fix(tools): log[level] compatible with when first argument is undefin…

…ed (fix #1555) (#1565)

lib/tools.js

@@ -53,7 +53,7 @@ function genLog (level, hook) {
       }
       this[writeSym](o, format(msg, formatParams, this[formatOptsSym]), level)
     } else {
-      this[writeSym](null, format(o, n, this[formatOptsSym]), level)
+      this[writeSym](null, format(o === undefined ? n.shift() : o, n, this[formatOptsSym]), level)
     }
   }
 }

test/basic.test.js

@@ -109,6 +109,22 @@ function levelTest (name, level) {
     same(Object.keys(obj), ['hello'])
   })
 
+  test(`passing a undefined and a string at level ${name}`, async ({ equal, same }) => {
+    const stream = sink()
+    const instance = pino(stream)
+    instance.level = name
+    instance[name](undefined, 'a string')
+    const result = await once(stream, 'data')
+    equal(new Date(result.time) <= new Date(), true, 'time is greater than Date.now()')
+    delete result.time
+    same(result, {
+      pid,
+      hostname,
+      level,
+      msg: 'a string'
+    })
+  })
+
   test(`overriding object key by string at level ${name}`, async ({ equal, same }) => {
     const stream = sink()
     const instance = pino(stream)