extend existing sourcemaps

src/assets/JSAsset.js

@@ -12,6 +12,9 @@ const babel = require('../transforms/babel');
 const generate = require('babel-generator').default;
 const uglify = require('../transforms/uglify');
 const SourceMap = require('../SourceMap');
+const path = require('path');
+const fs = require('../utils/fs');
+const logger = require('../Logger');
 
 const IMPORT_RE = /\b(?:import\b|export\b|require\s*\()/;
 const ENV_RE = /\b(?:process\.env)\b/;
@@ -94,6 +97,25 @@ class JSAsset extends Asset {
   }
 
   async pretransform() {
+    // Get original sourcemap if there is any
+    let sourceMapLine = this.contents.lastIndexOf('//# sourceMappingURL=');
+    if (sourceMapLine > -1) {
+      let sourceMapReference = this.contents.substring(sourceMapLine + 21);
+      this.contents = this.contents.substring(0, sourceMapLine);
+
+      try {
+        this.sourceMap = JSON.parse(
+          await fs.readFile(
+            path.join(path.dirname(this.name), sourceMapReference)
+          )
+        );
+      } catch (e) {
+        logger.warn(
+          `Could not load existing sourcemap of ${this.relativeName}.`
+        );
+      }
+    }
+
     await babel(this);
 
     // Inline environment variables

test/integration/sourcemap-existing/index.js

@@ -0,0 +1,5 @@
+const sum = require('./sum');
+
+module.exports = function() {
+  return sum(1, 2);
+}

test/integration/sourcemap-existing/sum.map

@@ -0,0 +1,19 @@
+{
+  "version": 3,
+  "sources": [
+    "sum.js"
+  ],
+  "names": [
+    "sum",
+    "a",
+    "b",
+    "module",
+    "exports"
+  ],
+  "mappings": ";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;AAAA,SAASA,GAAT,CAAaC,CAAb,EAAgBC,CAAhB,EAAmB;AACjB,SAAOD,IAAIC,CAAX;AACD;;AAEDC,OAAOC,OAAP,GAAiBJ,GAAjB",
+  "file": "sum.map",
+  "sourceRoot": '',
+  "sourcesContent": [
+    "function sum(a, b) {\n  return a + b;\n}\n\nmodule.exports = sum;\n\n"
+  ]
+}

test/sourcemaps.js

@@ -204,4 +204,40 @@ describe('sourcemaps', function() {
     let map = fs.readFileSync(path.join(sourcemapReference)).toString();
     mapValidator(jsOutput, map);
   });
+
+  it('should load existing sourcemaps of libraries', async function() {
+    let b = await bundle(
+      __dirname + '/integration/sourcemap-existing/index.js'
+    );
+
+    assertBundleTree(b, {
+      name: 'index.js',
+      assets: ['index.js', 'sum.js'],
+      childBundles: [
+        {
+          type: 'map'
+        }
+      ]
+    });
+
+    let jsOutput = fs.readFileSync(b.name).toString();
+
+    let sourcemapReference = path.join(
+      __dirname,
+      '/dist/',
+      jsOutput.substring(jsOutput.lastIndexOf('//# sourceMappingURL') + 22)
+    );
+
+    assert(
+      fs.existsSync(path.join(sourcemapReference)),
+      'referenced sourcemap should exist'
+    );
+
+    let map = fs.readFileSync(path.join(sourcemapReference)).toString();
+    assert(
+      map.indexOf('return a + b;') > -1,
+      'Sourcemap should contain the existing sourcemap'
+    );
+    mapValidator(jsOutput, map);
+  });
 });