use main field over module for runtime (#3448)

* use main field over module for runtime

* move flag to `Resolver`

* set `prefer_module_field` in `initWithModuleGraph`

3 files changed, 210 insertions, 1 deletions

diff --git a/src/bun.js/javascript.zig b/src/bun.js/javascript.zig
index 5c58eff60..cf6a65841 100644
--- a/src/bun.js/javascript.zig
+++ b/src/bun.js/javascript.zig
@@ -801,6 +801,7 @@ pub const VirtualMachine = struct {
 
         vm.bundler.macro_context = null;
         vm.bundler.resolver.store_fd = false;
+        vm.bundler.resolver.prefer_module_field = false;
 
         vm.bundler.resolver.onWakePackageManager = .{
             .context = &vm.modules,
@@ -898,6 +899,7 @@ pub const VirtualMachine = struct {
 
         vm.bundler.macro_context = null;
         vm.bundler.resolver.store_fd = store_fd;
+        vm.bundler.resolver.prefer_module_field = false;
 
         vm.bundler.resolver.onWakePackageManager = .{
             .context = &vm.modules,
diff --git a/src/resolver/resolver.zig b/src/resolver/resolver.zig
index d9f4dc887..e1e83ba4f 100644
--- a/src/resolver/resolver.zig
+++ b/src/resolver/resolver.zig
@@ -533,6 +533,10 @@ pub const Resolver = struct {
     // all parent directories
     dir_cache: *DirInfo.HashMap,
 
+    /// This is set to false for the runtime. The runtime should choose "main"
+    /// over "module" in package.json
+    prefer_module_field: bool = true,
+
     pub fn getPackageManager(this: *Resolver) *PackageManager {
         return this.package_manager orelse brk: {
             bun.HTTPThead.init() catch unreachable;
@@ -3408,7 +3412,10 @@ pub const Resolver = struct {
                             // with this same path. The goal of this code is to avoid having
                             // both the "module" file and the "main" file in the bundle at the
                             // same time.
-                            if (kind != ast.ImportKind.require) {
+                            //
+                            // Additionally, if this is for the runtime, use the "main" field.
+                            // If it doesn't exist, the "module" field will be used.
+                            if (r.prefer_module_field and kind != ast.ImportKind.require) {
                                 if (r.debug_logs) |*debug| {
                                     debug.addNoteFmt("Resolved to \"{s}\" using the \"module\" field in \"{s}\"", .{ auto_main_result.path_pair.primary.text, pkg_json.source.key_path.text });
 
