fix #3700: json loader preserves __proto__ keys

CHANGELOG.md

@@ -32,6 +32,36 @@
     }();
     ```
 
+* JSON loader now preserves `__proto__` properties ([#3700](https://github.com/evanw/esbuild/issues/3700))
+
+    Copying JSON source code into a JavaScript file will change its meaning if a JSON object contains the `__proto__` key. A literal `__proto__` property in a JavaScript object literal sets the prototype of the object instead of adding a property named `__proto__`, while a literal `__proto__` property in a JSON object literal just adds a property named `__proto__`. With this release, esbuild will now work around this problem by converting JSON to JavaScript with a computed property key in this case:
+
+    ```js
+    // Original code
+    import data from 'data:application/json,{"__proto__":{"fail":true}}'
+    if (Object.getPrototypeOf(data)?.fail) throw 'fail'
+
+    // Old output (with --bundle)
+    (() => {
+      // <data:application/json,{"__proto__":{"fail":true}}>
+      var json_proto_fail_true_default = { __proto__: { fail: true } };
+
+      // entry.js
+      if (Object.getPrototypeOf(json_proto_fail_true_default)?.fail)
+        throw "fail";
+    })();
+
+    // New output (with --bundle)
+    (() => {
+      // <data:application/json,{"__proto__":{"fail":true}}>
+      var json_proto_fail_true_default = { ["__proto__"]: { fail: true } };
+
+      // example.mjs
+      if (Object.getPrototypeOf(json_proto_fail_true_default)?.fail)
+        throw "fail";
+    })();
+    ```
+
 * Improve dead code removal of `switch` statements ([#3659](https://github.com/evanw/esbuild/issues/3659))
 
     With this release, esbuild will now remove `switch` statements in branches when minifying if they are known to never be evaluated:

internal/bundler/bundler.go

@@ -244,7 +244,9 @@ func parseFile(args parseArgs) {
 		result.ok = true
 
 	case config.LoaderJSON, config.LoaderWithTypeJSON:
-		expr, ok := args.caches.JSONCache.Parse(args.log, source, js_parser.JSONOptions{})
+		expr, ok := args.caches.JSONCache.Parse(args.log, source, js_parser.JSONOptions{
+			UnsupportedJSFeatures: args.options.UnsupportedJSFeatures,
+		})
 		ast := js_parser.LazyExportAST(args.log, source, js_parser.OptionsFromConfig(&args.options), expr, "")
 		if loader == config.LoaderWithTypeJSON {
 			// The exports kind defaults to "none", in which case the linker picks

internal/bundler_tests/bundler_loader_test.go

@@ -1632,3 +1632,46 @@ func TestLoaderBundleWithTypeJSONOnlyDefaultExport(t *testing.T) {
 `,
 	})
 }
+
+func TestLoaderJSONPrototype(t *testing.T) {
+	loader_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				import data from "./data.json"
+				console.log(data)
+			`,
+			"/data.json": `{
+				"": "The property below should be converted to a computed property:",
+				"__proto__": { "foo": "bar" }
+			}`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:          config.ModeBundle,
+			AbsOutputFile: "/out.js",
+			MinifySyntax:  true,
+		},
+	})
+}
+
+func TestLoaderJSONPrototypeES5(t *testing.T) {
+	loader_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				import data from "./data.json"
+				console.log(data)
+			`,
+			"/data.json": `{
+				"": "The property below should NOT be converted to a computed property for ES5:",
+				"__proto__": { "foo": "bar" }
+			}`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:                  config.ModeBundle,
+			AbsOutputFile:         "/out.js",
+			MinifySyntax:          true,
+			UnsupportedJSFeatures: es(5),
+		},
+	})
+}

internal/bundler_tests/snapshots/snapshots_loader.txt

@@ -899,6 +899,30 @@ TestLoaderJSONNoBundleIIFE
   require_test();
 })();
 
+================================================================================
+TestLoaderJSONPrototype
+---------- /out.js ----------
+// data.json
+var data_default = {
+  "": "The property below should be converted to a computed property:",
+  ["__proto__"]: { foo: "bar" }
+};
+
+// entry.js
+console.log(data_default);
+
+================================================================================
+TestLoaderJSONPrototypeES5
+---------- /out.js ----------
+// data.json
+var data_default = {
+  "": "The property below should NOT be converted to a computed property for ES5:",
+  __proto__: { foo: "bar" }
+};
+
+// entry.js
+console.log(data_default);
+
 ================================================================================
 TestLoaderJSONSharedWithMultipleEntriesIssue413
 ---------- /out/a.js ----------

internal/js_parser/json_parser.go

@@ -3,6 +3,7 @@ package js_parser
 import (
 	"fmt"
 
+	"github.com/evanw/esbuild/internal/compat"
 	"github.com/evanw/esbuild/internal/helpers"
 	"github.com/evanw/esbuild/internal/js_ast"
 	"github.com/evanw/esbuild/internal/js_lexer"
@@ -142,6 +143,14 @@ func (p *jsonParser) parseExpr() js_ast.Expr {
 				Key:        key,
 				ValueOrNil: value,
 			}
+
+			// The key "__proto__" must not be a string literal in JavaScript because
+			// that actually modifies the prototype of the object. This can be
+			// avoided by using a computed property key instead of a string literal.
+			if helpers.UTF16EqualsString(keyString, "__proto__") && !p.options.UnsupportedJSFeatures.Has(compat.ObjectExtensions) {
+				property.Flags |= js_ast.PropertyIsComputed
+			}
+
 			properties = append(properties, property)
 		}
 
@@ -163,8 +172,9 @@ func (p *jsonParser) parseExpr() js_ast.Expr {
 }
 
 type JSONOptions struct {
-	Flavor      js_lexer.JSONFlavor
-	ErrorSuffix string
+	UnsupportedJSFeatures compat.JSFeature
+	Flavor                js_lexer.JSONFlavor
+	ErrorSuffix           string
 }
 
 func ParseJSON(log logger.Log, source logger.Source, options JSONOptions) (result js_ast.Expr, ok bool) {

scripts/end-to-end-tests.js

@@ -3528,6 +3528,43 @@ for (const minify of [[], ['--minify-syntax']]) {
       `,
     }),
   )
+
+  // https://github.com/evanw/esbuild/issues/3700
+  tests.push(
+    test(['in.js', '--bundle', '--outfile=node.js'].concat(minify), {
+      'in.js': `
+        import imported from './data.json'
+        const native = JSON.parse(\`{
+          "hello": "world",
+          "__proto__": {
+            "sky": "universe"
+          }
+        }\`)
+        const literal1 = {
+          "hello": "world",
+          "__proto__": {
+            "sky": "universe"
+          }
+        }
+        const literal2 = {
+          "hello": "world",
+          ["__proto__"]: {
+            "sky": "universe"
+          }
+        }
+        if (Object.getPrototypeOf(native)?.sky) throw 'fail: native'
+        if (!Object.getPrototypeOf(literal1)?.sky) throw 'fail: literal1'
+        if (Object.getPrototypeOf(literal2)?.sky) throw 'fail: literal2'
+        if (Object.getPrototypeOf(imported)?.sky) throw 'fail: imported'
+      `,
+      'data.json': `{
+        "hello": "world",
+        "__proto__": {
+          "sky": "universe"
+        }
+      }`,
+    }),
+  )
 }
 
 // Test minification of top-level symbols