Make `page` and `index` fields as optional

These fields may not be missing from the response of VitalSource's
`getCurrentPage` API if the book is an EPUB that does not have page numbers. As
far as I know all PDF-based books should have page numbers.

src/annotator/integrations/test/vitalsource-test.js

@@ -405,7 +405,7 @@ describe('annotator/integrations/vitalsource', () => {
       });
     });
 
-    ['absoluteURL', 'cfi', 'index', 'page'].forEach(field => {
+    ['absoluteURL', 'cfi'].forEach(field => {
       it(`throws if page info field "${field}" is missing`, async () => {
         fakeBookElement.selectPDFBook();
 
@@ -428,7 +428,7 @@ describe('annotator/integrations/vitalsource', () => {
         assert.instanceOf(error, Error);
         assert.equal(
           error.message,
-          `Chapter metadata field "${field}" is missing`
+          `Page metadata field "${field}" is missing`
         );
       });
     });

src/annotator/integrations/vitalsource.ts

@@ -337,7 +337,7 @@ export class VitalSourceContentIntegration
     // currently limit page number selectors to PDFs until more is understood
     // about when EPUB page numbers are reliable/likely to remain stable.
     const bookInfo = this._bookElement.getBookInfo();
-    if (bookInfo.format === 'pbk') {
+    if (bookInfo.format === 'pbk' && typeof pageIndex === 'number') {
       extraSelectors.push({
         type: 'PageSelector',
         index: pageIndex,
@@ -433,12 +433,12 @@ export class VitalSourceContentIntegration
     // If changes in VitalSource ever mean that critical chapter/page metadata
     // fields are missing, fail loudly. Otherwise we might create annotations
     // that cannot be re-anchored in future.
-    const expectedFields = ['absoluteURL', 'cfi', 'index', 'page'];
-    for (const field of expectedFields) {
+    const requiredFields = ['absoluteURL', 'cfi'];
+    for (const field of requiredFields) {
       // nb. We intentionally allow properties anywhere on the prototype chain,
       // rather than requiring `hasOwnProperty`.
       if (!(field in pageInfo)) {
-        throw new Error(`Chapter metadata field "${field}" is missing`);
+        throw new Error(`Page metadata field "${field}" is missing`);
       }
     }
 

src/types/vitalsource.ts

@@ -65,13 +65,13 @@ export type PageInfo = {
    * that is displayed in the VitalSource navigation controls when the
    * chapter is scrolled to the top.
    */
-  page: string;
+  page?: string;
 
   /**
    * Index of the current segment within the sequence of pages or content
    * documents that make up the book.
    */
-  index: number;
+  index?: number;
 
   /**
    * Title of the entry in the table of contents that refers to the current