test: correct and complete unit test of diff()

Should be about as near full coverage as possible

Author: NeonSpork

src/danfojs-node/test/core/frame.test.ts

@@ -993,6 +993,11 @@ describe("DataFrame", function () {
     });
 
     describe("diff", function () {
+        it("Return same DataFrame if other === 0", function () {
+            const data = [[0, 2, 4], [10, 10, 10], [1, 2, 3]];
+            const df = new DataFrame(data);
+            assert.deepEqual((df.diff(0) as DataFrame).values, [[0, 2, 4], [10, 10, 10], [1, 2, 3]]);
+        });
         it("Return difference of DataFrame with previous row", function () {
             const data = [[0, 2, 4], [10, 10, 10], [1, 2, 3]];
             const df = new DataFrame(data);
@@ -1001,7 +1006,7 @@ describe("DataFrame", function () {
         it("Return difference of DataFrame with following row", function () {
             const data = [[0, 2, 4], [10, 10, 10], [1, 2, 3]];
             const df = new DataFrame(data);
-            assert.deepEqual((df.diff(-1) as DataFrame).values, [[-2, 0, 2], [8, 8, 8], [-1, 0, 1]]);
+            assert.deepEqual((df.diff(-1) as DataFrame).values, [[-10, -8, -6], [9, 8, 7], [NaN, NaN, NaN]]);
         });
         it("Return difference of a DataFrame with a Series along default axis 1", function () {
             const data = [[0, 2, 4], [10, 10, 10], [1, 2, 3]];
@@ -1017,17 +1022,18 @@ describe("DataFrame", function () {
         it("Return difference of a DataFrame with along axis 0 (column-wise), following column", function () {
             const data = [[0, 2, 4], [10, 10, 10], [1, 2, 3]];
             const df = new DataFrame(data);
-            assert.deepEqual((df.diff(1, { axis: 0 }) as DataFrame).values, [[-2, 2, NaN], [0, 0, NaN], [1, 1, NaN]]);
+            assert.deepEqual((df.diff(-1, { axis: 0 }) as DataFrame).values, [[-2, -2, NaN], [0, 0, NaN], [-1, -1, NaN]]);
         });
         it("Return difference of a DataFrame with another DataFrame along default axis 1", function () {
             const df1 = new DataFrame([[0, 2, 4], [3, 10, 4]]);
-            const df2 = new DataFrame([[1, 2, 4], [10, 5, 0]]);
-            assert.deepEqual((df1.diff(df2) as DataFrame).values, [[-1, 0, 0], [-7, 5, 4]]);
-        });
-        it("Return difference of a DataFrame with another DataFrame along axis 0", function () {
-            const df1 = new DataFrame([[0, 2, 4], [3, 10, 4]]);
-            const df2 = new DataFrame([[1, 2, 4], [10, 5, 0]]);
-            assert.deepEqual((df1.diff(df2, { axis: 0 }) as DataFrame).values, [[-1, 0, 0], [-7, 5, 4]]);
+            const df2 = new DataFrame([[-1, -2, 4], [10, 5, 0]]);
+            assert.deepEqual((df1.diff(df2) as DataFrame).values, [[1, 4, 0], [-7, 5, 4]]);
+        });
+        it("Throw error if DataFrame for diff contains string", function () {
+            const df = new DataFrame([["words", "words", "words"], ["words", "words", "words"]]);
+            assert.throws(() => {
+                df.diff(1);
+            }, Error, "TypeError: diff operation is not supported for string dtypes");
         });
     });