LibJS: Support computed member expressions in nullish object exceptions

Author: https://github.com/trflynn89
Commit: 2d603c7c3f
Pull-request: https://github.com/SerenityOS/serenity/pull/23802

Userland/Libraries/LibJS/Bytecode/Generator.cpp

@@ -461,9 +461,10 @@ static Optional<ByteString> expression_identifier(Expression const& expression)
             builder.append(*identifer);
 
         if (auto identifer = expression_identifier(member_expression.property()); identifer.has_value()) {
-            if (!builder.is_empty())
-                builder.append('.');
-            builder.append(*identifer);
+            if (member_expression.is_computed())
+                builder.appendff("[{}]", *identifer);
+            else
+                builder.appendff(".{}", *identifer);
         }
 
         return builder.to_byte_string();

Userland/Libraries/LibJS/Tests/null-or-undefined-access.js

@@ -43,6 +43,7 @@ test("null/undefined object key", () => {
 test("null/undefined array index", () => {
     [null, undefined].forEach(value => {
         let foo = [value];
+        let index = 0;
 
         expect(() => {
             foo[0].bar;
@@ -51,5 +52,19 @@ test("null/undefined array index", () => {
         expect(() => {
             foo[0].bar = 1;
         }).toThrowWithMessage(TypeError, `Cannot access property "bar" on ${value} object`);
+
+        expect(() => {
+            foo[index].bar;
+        }).toThrowWithMessage(
+            TypeError,
+            `Cannot access property "bar" on ${value} object "foo[index]"`
+        );
+
+        expect(() => {
+            foo[index].bar = 1;
+        }).toThrowWithMessage(
+            TypeError,
+            `Cannot access property "bar" on ${value} object "foo[index]"`
+        );
     });
 });