Ensure field data is preserved on write

Because we're continuing to store field data that isn't unloaded back
into the DOM we must make sure that we preserve the existing data on new
writes.

Author: jklina

src/index.ts

@@ -24,7 +24,18 @@ export function persistResumableFields(id: string, options?: PersistOptions): vo
 
   if (fields.length) {
     try {
-      sessionStorage.setItem(key, JSON.stringify(fields))
+      const previouslyStoredFieldsJson = sessionStorage.getItem(key)
+      let allFields: string[][] = fields
+
+      if (previouslyStoredFieldsJson !== null) {
+        const previouslyStoredFields: string[][] = JSON.parse(previouslyStoredFieldsJson)
+        const fieldsNotReplaced: string[][] = previouslyStoredFields.filter(function (oldField) {
+          return !allFields.some(field => field[0] === oldField[0])
+        })
+        allFields = allFields.concat(fieldsNotReplaced)
+      }
+
+      sessionStorage.setItem(key, JSON.stringify(allFields))
     } catch {
       // Ignore browser private mode error.
     }

test/test.js

@@ -67,13 +67,22 @@ describe('session-resume', function () {
 
   describe('persistResumableFields', function () {
     it('persist fields values to session storage', function () {
+      sessionStorage.setItem(
+        'session-resume:test-persist',
+        JSON.stringify([
+          ['my-first-field', 'old data'],
+          ['non-existant-field', 'test3']
+        ])
+      )
       document.querySelector('#my-first-field').value = 'test1'
       document.querySelector('#my-second-field').value = 'test2'
+
       persistResumableFields('test-persist')
 
       assert.deepEqual(JSON.parse(sessionStorage.getItem('session-resume:test-persist')), [
         ['my-first-field', 'test1'],
-        ['my-second-field', 'test2']
+        ['my-second-field', 'test2'],
+        ['non-existant-field', 'test3']
       ])
     })
   })