Allow array as value in externals object

lib/ExternalModule.js

@@ -74,7 +74,9 @@ class ExternalModule extends Module {
 			.slice(1)
 			.map(r => `[${JSON.stringify(r)}]`)
 			.join("");
-		return `module.exports = require(${moduleName})${objectLookup};`;
+		return `module.exports = require(${JSON.stringify(
+			moduleName
+		)})${objectLookup};`;
 	}
 
 	checkExternalVariable(variableToCheck, request) {
@@ -94,15 +96,25 @@ class ExternalModule extends Module {
 	}
 
 	getSourceForDefaultCase(optional, request) {
+		if (!Array.isArray(request)) {
+			// make it an array as the look up works the same basically
+			request = [request];
+		}
+
+		const variableName = request[0];
 		const missingModuleError = optional
-			? this.checkExternalVariable(request, request)
+			? this.checkExternalVariable(variableName, request.join("."))
 			: "";
-		return `${missingModuleError}module.exports = ${request};`;
+		const objectLookup = request
+			.slice(1)
+			.map(r => `[${JSON.stringify(r)}]`)
+			.join("");
+		return `${missingModuleError}module.exports = ${variableName}${objectLookup};`;
 	}
 
 	getSourceString(runtime) {
 		const request =
-			typeof this.request === "object"
+			typeof this.request === "object" && !Array.isArray(this.request)
 				? this.request[this.externalType]
 				: this.request;
 		switch (this.externalType) {

schemas/WebpackOptions.json

@@ -123,6 +123,9 @@
               {
                 "type": "object"
               },
+              {
+                "$ref": "#/definitions/common.arrayOfStringValues"
+              },
               {
                 "type": "boolean"
               }

test/configCases/externals/externals-array/index.js

@@ -0,0 +1,4 @@
+it("should not fail on optional externals", function() {
+	const external = require("external");
+	expect(external).toBe(EXPECTED);
+});

test/configCases/externals/externals-array/webpack.config.js

@@ -0,0 +1,26 @@
+const webpack = require("../../../../");
+module.exports = [
+	{
+		output: {
+			libraryTarget: "commonjs2"
+		},
+		externals: {
+			external: ["webpack", "version"]
+		},
+		plugins: [
+			new webpack.DefinePlugin({
+				EXPECTED: JSON.stringify(webpack.version)
+			})
+		]
+	},
+	{
+		externals: {
+			external: ["Array", "isArray"]
+		},
+		plugins: [
+			new webpack.DefinePlugin({
+				EXPECTED: "Array.isArray"
+			})
+		]
+	}
+];