diff --git a/lib/build_types/minify_js.js b/lib/build_types/minify_js.js
index 392f284b..9ab341ba 100644
--- a/lib/build_types/minify_js.js
+++ b/lib/build_types/minify_js.js
@@ -22,24 +22,31 @@ function uglify(source, options) {
 	var envify = require("loose-envify/replace");
 
 	var code = source.code;
-	var opts = assign({}, options ? options.uglifyOptions : {});
+	var existingSourceMap = source.map;
+	var uglifyOptions = assign({}, options ? options.uglifyOptions : {});
 
-	if (source.map) {
-		var content = source.map.toJSON();
-		var filename = content.sources && content.sources[0];
+	if (options.sourceMaps) {
+		var sourceMap = uglifyOptions.sourceMap || {};
 
-		opts.sourceMap = opts.sourceMap || {};
-		opts.sourceMap.filename = filename;
-		opts.sourceMap.content = content;
+		if (existingSourceMap) {
+			var content = getRawSourceMap(existingSourceMap);
+			var filename = content.sources && content.sources[0];
 
-		if (options.sourceMapsContent) {
-			opts.sourceMap.includeSources = true;
+			sourceMap.filename = filename;
+			sourceMap.content = content;
 		}
+
+		sourceMap.includeSources = !!options.sourceMapsContent;
+		uglifyOptions.sourceMap = sourceMap;
 	}
 
 	if (options.envify) {
 		code = envify(code, [process.env]);
 	}
 
-	return UglifyJS.minify(code, opts);
+	return UglifyJS.minify(code, uglifyOptions);
+}
+
+function getRawSourceMap(map) {
+	return isFunction(map.toJSON) ? map.toJSON() : map;
 }
diff --git a/test/minify_js_test.js b/test/minify_js_test.js
new file mode 100644
index 00000000..367400c3
--- /dev/null
+++ b/test/minify_js_test.js
@@ -0,0 +1,45 @@
+var assert = require("assert");
+var minify = require("../lib/build_types/minify_js").sync;
+
+describe("minify js", function() {
+	it("generates sourcemaps when option is set", function() {
+		var result = minify({ code: "var a = 1;" }, { sourceMaps: true });
+		assert.ok(result.map, "should return a map");
+	});
+
+	it("map includes the source when option is set", function() {
+		var result = minify({ code: "var a = 1;" }, {
+			sourceMaps: true,
+			sourceMapsContent: true
+		});
+
+		assert.ok(
+			JSON.parse(result.map).sourcesContent,
+			"should include sourcesContent property"
+		);
+	});
+
+	it("sets input sourcemap", function() {
+		var map = {
+			"version": 3,
+			"sources": ["index.js"],
+			"names": [],
+			"mappings": ";;AAAA,IAAI,MAAM,SAAN,GAAM;AAAA,SAAK,SAAS,CAAd;AAAA,CAAV;AACA,QAAQ,GAAR,CAAY,IAAI,KAAJ,CAAZ",
+			"file": "bundle.js",
+			"sourcesContent": ["let foo = x => \"foo \" + x;\nconsole.log(foo(\"bar\"));"]
+		};
+
+        var transpiled = '"use strict";\n\n' +
+            'var foo = function foo(x) {\n  return "foo " + x;\n};\n' +
+            'console.log(foo("bar"));\n\n' +
+            '//# sourceMappingURL=bundle.js.map';
+
+        var result = minify({ code: transpiled, map: map }, { sourceMaps: true });
+
+		assert.equal(
+			JSON.parse(result.map).file,
+			"index.js",
+			"minify should compose the existing map"
+		);
+	 });
+});
diff --git a/test/test.js b/test/test.js
index a27db9ab..501ea024 100644
--- a/test/test.js
+++ b/test/test.js
@@ -15,6 +15,7 @@ require("./cli/cmd_live_test");
 require("./get_es_module_imports_test");
 require("./cli/make_build_options_test");
 require("./slim_build_conditionals_test");
+require("./minify_js_test");
 
 // Integration tests
 require("./test_cli");