Don't mutate options object. Fixes terser#1341

lib/minify.js

@@ -280,10 +280,13 @@ async function minify(files, options, _fs_module) {
     if (options.format.spidermonkey) {
         result.ast = toplevel.to_mozilla_ast();
     }
+    let format_options;
     if (!HOP(options.format, "code") || options.format.code) {
-        if (!options.format.ast) {
+        // Make a shallow copy so that we can modify without mutating the user's input.
+        format_options = {...options.format};
+        if (!format_options.ast) {
             // Destroy stuff to save RAM. (unless the deprecated `ast` option is on)
-            options.format._destroy_ast = true;
+            format_options._destroy_ast = true;
 
             walk(toplevel, node => {
                 if (node instanceof AST_Scope) {
@@ -303,25 +306,25 @@ async function minify(files, options, _fs_module) {
             if (options.sourceMap.includeSources && files instanceof AST_Toplevel) {
                 throw new Error("original source content unavailable");
             }
-            options.format.source_map = await SourceMap({
+            format_options.source_map = await SourceMap({
                 file: options.sourceMap.filename,
                 orig: options.sourceMap.content,
                 root: options.sourceMap.root,
                 files: options.sourceMap.includeSources ? files : null,
             });
         }
-        delete options.format.ast;
-        delete options.format.code;
-        delete options.format.spidermonkey;
-        var stream = OutputStream(options.format);
+        delete format_options.ast;
+        delete format_options.code;
+        delete format_options.spidermonkey;
+        var stream = OutputStream(format_options);
         toplevel.print(stream);
         result.code = stream.get();
         if (options.sourceMap) {
             Object.defineProperty(result, "map", {
                 configurable: true,
                 enumerable: true,
                 get() {
-                    const map = options.format.source_map.getEncoded();
+                    const map = format_options.source_map.getEncoded();
                     return (result.map = options.sourceMap.asObject ? map : JSON.stringify(map));
                 },
                 set(value) {
@@ -331,7 +334,7 @@ async function minify(files, options, _fs_module) {
                     });
                 }
             });
-            result.decoded_map = options.format.source_map.getDecoded();
+            result.decoded_map = format_options.source_map.getDecoded();
             if (options.sourceMap.url == "inline") {
                 var sourceMap = typeof result.map === "object" ? JSON.stringify(result.map) : result.map;
                 result.code += "\n//# sourceMappingURL=data:application/json;charset=utf-8;base64," + to_base64(sourceMap);
@@ -346,8 +349,8 @@ async function minify(files, options, _fs_module) {
             options.nameCache.props = cache_to_json(options.mangle.properties.cache);
         }
     }
-    if (options.format && options.format.source_map) {
-        options.format.source_map.destroy();
+    if (format_options && format_options.source_map) {
+        format_options.source_map.destroy();
     }
     if (timings) {
         timings.end = Date.now();

test/mocha/no-mutate-input.js

@@ -0,0 +1,34 @@
+import assert from "assert";
+import { minify } from "../../main.js";
+
+describe("no-mutate-input", function() {
+
+    it("does not modify the options object", async function() {
+        const config = {
+            format: {},
+            sourceMap: true,
+        };
+
+        await minify('"foo";', config);
+
+        assert.deepEqual(config, {
+            format: {},
+            sourceMap: true,
+        });
+    });
+
+    it("does not clobber source maps with a subsequent minification", async function() {
+        const config = {
+            format: {},
+            sourceMap: true,
+        };
+
+        const fooResult = await minify('"foo";', config);
+        const barResult = await minify('module.exports = "bar";', config);
+
+        const fooMap = fooResult.map;
+        const barMap = barResult.map;
+
+        assert.notEqual(barMap, fooMap);
+    });
+});