feat: Better print for sparse arrays

packages/pretty-format/src/__tests__/prettyFormat.test.ts

@@ -41,6 +41,26 @@ describe('prettyFormat()', () => {
     expect(prettyFormat(val)).toEqual('Array [\n  1,\n  2,\n  3,\n]');
   });
 
+  it('prints a sparse array with only holes', () => {
+    // eslint-disable-next-line no-sparse-arrays
+    const val = [, , ,];
+    expect(prettyFormat(val)).toEqual('Array [\n  ,\n  ,\n  ,\n]');
+  });
+
+  it('prints a sparse array with items', () => {
+    // eslint-disable-next-line no-sparse-arrays
+    const val = [1, , , 4];
+    expect(prettyFormat(val)).toEqual('Array [\n  1,\n  ,\n  ,\n  4,\n]');
+  });
+
+  it('prints a sparse array also containing undefined values', () => {
+    // eslint-disable-next-line no-sparse-arrays
+    const val = [1, , undefined, undefined, , 4];
+    expect(prettyFormat(val)).toEqual(
+      'Array [\n  1,\n  ,\n  undefined,\n  undefined,\n  ,\n  4,\n]',
+    );
+  });
+
   it('prints a empty typed array', () => {
     const val = new Uint32Array(0);
     expect(prettyFormat(val)).toEqual('Uint32Array []');

packages/pretty-format/src/collections.ts

@@ -142,9 +142,11 @@ export function printListItems(
     const indentationNext = indentation + config.indent;
 
     for (let i = 0; i < list.length; i++) {
-      result +=
-        indentationNext +
-        printer(list[i], config, indentationNext, depth, refs);
+      result += indentationNext;
+
+      if (i in list) {
+        result += printer(list[i], config, indentationNext, depth, refs);
+      }
 
       if (i < list.length - 1) {
         result += ',' + config.spacingInner;