Fix #1516 (#2089)

`FileSystemFlags.fromJS` was hardcoded to return O_RDONLY when the flag value
 was null or undefined and this caused breakage when used with write functions.
Updated the function to take a `default` argument so that the caller can specify
 a sane default for their use.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/bun.js/fs.test.js b/test/bun.js/fs.test.js
index 086f19211..5e4827672 100644
--- a/test/bun.js/fs.test.js
+++ b/test/bun.js/fs.test.js
@@ -987,3 +987,10 @@ it("fs.Dirent", () => {
 it("fs.Stats", () => {
   expect(Stats).toBeDefined();
 });
+
+it("repro 1516: can use undefined/null to specify default flag", () => {
+  const path = "/tmp/repro_1516.txt";
+  writeFileSync(path, "b", { flag: undefined });
+  expect(readFileSync(path, { encoding: "utf8", flag: null })).toBe("b");
+  rmSync(path);
+});