Update externals definition for Webpack V4 (#29345)

* Update externals definition for Webpack V4

 - Externals should accept objects as their values (see https://webpack.js.org/configuration/externals/\#object);

* fix linting errors

* update types, add additional test

types/webpack/index.d.ts

@@ -13,6 +13,7 @@
 //                 Christophe Hurpeau <https://github.com/christophehurpeau>
 //                 ZSkycat <https://github.com/ZSkycat>
 //                 John Reilly <https://github.com/johnnyreilly>
+//                 Ryan Waskiewicz <https://github.com/rwaskiewicz>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -350,7 +351,7 @@ declare namespace webpack {
     type ExternalsElement = string | RegExp | ExternalsObjectElement | ExternalsFunctionElement;
 
     interface ExternalsObjectElement {
-        [key: string]: boolean | string;
+        [key: string]: boolean | string | string[] | Record<string, string | string[]>;
     }
 
     type ExternalsFunctionElement = (context: any, request: any, callback: (error: any, result: any) => void) => any;

types/webpack/webpack-tests.ts

@@ -79,6 +79,91 @@ configuration = {
     }))
 };
 
+//
+// https://webpack.js.org/configuration/externals/
+//
+configuration = {
+    externals : {
+        react: 'react'
+    },
+};
+
+configuration = {
+    externals : {
+        lodash : {
+            commonjs: 'lodash',
+            amd: 'lodash',
+            root: '_' // indicates global variable
+        }
+      },
+};
+
+configuration = {
+    externals : {
+        subtract : {
+            root: ['math', 'subtract']
+        }
+    }
+};
+
+configuration = {
+    externals: [
+        // Disable TSLint for allowing non-arrow functions
+        /* tslint:disable-next-line */
+        function(context, request, callback) {
+          if (/^yourregex$/.test(request)) {
+            // Disable TSLint for bypassing 'no-void-expression' to align with Webpack documentation
+            /* tslint:disable-next-line */
+            return callback(null, 'commonjs ' + request);
+          }
+          callback({}, {});
+        }
+      ]
+};
+
+configuration = {
+    externals: [
+        {
+            // String
+            react: 'react',
+            // Object
+            lodash : {
+                commonjs: 'lodash',
+                amd: 'lodash',
+                root: '_' // indicates global variable
+            },
+            // Array
+            subtract: ['./math', 'subtract']
+            },
+            // Disable TSLint for allowing non-arrow functions
+            /* tslint:disable-next-line */
+            function(context, request, callback) {
+              if (/^yourregex$/.test(request)) {
+                // Disable TSLint for bypassing 'no-void-expression' to align with Webpack documentation
+                /* tslint:disable-next-line */
+                return callback(null, 'commonjs ' + request);
+              }
+              callback({}, {});
+            },
+            // Regex
+            /^(jquery|\$)$/i
+    ]
+};
+
+configuration = {
+    externals: [
+        "add",
+        {
+            subtract: {
+                root: "subtract",
+                commonjs2: "./subtract",
+                commonjs: ["./math", "subtract"],
+                amd: "subtract"
+            }
+        }
+    ]
+};
+
 //
 // https://webpack.github.io/docs/optimization.html
 //