convertComments");
+const toTokens = require("./toTokens");
+const toAST = require("./toAST");
 
 module.exports = function(ast, traverse, tt, code) {
   // convert tokens
diff --git a/eslint/babel-eslint-parser/lib/babylon-to-espree/toAST.js b/eslint/babel-eslint-parser/lib/babylon-to-espree/toAST.js
index b3da41f0cb..e375f69dd1 100644
--- a/eslint/babel-eslint-parser/lib/babylon-to-espree/toAST.js
+++ b/eslint/babel-eslint-parser/lib/babylon-to-espree/toAST.js
@@ -1,10 +1,10 @@
 "use strict";
 
-var t = require("@babel/types");
-var convertComments = require("./convertComments");
+const t = require("@babel/types");
+const convertComments = require("./convertComments");
 
 module.exports = function(ast, traverse, code) {
-  var state = { source: code };
+  const state = { source: code };
 
   // Monkey patch visitor keys in order to be able to traverse the estree nodes
   t.VISITOR_KEYS.Property = t.VISITOR_KEYS.ObjectProperty;
@@ -22,10 +22,10 @@ module.exports = function(ast, traverse, code) {
   delete t.VISITOR_KEYS.MethodDefinition;
 };
 
-var astTransformVisitor = {
+const astTransformVisitor = {
   noScope: true,
   enter(path) {
-    var node = path.node;
+    const node = path.node;
 
     // private var to track original node type
     node._babelType = node.type;
@@ -44,7 +44,7 @@ var astTransformVisitor = {
     }
   },
   exit(path) {
-    var node = path.node;
+    const node = path.node;
 
     if (path.isJSXText()) {
       node.type = "Literal";
@@ -98,8 +98,8 @@ var astTransformVisitor = {
 
     // template string range fixes
     if (path.isTemplateLiteral()) {
-      for (var j = 0; j < node.quasis.length; j++) {
-        var q = node.quasis[j];
+      for (let j = 0; j < node.quasis.length; j++) {
+        const q = node.quasis[j];
         q.range[0] -= 1;
         if (q.tail) {
           q.range[1] += 1;
diff --git a/eslint/babel-eslint-parser/lib/babylon-to-espree/toToken.js b/eslint/babel-eslint-parser/lib/babylon-to-espree/toToken.js
index 44c73529a1..3e0ba453c9 100644
--- a/eslint/babel-eslint-parser/lib/babylon-to-espree/toToken.js
+++ b/eslint/babel-eslint-parser/lib/babylon-to-espree/toToken.js
@@ -1,7 +1,7 @@
 "use strict";
 
 module.exports = function(token, tt, source) {
-  var type = token.type;
+  const type = token.type;
   token.range = [token.start, token.end];
 
   if (type === tt.name) {
@@ -72,7 +72,7 @@ module.exports = function(token, tt, source) {
     token.value = source.slice(token.start, token.end);
   } else if (type === tt.regexp) {
     token.type = "RegularExpression";
-    var value = token.value;
+    const value = token.value;
     token.regex = {
       pattern: value.pattern,
       flags: value.flags,
diff --git a/eslint/babel-eslint-parser/lib/babylon-to-espree/toTokens.js b/eslint/babel-eslint-parser/lib/babylon-to-espree/toTokens.js
index bb30819bac..2927c4b011 100644
--- a/eslint/babel-eslint-parser/lib/babylon-to-espree/toTokens.js
+++ b/eslint/babel-eslint-parser/lib/babylon-to-espree/toTokens.js
@@ -1,7 +1,7 @@
 "use strict";
 
-var convertTemplateType = require("./convertTemplateType");
-var toToken = require("./toToken");
+const convertTemplateType = require("./convertTemplateType");
+const toToken = require("./toToken");
 
 module.exports = function(tokens, tt, code) {
   return convertTemplateType(tokens, tt)
diff --git a/eslint/babel-eslint-parser/lib/parse.js b/eslint/babel-eslint-parser/lib/parse.js
index b23b9dc9fc..0817df4b07 100644
--- a/eslint/babel-eslint-parser/lib/parse.js
+++ b/eslint/babel-eslint-parser/lib/parse.js
@@ -1,16 +1,16 @@
 "use strict";
 