diff --git a/test/bundler/esbuild/default.test.ts b/test/bundler/esbuild/default.test.ts
index 4c4cf87be..20856ecdf 100644
--- a/test/bundler/esbuild/default.test.ts
+++ b/test/bundler/esbuild/default.test.ts
@@ -6565,4 +6565,204 @@ describe("bundler", () => {
       api.expectFile("/out.js").not.toContain("data = 123");
     },
   });
+  itBundled("default/BundlerUsesModuleFieldForEsm", {
+    files: {
+      "/entry.js": `
+        import { foo } from 'foo';
+        console.log(foo);
+      `,
+      "/node_modules/foo/package.json": `
+        {
+          "name": "foo",
+          "version": "2.0.0",
+          "module": "index.esm.js",
+          "main": "index.cjs.js"
+        }
+      `,
+      "/node_modules/foo/index.cjs.js": `
+        module.exports.foo = "hello index.cjs.js";
+      `,
+      "/node_modules/foo/index.esm.js": `
+        export const foo = "hello index.esm.js";
+      `,
+    },
+    run: {
+      stdout: "hello index.esm.js",
+    },
+  });
+  itBundled("default/BundlerUsesMainFieldForCjs", {
+    files: {
+      "/entry.js": `
+        const { foo } = require('foo');
+        console.log(foo);
+      `,
+      "/node_modules/foo/package.json": `
+        {
+          "name": "foo",
+          "version": "2.0.0",
+          "module": "index.esm.js",
+          "main": "index.cjs.js"
+        }
+      `,
+      "/node_modules/foo/index.cjs.js": `
+        module.exports.foo = "hello index.cjs.js";
+      `,
+      "/node_modules/foo/index.esm.js": `
+        export const foo = "hello index.esm.js";
+      `,
+    },
+    run: {
+      stdout: "hello index.cjs.js",
+    },
+  });
+  itBundled("default/RuntimeUsesMainFieldForCjs", {
+    files: {
+      "/entry.js": `
+        const { foo } = require('foo');
+        console.log(foo);
+      `,
+      "/node_modules/foo/package.json": `
+        {
+          "name": "foo",
+          "version": "2.0.0",
+          "module": "index.esm.js",
+          "main": "index.cjs.js"
+        }
+      `,
+      "/node_modules/foo/index.cjs.js": `
+        module.exports.foo = "hello index.cjs.js";
+      `,
+      "/node_modules/foo/index.esm.js": `
+        export const foo = "hello index.esm.js";
+      `,
+    },
+    bundling: false,
+    run: {
+      stdout: "hello index.cjs.js",
+    },
+  });
+  itBundled("default/RuntimeUsesMainFieldForEsm", {
+    files: {
+      "/entry.js": `
+        import { foo } from 'foo';
+        console.log(foo);
+      `,
+      "/node_modules/foo/package.json": `
+        {
+          "name": "foo",
+          "version": "2.0.0",
+          "module": "index.esm.js",
+          "main": "index.cjs.js"
+        }
+      `,
+      "/node_modules/foo/index.cjs.js": `
+        module.exports.foo = "hello index.cjs.js";
+      `,
+      "/node_modules/foo/index.esm.js": `
+        export const foo = "hello index.esm.js";
+      `,
+    },
+    bundling: false,
+    run: {
+      stdout: "hello index.cjs.js",
+    },
+  });
+  itBundled("default/BundlerUsesModuleFieldIfMainDoesNotExistCjs", {
+    files: {
+      "/entry.js": `
+        const { foo } = require('foo');
+        console.log(foo);
+      `,
+      "/node_modules/foo/package.json": `
+        {
+          "name": "foo",
+          "version": "2.0.0",
+          "module": "index.esm.js"
+        }
+      `,
+      "/node_modules/foo/index.cjs.js": `
+        module.exports.foo = "hello index.cjs.js";
+      `,
+      "/node_modules/foo/index.esm.js": `
+        export const foo = "hello index.esm.js";
+      `,
+    },
+    run: {
+      stdout: "hello index.esm.js",
+    },
+  });
+  itBundled("default/BundlerUsesModuleFieldIfMainDoesNotExistEsm", {
+    files: {
+      "/entry.js": `
+        import { foo } from 'foo';
+        console.log(foo);
+      `,
+      "/node_modules/foo/package.json": `
+        {
+          "name": "foo",
+          "version": "2.0.0",
+          "module": "index.esm.js"
+        }
+      `,
+      "/node_modules/foo/index.cjs.js": `
+        module.exports.foo = "hello index.cjs.js";
+      `,
+      "/node_modules/foo/index.esm.js": `
+        export const foo = "hello index.esm.js";
+      `,
+    },
+    run: {
+      stdout: "hello index.esm.js",
+    },
+  });
+  itBundled("default/RuntimeUsesModuleFieldIfMainDoesNotExistCjs", {
+    files: {
+      "/entry.js": `
+        const { foo } = require('foo');
+        console.log(foo);
+      `,
+      "/node_modules/foo/package.json": `
+        {
+          "name": "foo",
+          "version": "2.0.0",
+          "module": "index.esm.js"
+        }
+      `,
+      "/node_modules/foo/index.cjs.js": `
+        module.exports.foo = "hello index.cjs.js";
+      `,
+      "/node_modules/foo/index.esm.js": `
+        export const foo = "hello index.esm.js";
+      `,
+    },
+    bundling: false,
+    run: {
+      stdout: "hello index.esm.js",
+    },
+  });
+  itBundled("default/RuntimeUsesModuleFieldIfMainDoesNotExistEsm", {
+    files: {
+      "/entry.js": `
+        import { foo } from 'foo';
+        console.log(foo);
+      `,
+      "/node_modules/foo/package.json": `
+        {
+          "name": "foo",
+          "version": "2.0.0",
+          "module": "index.esm.js"
+        }
+      `,
+      "/node_modules/foo/index.cjs.js": `
+        module.exports.foo = "hello index.cjs.js";
+      `,
+      "/node_modules/foo/index.esm.js": `
+        export const foo = "hello index.esm.js";
+      `,
+    },
+    bundling: false,
+    run: {
+      stdout: "hello index.esm.js",
+    },
+  });
 });