fix(std/log): print "{msg}" when log an empty line (#6381)

2 files changed, 18 insertions, 1 deletions

diff --git a/std/log/handlers.ts b/std/log/handlers.ts
index 1b152c273..0efa7cc26 100644
--- a/std/log/handlers.ts
+++ b/std/log/handlers.ts
@@ -41,7 +41,7 @@ export class BaseHandler {
       const value = logRecord[p1 as keyof LogRecord];
 
       // do not interpolate missing values
-      if (!value) {
+      if (value == null) {
         return match;
       }
 
diff --git a/std/log/handlers_test.ts b/std/log/handlers_test.ts
index 7ff4c4c90..ef4b69000 100644
--- a/std/log/handlers_test.ts
+++ b/std/log/handlers_test.ts
@@ -94,6 +94,23 @@ Deno.test("testFormatterAsString", function (): void {
   assertEquals(handler.messages, ["test DEBUG Hello, world!"]);
 });
 
+Deno.test("testFormatterWithEmptyMsg", function () {
+  const handler = new TestHandler("DEBUG", {
+    formatter: "test {levelName} {msg}",
+  });
+
+  handler.handle(
+    new LogRecord({
+      msg: "",
+      args: [],
+      level: LogLevels.DEBUG,
+      loggerName: "default",
+    })
+  );
+
+  assertEquals(handler.messages, ["test DEBUG "]);
+});
+
 Deno.test("testFormatterAsFunction", function (): void {
   const handler = new TestHandler("DEBUG", {
     formatter: (logRecord): string =>