-var babylonToEspree = require("./babylon-to-espree");
-var parse = require("@babel/parser").parse;
-var tt = require("@babel/parser").tokTypes;
-var traverse = require("@babel/traverse").default;
-var codeFrameColumns = require("@babel/code-frame").codeFrameColumns;
+const babylonToEspree = require("./babylon-to-espree");
+const parse = require("@babel/parser").parse;
+const tt = require("@babel/parser").tokTypes;
+const traverse = require("@babel/traverse").default;
+const codeFrameColumns = require("@babel/code-frame").codeFrameColumns;
 
 module.exports = function(code, options) {
   const legacyDecorators =
     options.ecmaFeatures && options.ecmaFeatures.legacyDecorators;
 
-  var opts = {
+  const opts = {
     codeFrame: options.hasOwnProperty("codeFrame") ? options.codeFrame : true,
     sourceType: options.sourceType,
     allowImportExportEverywhere: options.allowImportExportEverywhere, // consistent with espree
@@ -51,7 +51,7 @@ module.exports = function(code, options) {
     ],
   };
 
-  var ast;
+  let ast;
   try {
     ast = parse(code, opts);
   } catch (err) {
diff --git a/eslint/babel-eslint-parser/test/babel-eslint.js b/eslint/babel-eslint-parser/test/babel-eslint.js
index 99b03f25c7..ce270f113b 100644
--- a/eslint/babel-eslint-parser/test/babel-eslint.js
+++ b/eslint/babel-eslint-parser/test/babel-eslint.js
@@ -1,10 +1,12 @@
-var assert = require("assert");
-var babelEslint = require("..");
-var espree = require("espree");
-var escope = require("eslint-scope");
-var util = require("util");
-var unpad = require("dedent");
-var assertImplementsAST = require("./fixtures/assert-implements-ast");
+"use strict";
+
+const assert = require("assert");
+const babelEslint = require("..");
+const espree = require("espree");
+const escope = require("eslint-scope");
+const util = require("util");
+const unpad = require("dedent");
+const assertImplementsAST = require("./fixtures/assert-implements-ast");
 
 function lookup(obj, keypath, backwardsDepth) {
   if (!keypath) {
@@ -21,7 +23,7 @@ function lookup(obj, keypath, backwardsDepth) {
 
