Replace a map method with reduce in jest-diff (#4528)

packages/jest-diff/src/__tests__/__snapshots__/diff.test.js.snap

@@ -71,7 +71,7 @@ exports[`collapses big diffs to patch format 1`] = `
 <red>+ Received</>
 
 @@ -6,9 +6,9 @@</>
-</><dim>      4,
+<dim>      4,
 <dim>      5,
 <dim>      6,
 <dim>      7,
@@ -104,7 +104,7 @@ exports[`context number of lines: -1 (5 default) 1`] = `
 <red>+ Received</>
 
 @@ -6,9 +6,9 @@</>
-</><dim>      4,
+<dim>      4,
 <dim>      5,
 <dim>      6,
 <dim>      7,
@@ -121,17 +121,17 @@ exports[`context number of lines: 0  1`] = `
 <red>+ Received</>
 
 @@ -11,0 +11,1 @@</>
-</><red>+     10,</>
+<red>+     10,</>
 @@ -12,1 +13,0 @@</>
-</><green>-     10,</>"
+<green>-     10,</>"
 `;
 
 exports[`context number of lines: 1  1`] = `
 "<green>- Expected</>
 <red>+ Received</>
 
 @@ -10,4 +10,4 @@</>
-</><dim>      8,
+<dim>      8,
 <red>+     10,</>
 <dim>      9,
 <green>-     10,</>
@@ -143,7 +143,7 @@ exports[`context number of lines: 2  1`] = `
 <red>+ Received</>
 
 @@ -9,6 +9,6 @@</>
-</><dim>      7,
+<dim>      7,
 <dim>      8,
 <red>+     10,</>
 <dim>      9,
@@ -157,7 +157,7 @@ exports[`context number of lines: null (5 default) 1`] = `
 <red>+ Received</>
 
 @@ -6,9 +6,9 @@</>
-</><dim>      4,
+<dim>      4,
 <dim>      5,
 <dim>      6,
 <dim>      7,

packages/jest-diff/src/diff_strings.js

@@ -189,7 +189,7 @@ const shouldShowPatchMarks = (hunk: Hunk, oldLinesCount: number): boolean =>
 const createPatchMark = (hunk: Hunk): string => {
   const markOld = `-${hunk.oldStart},${hunk.oldLines}`;
   const markNew = `+${hunk.newStart},${hunk.newLines}`;
-  return chalk.yellow(`@@ ${markOld} ${markNew} @@\n`);
+  return chalk.yellow(`@@ ${markOld} ${markNew} @@`);
 };
 
 // Given original lines, return callback function which given indexes for hunk,
@@ -232,18 +232,20 @@ const formatHunks = (
   const getter = original && getterForHunks(original);
   const oldLinesCount = (a.match(/\n/g) || []).length;
   return hunks
-    .map((hunk: Hunk) => {
+    .reduce((lines, hunk: Hunk) => {
+      if (shouldShowPatchMarks(hunk, oldLinesCount)) {
+        lines.push(createPatchMark(hunk));
+      }
+
       // Hunk properties are one-based but index args are zero-based.
       const getOriginal =
         getter && getter(hunk.oldStart - 1, hunk.newStart - 1);
-      const lines = hunk.lines
-        .map(line => formatLine(line[0], line.slice(1), getOriginal))
-        .join('\n');
-
-      return shouldShowPatchMarks(hunk, oldLinesCount)
-        ? createPatchMark(hunk) + lines
-        : lines;
-    })
+      hunk.lines.forEach(line => {
+        lines.push(formatLine(line[0], line.slice(1), getOriginal));
+      });
+
+      return lines;
+    }, [])
     .join('\n');
 };