fix(winston): do not remove info.level field

docs/winston.asciidoc

@@ -339,11 +339,22 @@ const logger = winston.createLogger({
 ** `eventDataset` +{type-string}+ A "event.dataset" value. If specified this overrides the default of using `${serviceVersion}`.
 
 Create a formatter for winston that converts fields on the log record info
-objecct to ECS Logging format.
+object to ECS Logging format.
 
 [float]
 [[winston-ref-ecsStringify]]
 ==== `ecsStringify([options])`
 
 Create a formatter for winston that stringifies/serializes the log record to
-JSON. (This is very similar to `logform.json()`.)
+JSON.
+
+This is similar to `logform.json()`. They both use the `safe-stable-stringify`
+package to produce the JSON. Some differences:
+
+* This stringifier skips serializing the `level` field, because it is not
+  an ECS field.
+* Winston provides a `replacer` that converts bigints to strings The argument
+  *for* doing so is that a *JavaScript* JSON parser looses precision when
+  parsing a bigint. The argument against is that a BigInt changes type to a
+  string rather than a number. For now this stringifier does not convert
+  BitInts to strings.

packages/ecs-winston-format/CHANGELOG.md

@@ -1,5 +1,12 @@
 # @elastic/ecs-winston-format Changelog
 
+## v1.5.2
+
+- Fix the Winston transformers to *not* delete the `level` property from the
+  `info` object, because https://github.com/winstonjs/logform#info-objects says
+  "Every `info` must have at least the `level` and `message` properties".
+  (https://github.com/elastic/ecs-logging-nodejs/issues/173)
+
 ## v1.5.0
 
 - Add `ecsFields` and `ecsStringify` exports that are winston formatters

packages/ecs-winston-format/index.js

@@ -35,16 +35,6 @@ try {
   // Silently ignore.
 }
 
-// Comparison of Winston's `logform.json()` for JSON serialization and
-// `ecsStringify()`.
-// - They both use `safe-stable-stringify`.
-// - Winston's exposes its `safe-stable-stringify` options, but doesn't document
-//   this. (https://github.com/winstonjs/logform#json)
-// - Winston provides a `replacer` that converts bigints to strings. Doing
-//   that is debatable. The argument *for* is that a *JavaScript* JSON parser
-//   looses precision when parsing a bigint. The argument against is that a
-//   BigInt changes type to a string rather than a number.
-// TODO: These differences should make it to docs somewhere.
 const stringify = safeStableStringify.configure()
 
 /**
@@ -183,13 +173,10 @@ class EcsFieldsTransform {
     // Core ECS logging fields.
     info['@timestamp'] = new Date().toISOString()
     info['log.level'] = info.level
-    // Removing 'level' might cause trouble for downstream winston formatters
-    // given that https://github.com/winstonjs/logform#info-objects says:
-    //
-    // > Every info must have at least the level and message properties:
-    //
-    // However info still has a `info[Symbol.for('level')]` for more reliable use.
-    delete info.level
+    // Note: We do *not* remove `info.level`, even though it is not an ECS
+    // field, because https://github.com/winstonjs/logform#info-objects says:
+    // "Every info must have at least the level and message properties".
+    // Instead, it will be excluded from serialization in `EcsStringifyTransform`.
     info['ecs.version'] = version
 
     let apm = null
@@ -290,7 +277,11 @@ class EcsStringifyTransform {
   }
 
   transform (info, opts) {
-    info[MESSAGE] = stringify(info)
+    // `info.level` must stay (see note above), but we don't want to serialize
+    // it, so exclude it from the stringified fields. There *is* a perf cost
+    // for this.
+    const { level, ...infoSansLevel } = info
+    info[MESSAGE] = stringify(infoSansLevel)
     return info
   }
 }

packages/ecs-winston-format/test/basic.test.js

@@ -85,6 +85,38 @@ test('Should set expected @timestamp, "log.level", message', t => {
   t.end()
 })
 
+// Per https://github.com/winstonjs/logform#info-objects
+// "Every info must have at least the level and message properties"
+test('Log record "info" objects should still have info.level and info.message', t => {
+  class CheckInfoTransform {
+    constructor (opts) {
+      this.options = opts
+    }
+
+    transform (info, _opts) {
+      t.ok(info.message, 'info.message exists')
+      t.ok(info.level, 'info.level exists')
+      return info
+    }
+  }
+
+  const cap = new CaptureTransport()
+  const logger = winston.createLogger({
+    format: winston.format.combine(
+      ecsFormat(),
+      new CheckInfoTransform()
+    ),
+    transports: [cap]
+  })
+
+  logger.info('hi')
+  // Should *not* have a 'level' field.
+  const rec = cap.records[0]
+  t.notOk(rec.level, 'should not have a "level" field')
+
+  t.end()
+})
+
 test('Should append additional fields to the log record', t => {
   const cap = new CaptureTransport()
   const logger = winston.createLogger({