feat: add language target to control transpilation level

.npmrc

@@ -0,0 +1 @@
+package-lock=false

package.json

@@ -112,4 +112,4 @@
     "url": "https://opencollective.com/fuse-box",
     "logo": "https://opencollective.com/opencollective/logo.txt"
   }
-}
+}

src/analysis/plugins/ImportDeclaration.ts

@@ -1,4 +1,4 @@
-import { File } from "../../core/File";
+import { File, ScriptTarget } from "../../core/File";
 /**
  * Handles require and ImportDeclarations
  * At the moment does not transpile
@@ -25,7 +25,7 @@ export class ImportDeclaration {
         }
 
         if (node.async === true) {
-            file.es6module = true;
+            file.setLanguageLevel(ScriptTarget.ES2017)
         }
 
         if (node.type === "ExportDefaultDeclaration") {
@@ -50,17 +50,18 @@ export class ImportDeclaration {
     }
 
     public static onEnd(file: File) {
-        // We detect that imports are used
-        // Now we need to transpile the code
-        if (file.es6module) {
+        const target = file.context.languageLevel
+        const downlevelTranspile = file.languageLevel > target
+
+        if (file.es6module || downlevelTranspile) {
             file.context.log.magicReason(
                 'used typescript to compile because an import was used',
                 file.info.fuseBoxPath);
             const ts = require("typescript");
             let tsconfg: any = {
                 compilerOptions: {
                     module: "commonjs",
-                    target: "es5"
+                    target
                 },
             };;
             let result = ts.transpileModule(file.contents, tsconfg);

src/core/File.ts

@@ -8,6 +8,17 @@ import * as fs from "fs";
 import * as path from "path";
 import { ensureFuseBoxPath, readFuseBoxModule, isStylesheetExtension } from "../Utils";
 
+/**
+ * Same Target Enumerator used in TypeScript
+ */
+export enum ScriptTarget {
+    ES5 = 1,
+    ES2015 = 2,
+    ES2016 = 3,
+    ES2017 = 4,
+    ESNext = 5
+}
+
 /**
  *
  *
@@ -18,6 +29,8 @@ export class File {
 
     public isFuseBoxBundle = false;
 
+    public languageLevel = ScriptTarget.ES5
+
     public es6module = false;
     /**
      * In order to keep bundle in a bundle
@@ -166,6 +179,12 @@ export class File {
         return file;
     }
 
+    public setLanguageLevel (level: ScriptTarget) {
+        if (this.languageLevel < level) {
+            this.languageLevel = level
+        }
+    }
+
     public addProperty(key: string, obj: any) {
         this.properties.set(key, obj);
     }

src/core/FuseBox.ts

@@ -16,6 +16,7 @@ import { UserOutput } from "./UserOutput";
 import { BundleProducer } from "./BundleProducer";
 import { Bundle } from "./Bundle";
 import { SplitConfig } from "./BundleSplit";
+import { ScriptTarget } from "./File";
 
 const isWin = /^win/.test(process.platform);
 const appRoot = require("app-root-path");
@@ -26,7 +27,16 @@ export interface FuseBoxOptions {
     tsConfig?: string;
     package?: any;
     cache?: boolean;
-    target?: "browser" | "server" | "universal" | "electron",
+    /**
+     * "browser" | "server" | "universal" | "electron"
+     *
+     * Combine target and language version with an '@'
+     *
+     * eg. server@es2017
+     *
+     * default: "universal@es5"
+     */
+    target?: string,
     log?: boolean;
     globals?: { [packageName: string]: /** Variable name */ string };
     plugins?: Plugin[] | [Plugin[]];
@@ -91,7 +101,11 @@ export class FuseBox {
         }
 
         if (opts.target !== undefined) {
-            this.context.target = opts.target;
+            const [target, languageLevel] = opts.target.toLowerCase().split('@')
+            this.context.target = target
+            const level = languageLevel && Object.keys(ScriptTarget)
+                .find(t => t.toLowerCase() === languageLevel)
+            this.context.languageLevel = ScriptTarget[level] || ScriptTarget.ES5
         }
         if (opts.polyfillNonStandardDefaultUsage !== undefined) {
             this.context.polyfillNonStandardDefaultUsage = opts.polyfillNonStandardDefaultUsage;

src/core/WorkflowContext.ts

@@ -1,7 +1,7 @@
 import * as path from "path";
 import * as escodegen from "escodegen";
 import { BundleSource } from "../BundleSource";
-import { File } from "./File";
+import { File, ScriptTarget } from "./File";
 import { Log } from "../Log";
 import { IPackageInformation, IPathInformation, AllowedExtenstions } from "./PathMaster";
 import { ModuleCollection } from "./ModuleCollection";
@@ -97,6 +97,8 @@ export class WorkFlowContext {
 
     public pendingPromises: Promise<any>[] = [];
 
+    public languageLevel: ScriptTarget;
+
     public polyfillNonStandardDefaultUsage: boolean | string[] = false;
 
     public customAPIFile: string;
@@ -558,6 +560,9 @@ export class WorkFlowContext {
         }
 
         config.compilerOptions.module = "commonjs";
+        if (!('target' in config.compilerOptions)) {
+            config.compilerOptions.target = this.languageLevel
+        }
 
         if (this.useSourceMaps) {
             config.compilerOptions.sourceMap = true;