Preserve trailing newline when updating package.json

Fixes https://github.com/oven-sh/bun/issues/1375

2 files changed, 14 insertions, 0 deletions

diff --git a/src/install/install.zig b/src/install/install.zig
index 7ea14acf6..683261e50 100644
--- a/src/install/install.zig
+++ b/src/install/install.zig
@@ -4541,6 +4541,12 @@ pub const PackageManager = struct {
             current_package_json_buf[0..current_package_json_contents_len],
         );
 
+        // If there originally was a newline at the end of their package.json, preserve it
+        // so that we don't cause unnecessary diffs in their git history.
+        // https://github.com/oven-sh/bun/issues/1375
+        const preserve_trailing_newline_at_eof_for_package_json = current_package_json_contents_len > 0 and
+            current_package_json_buf[current_package_json_contents_len - 1] == '\n';
+
         initializeStore();
         var current_package_json = json_parser.ParseJSONUTF8(&package_json_source, ctx.log, manager.allocator) catch |err| {
             if (Output.enable_ansi_colors) {
@@ -4691,6 +4697,8 @@ pub const PackageManager = struct {
             try PackageJSONEditor.edit(ctx.allocator, updates, &current_package_json, dependency_list);
             var buffer_writer_two = try JSPrinter.BufferWriter.init(ctx.allocator);
             try buffer_writer_two.buffer.list.ensureTotalCapacity(ctx.allocator, new_package_json_source.len + 1);
+            buffer_writer_two.append_newline =
+                preserve_trailing_newline_at_eof_for_package_json;
             var package_json_writer_two = JSPrinter.BufferPrinter.init(buffer_writer_two);
 
             written = JSPrinter.printJSON(
diff --git a/src/js_printer.zig b/src/js_printer.zig
index 707161ffd..bd5573a17 100644
--- a/src/js_printer.zig
+++ b/src/js_printer.zig
@@ -4899,6 +4899,7 @@ pub const BufferWriter = struct {
     written: []u8 = "",
     sentinel: [:0]u8 = "",
     append_null_byte: bool = false,
+    append_newline: bool = false,
     approximate_newline_count: usize = 0,
 
     pub fn getWritten(this: *BufferWriter) []u8 {
@@ -4963,6 +4964,11 @@ pub const BufferWriter = struct {
     pub fn done(
         ctx: *BufferWriter,
     ) anyerror!void {
+        if (ctx.append_newline) {
+            ctx.append_newline = false;
+            try ctx.buffer.appendChar('\n');
+        }
+
         if (ctx.append_null_byte) {
             ctx.sentinel = ctx.buffer.toOwnedSentinelLeaky();
             ctx.written = ctx.buffer.toOwnedSliceLeaky();