 function parseAndAssertSame(code) {
   code = unpad(code);
-  var esAST = espree.parse(code, {
+  const esAST = espree.parse(code, {
     ecmaFeatures: {
       // enable JSX parsing
       jsx: true,
@@ -40,14 +42,14 @@ function parseAndAssertSame(code) {
     ecmaVersion: 2018,
     sourceType: "module",
   });
-  var babylonAST = babelEslint.parseForESLint(code, {
+  const babylonAST = babelEslint.parseForESLint(code, {
     eslintVisitorKeys: true,
     eslintScopeManager: true,
   }).ast;
   try {
     assertImplementsAST(esAST, babylonAST);
   } catch (err) {
-    var traversal = err.message.slice(3, err.message.indexOf(":"));
+    const traversal = err.message.slice(3, err.message.indexOf(":"));
     err.message += unpad(`
       espree:
       ${util.inspect(lookup(esAST, traversal, 2), {
@@ -68,7 +70,7 @@ function parseAndAssertSame(code) {
 describe("babylon-to-espree", () => {
   describe("compatibility", () => {
     it("should allow ast.analyze to be called without options", function() {
-      var esAST = babelEslint.parseForESLint("`test`", {
+      const esAST = babelEslint.parseForESLint("`test`", {
         eslintScopeManager: true,
         eslintVisitorKeys: true,
       }).ast;
@@ -271,7 +273,7 @@ describe("babylon-to-espree", () => {
   // Espree doesn't support the optional chaining operator yet
   it("optional chaining operator (token)", () => {
     const code = "foo?.bar";
-    var babylonAST = babelEslint.parseForESLint(code, {
+    const babylonAST = babelEslint.parseForESLint(code, {
       eslintVisitorKeys: true,
       eslintScopeManager: true,
     }).ast;
@@ -281,7 +283,7 @@ describe("babylon-to-espree", () => {
   // Espree doesn't support the nullish coalescing operator yet
   it("nullish coalescing operator (token)", () => {
     const code = "foo ?? bar";
-    var babylonAST = babelEslint.parseForESLint(code, {
+    const babylonAST = babelEslint.parseForESLint(code, {
       eslintVisitorKeys: true,
       eslintScopeManager: true,
     }).ast;
@@ -291,7 +293,7 @@ describe("babylon-to-espree", () => {
   // Espree doesn't support the pipeline operator yet
   it("pipeline operator (token)", () => {
     const code = "foo |> bar";
-    var babylonAST = babelEslint.parseForESLint(code, {
+    const babylonAST = babelEslint.parseForESLint(code, {
       eslintVisitorKeys: true,
       eslintScopeManager: true,
     }).ast;
diff --git a/eslint/babel-eslint-parser/test/fixtures/assert-implements-ast.js b/eslint/babel-eslint-parser/test/fixtures/assert-implements-ast.js
index 61e77e21b0..90dceb0dba 100644
--- a/eslint/babel-eslint-parser/test/fixtures/assert-implements-ast.js
+++ b/eslint/babel-eslint-parser/test/fixtures/assert-implements-ast.js
@@ -1,3 +1,5 @@
+"use strict"
+
 // Checks if the source ast implements the target ast. Ignores extra keys on source ast
 module.exports = function assertImplementsAST(target, source, path) {
   if (!path) {
@@ -5,13 +7,13 @@ module.exports = function assertImplementsAST(target, source, path) {
   }
 
   function error(text) {
-    var err = new Error(`At ${path.join(".")}: ${text}:`);
+    const err = new Error(`At ${path.join(".")}: ${text}:`);
     err.depth = path.length + 1;
     throw err;
   }
 
-  var typeA = target === null ? "null" : typeof target;
-  var typeB = source === null ? "null" : typeof source;
+  const typeA = target === null ? "null" : typeof target;
+  const typeB = source === null ? "null" : typeof source;
   if (typeA !== typeB) {
     error(
       `have different types (${typeA} !== ${typeB}) (${target} !== ${source})`
@@ -26,9 +28,9 @@ module.exports = function assertImplementsAST(target, source, path) {
         .name} !== ${source.constructor.name}`
     );
   } else if (typeA === "object") {
-    var keysTarget = Object.keys(target);
-    for (var i in keysTarget) {
-      var key = keysTarget[i];
+    const keysTarget = Object.keys(target);
+    for (const i in keysTarget) {
+      const key = keysTarget[i];
       path.push(key);
       assertImplementsAST(target[key], source[key], path);
       path.pop();
diff --git a/eslint/babel-eslint-parser/test/fixtures/preprocess-to-patch.js b/eslint/babel-eslint-parser/test/fixtures/preprocess-to-patch.js
index 1dbfc172a1..8cb4c54fb6 100644
--- a/eslint/babel-eslint-parser/test/fixtures/preprocess-to-patch.js
+++ b/eslint/babel-eslint-parser/test/fixtures/preprocess-to-patch.js
@@ -1,4 +1,5 @@
 "use strict"
+
 const babelEslint = require("../..")
 
 // Apply monkeypatch to eslint-scope.
diff --git a/eslint/babel-eslint-parser/test/integration.js b/eslint/babel-eslint-parser/test/integration.js
index 09e4af9d64..2462cd5d38 100644
--- a/eslint/babel-eslint-parser/test/integration.js
+++ b/eslint/babel-eslint-parser/test/integration.js
@@ -1,16 +1,18 @@
-var assert = require("assert");
-var eslint = require("eslint");
-var fs = require("fs");
-var path = require("path");
+"use strict";
 
-var paths = {
+const assert = require("assert");
+const eslint = require("eslint");
+const fs = require("fs");
+const path = require("path");
+
+const paths = {
   fixtures: path.join(__dirname, "fixtures", "rules"),
 };
 
