Export getOffset, and give it a more descriptive name

In building a schema using `connectionFromArraySlice`, I've found it
useful to have access to functionality like `getOffset`. This commit
exposes it as an export, after changing the name to be more descriptive,
do further distinguish it from the existing `cursorToOffset`.

Here's an example of the function in use:

    resolve: async (_, args) => {
      const offset = getOffsetWithDefault(args.after, -1) + 1;
      const [articles, totalCount] = await articleLoader.readIndex(
        args.first, offset
      );
      return connectionFromPromisedArraySlice(
        articleLoader.loadMany(articles),
        args,
        {
          sliceStart: offset,
          arrayLength: totalCount,
        },
      );
    },

Without the function, I end up reimplementing equivalent logic, such as:

    const after = args.after && cursorToOffset(args.after);
    const offset = isNaN(after) ? 0 : after + 1;

Author: wincent

src/connection/arrayconnection.js

@@ -71,8 +71,8 @@ export function connectionFromArraySlice<T>(
   var {after, before, first, last} = args;
   var {sliceStart, arrayLength} = meta;
   var sliceEnd = sliceStart + arraySlice.length;
-  var beforeOffset = getOffset(before, arrayLength);
-  var afterOffset = getOffset(after, -1);
+  var beforeOffset = getOffsetWithDefault(before, arrayLength);
+  var afterOffset = getOffsetWithDefault(after, -1);
 
   var startOffset = Math.max(
     sliceStart - 1,
@@ -172,7 +172,10 @@ export function cursorForObjectInConnection<T>(
  * to use; if the cursor contains a valid offset, that will be used,
  * otherwise it will be the default.
  */
-function getOffset(cursor?: ?ConnectionCursor, defaultOffset: number): number {
+export function getOffsetWithDefault(
+  cursor?: ?ConnectionCursor,
+  defaultOffset: number
+): number {
   if (cursor == null) {
     return defaultOffset;
   }

src/index.js

@@ -21,6 +21,7 @@ export {
   connectionFromPromisedArraySlice,
   cursorForObjectInConnection,
   cursorToOffset,
+  getOffsetWithDefault,
 } from './connection/arrayconnection.js';
 
 // Helper for creating mutations with client mutation IDs