support loading default externs

Change-Id: I8e930d43061cbce9cf30fc01595910b19456b6d5

Author: samthor

.gitignore

@@ -1,5 +1,3 @@
 node_modules
 .DS_Store
-contrib
-externs
-compile.js
+build

README.md

@@ -25,12 +25,13 @@ Contributions are welcome.
 ### Native Node Usage
 
 The module provides `compile` as a low-level method to compile JavaScript.
+By default, this compiles ES6 to ES5 and includes the default set of EcmaScript externs files.
 
 ```js
 const compiler = require('closure-compiler-js').compiler;
 
 const flags = {
-  jsCode: [{source: 'var x = 1 + 2;'}],
+  jsCode: [{source: 'const x = 1 + 2;'}],
 };
 const out = compile(flags);
 console.info(out.compiledCode);  // will print 'var x = 3;\n'

build.js

@@ -19,6 +19,13 @@
 
 const spawn = require('child_process').spawnSync;
 const ncp = require('ncp');
+const fs = require('fs');
+
+try {
+  fs.mkdirSync('./build');
+} catch (e) {
+  // ignore, probably exists
+}
 
 const moduleName = 'com.google.javascript:closure-compiler-gwt';
 const compilerBuild = spawn('mvn', ['-DskipTests', '-pl', moduleName], {
@@ -31,12 +38,12 @@ if (compilerBuild.status !== 0) {
 }
 
 const targetPath = './closure-compiler/target/closure-compiler-gwt-1.0-SNAPSHOT/jscomp/jscomp.js';
-ncp(targetPath, './compile.js', err => {
+ncp(targetPath, './build/compile.js', err => {
   if (err) {
     throw new Error(err);
   }
   ['contrib', 'externs'].forEach(name => {
-    ncp('./closure-compiler/' + name, './' + name, function(err) {
+    ncp('./closure-compiler/' + name, './build/' + name, function(err) {
       if (err) {
         throw new Error(err);
       }

compile.js

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2016 The Closure Compiler Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * @fileoverview Wrapper for the natively built Closure Compiler GWT binary to
+ * work around a few quirks.
+ */
+
+'use strict';
+
+const compile = require('./build/compile.js');
+
+let externs;
+
+/**
+ * @return {!Array<{source: string}>} default externs files
+ */
+function loadExterns() {
+  const fs = require('fs');
+  const path = './build/externs';
+  const all = fs.readdirSync(path); 
+
+  return all.filter(x => x.endsWith('.js')).map(x => {
+    return {
+      source: fs.readFileSync(path + '/' + x, {encoding: 'UTF-8'}),
+    };
+  });
+}
+
+module.exports = function(flags) {
+  const clone = {
+    languageIn: 'ES6',  // TODO(samthor): assume this in upstream code
+  };
+  for (const k in flags) {
+    clone[k] = flags[k];
+  }
+
+  // add default externs
+  if (!externs) {
+    externs = loadExterns();
+  }
+  clone.externs = (clone.externs || []).concat(externs);
+
+  const out = compile(clone);
+
+  // hide weird GWT internals
+  out.warnings = [...out.warnings];
+  out.errors = [...out.errors];
+
+  return out;
+};