-var encoding = "utf8";
-var errorLevel = 2;
+const encoding = "utf8";
+const errorLevel = 2;
 
-var baseEslintOpts = {
+const baseEslintOpts = {
   parser: require.resolve(".."),
   parserOptions: {
     sourceType: "script",
@@ -48,10 +50,10 @@ describe("Rules:", () => {
 // describe
 
 function strictSuite() {
-  var ruleId = "strict";
+  const ruleId = "strict";
 
   describe("when set to 'never'", () => {
-    var eslintOpts = Object.assign({}, baseEslintOpts, {
+    const eslintOpts = Object.assign({}, baseEslintOpts, {
       rules: {},
     });
     eslintOpts.rules[ruleId] = [errorLevel, "never"];
@@ -76,7 +78,7 @@ function strictSuite() {
   // describe
 
   describe("when set to 'global'", () => {
-    var eslintOpts = Object.assign({}, baseEslintOpts, {
+    const eslintOpts = Object.assign({}, baseEslintOpts, {
       rules: {},
     });
     eslintOpts.rules[ruleId] = [errorLevel, "global"];
@@ -152,7 +154,7 @@ function strictSuite() {
   // describe
 
   describe("when set to 'function'", () => {
-    var eslintOpts = Object.assign({}, baseEslintOpts, {
+    const eslintOpts = Object.assign({}, baseEslintOpts, {
       rules: {},
     });
     eslintOpts.rules[ruleId] = [errorLevel, "function"];
diff --git a/eslint/babel-eslint-parser/test/non-regression.js b/eslint/babel-eslint-parser/test/non-regression.js
index 4c43b29f3f..fc1d3416be 100644
--- a/eslint/babel-eslint-parser/test/non-regression.js
+++ b/eslint/babel-eslint-parser/test/non-regression.js
@@ -1,7 +1,7 @@
-/*eslint-env mocha*/
 "use strict";
-var eslint = require("eslint");
-var unpad = require("dedent");
+
+const eslint = require("eslint");
+const unpad = require("dedent");
 
 function verifyAndAssertMessagesWithSpecificESLint(
   code,
@@ -11,7 +11,7 @@ function verifyAndAssertMessagesWithSpecificESLint(
   overrideConfig,
   linter
 ) {
-  var config = {
+  const config = {
     parser: require.resolve(".."),
     rules,
     env: {
@@ -30,12 +30,12 @@ function verifyAndAssertMessagesWithSpecificESLint(
   };
 
   if (overrideConfig) {
-    for (var key in overrideConfig) {
+    for (const key in overrideConfig) {
       config[key] = overrideConfig[key];
     }
   }
 
-  var messages = linter.verify(code, config);
+  const messages = linter.verify(code, config);
 
   if (messages.length !== expectedMessages.length) {
     throw new Error(
@@ -46,7 +46,7 @@ function verifyAndAssertMessagesWithSpecificESLint(
   }
 
   messages.forEach((message, i) => {
-    var formatedMessage = `${message.line}:${message.column} ${
+    const formatedMessage = `${message.line}:${message.column} ${
       message.message
     }${message.ruleId ? ` ${message.ruleId}` : ""}`;
     if (formatedMessage !== expectedMessages[i]) {
diff --git a/eslint/babel-eslint-parser/test/z_parser-for-eslint-after-patched.js b/eslint/babel-eslint-parser/test/z_parser-for-eslint-after-patched.js
index a7d6882d35..265fb5f040 100644
--- a/eslint/babel-eslint-parser/test/z_parser-for-eslint-after-patched.js
+++ b/eslint/babel-eslint-parser/test/z_parser-for-eslint-after-patched.js
@@ -4,7 +4,7 @@ const eslint = require("eslint");
 const assert = require("assert");
 const babelEslint = require("..");
 const espree = require("espree");
-var assertImplementsAST = require("./fixtures/assert-implements-ast");
+const assertImplementsAST = require("./fixtures/assert-implements-ast");
 
 describe("https://github.com/babel/babel-eslint/issues/558", () => {
   it("don't crash with eslint-plugin-import", () => {