Use arrow functions to preserve this

lib/persistence.js

@@ -195,12 +195,11 @@ class Persistence {
     let corruptItems = 0
 
     const lineStream = byline(rawStream)
-    const that = this
     let length = 0
 
-    lineStream.on('data', function (line) {
+    lineStream.on('data', (line) => {
       try {
-        const doc = model.deserialize(that.beforeDeserialization(line))
+        const doc = model.deserialize(this.beforeDeserialization(line))
         if (doc._id) {
           if (doc.$$deleted === true) delete dataById[doc._id]
           else dataById[doc._id] = doc
@@ -213,10 +212,10 @@ class Persistence {
       length++
     })
 
-    lineStream.on('end', function () {
+    lineStream.on('end', () => {
       // A bit lenient on corruption
-      if (length > 0 && corruptItems / length > that.corruptAlertThreshold) {
+      if (length > 0 && corruptItems / length > this.corruptAlertThreshold) {
-        const err = new Error('More than ' + Math.floor(100 * that.corruptAlertThreshold) + '% of the data file is corrupt, the wrong beforeDeserialization hook may be used. Cautiously refusing to start NeDB to prevent dataloss')
+        const err = new Error('More than ' + Math.floor(100 * this.corruptAlertThreshold) + '% of the data file is corrupt, the wrong beforeDeserialization hook may be used. Cautiously refusing to start NeDB to prevent dataloss')
         cb(err, null)
         return
       }