SQLiteCloudRowset.slice to take negative arguments

Author: gionatamettifogo

src/rowset.ts

@@ -104,20 +104,15 @@ export class SQLiteCloudRowset extends Array<SQLiteCloudRow> {
 
   /** Returns a subset of rows from this rowset */
   slice(start?: number, end?: number): SQLiteCloudRow[] {
-    start = start && start > 0 ? start : 0
-    if (end === undefined) {
-      end = this.numberOfRows
-    } else if (end < 0) {
-      end = this.numberOfRows + end
-    }
-    if (end < start) {
-      end = start
-    } else if (end > this.numberOfRows) {
-      end = this.numberOfRows
-    }
+    // validate and apply boundaries
+    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice
+    start = start === undefined ? 0 : start < 0 ? this.numberOfRows + start : start
+    start = Math.min(Math.max(start, 0), this.numberOfRows)
+    end = end === undefined ? this.numberOfRows : end < 0 ? this.numberOfRows + end : end
+    end = Math.min(Math.max(start, end), this.numberOfRows)
 
-    const slicedData = this.#data.slice(start * this.numberOfColumns, end * this.numberOfColumns)
     const slicedMetadata = { ...this.#metadata, numberOfRows: end - start }
+    const slicedData = this.#data.slice(start * this.numberOfColumns, end * this.numberOfColumns)
     return new SQLiteCloudRowset(slicedMetadata, slicedData)
   }
 }

test/rowset.test.ts

@@ -43,16 +43,32 @@ describe('rowset', () => {
       expect(Array.isArray(rowset)).toBeTruthy()
       expect(rowset).toHaveLength(50)
 
+      // slice with correct bounds
       const sliced = rowset.slice(10, 20)
       expect(sliced).toHaveLength(10)
       expect(sliced[0]).toMatchObject(rowset[10])
 
-      // slice more than actually exists!
+      // slice with end larger than length
       const largerSlice = rowset.slice(10, 100)
       expect(largerSlice).toHaveLength(40)
       expect(largerSlice[0]).toMatchObject(rowset[10])
       expect(largerSlice[39]).toMatchObject(rowset[49])
 
+      // slice last 3 elements
+      const negativeStartSlice = rowset.slice(-3)
+      expect(negativeStartSlice).toHaveLength(3)
+      expect(negativeStartSlice[0]).toMatchObject(rowset[rowset.length - 3])
+
+      // slice first 3 elements
+      const negativeEndSlice = rowset.slice(0, -3)
+      expect(negativeEndSlice).toHaveLength(rowset.length - 3)
+      expect(negativeEndSlice[0]).toMatchObject(rowset[0])
+      expect(negativeEndSlice[negativeEndSlice.length - 1]).toMatchObject(rowset[rowset.length - 4])
+
+      // slice to empty set
+      const emptySlice = rowset.slice(20, 10)
+      expect(emptySlice).toHaveLength(0)
+
       connection.close()
       done()
     })