Only download the compiler.jar if it doesn't already exist or it is older than the package version.

Author: ChadKillingsworth

build_compiler.js

@@ -3,19 +3,42 @@
 
 var spawn = require('child_process').spawnSync;
 var ncp = require('ncp');
-var version = require('./package.json').version
+var Semver = require('semver');
+var version = require('./package.json').version;
+var fs = require('fs');
+var packageVer = new Semver(version);
 
 var mavenVersion = 'v' + version.split('.')[0];
 var url =
     'https://repo1.maven.org/maven2/com/google/javascript/closure-compiler/'
     + mavenVersion + '/closure-compiler-' + mavenVersion + '.jar';
 
-var compilerBuild = spawn('wget', ['-O', './compiler.jar', url], {
-  stdio: 'inherit'
-});
+var shouldDownloadCompiler = true;
+var compilerJarStats = null;
+try {
+  compilerJarStats = fs.statSync('./compiler.jar');
+} catch (e) {}
+if (compilerJarStats && compilerJarStats.isFile()) {
+  var versionOutput = spawn('java',  ['-jar', 'compiler.jar', '--version']);
+  for (var line of versionOutput.output) {
+    if (line) {
+      var lineString = line.toString();
+      var versionParts = /^Version: v(\d+)$/m.exec(lineString);
+      if (versionParts) {
+        shouldDownloadCompiler = parseInt(versionParts[1], 10) < packageVer.major;
+      }
+    }
+  }
+}
+
+if (shouldDownloadCompiler) {
+   var compilerBuild = spawn('wget', ['-O', './compiler.jar', url], {
+    stdio: 'inherit'
+   });
 
-if (compilerBuild.status !== 0) {
-  throw new Error('Downloading compiler jar from Maven Central failed');
+   if (compilerBuild.status !== 0) {
+    throw new Error('Downloading compiler jar from Maven Central failed');
+   }
 }
 
 ncp('./compiler/contrib', './contrib', function(err) {