feat: add stablePatchmarks option (#72)

Solves #69.

I'm identifying patch marks with a simple regex that looks for a line starting with `@`. This seems safe since `jest-diff` appears to add whitespace at the start of all lines. Therefore, even a line of content that begins with `@` will end up padded with whitespace in the diff itself.

And rather than simply stripping out the patchmarks, I'm replacing them with `-------------`. Since the point of this is to avoid broken diffs, this solves that issue while still giving some indication that there are hidden lines in between those parts of the diff. Perhaps there's a better replacement indicator than that. We could certainly preserve the patch mark `@@` notation like `@@ ------------- @@` if we think that helps communicate the meaning of the new mark.

Author: ericbiewener

README.md

@@ -133,6 +133,8 @@ expect(valueA: any).toMatchDiffSnapshot(valueB: any, options?: Options, testName
 - `expand: boolean` (default: `false`) – expand the diff, so the whole information is preserved
 - `colors: boolean` (default: `false`) – preserve color information from Jest diff
 - `contextLines: number` (default: 5) - number of context lines to be shown at the beginning and at the end of a snapshot
+- `stablePatchmarks: boolean` (default: `false`) - prevent line number patch marks from appearing in
+diffs. This can be helpful when diffs are breaking only because of the patch marks. Changes `@@ -1,1 +1,2 @@` to `@@ --- --- @@`.
 - `aAnnotation: string` (default: `'First Value'`) - the annotation indicating from which serialization the `-` lines are
 - `bAnnotation: string` (default: `'Second Value'`) - the annotation indicating from which serialization the `+` lines are
 

__tests__/__snapshots__/snapshotDiff.test.js.snap

@@ -64,6 +64,37 @@ exports[`can use contextLines with React components 1`] = `
 +     my name"
 `;
 
+exports[`can use stablePatchmarks on diff 1`] = `
+"Snapshot Diff:
+- First value
++ Second value
+
+@@ --- --- @@
+  @some
+  @some
+  @some
+  @some
+  @some
+- @foo
++ @bar
+  @some
+  @some
+  @some
+  @some
+  @some
+@@ --- --- @@
+  @some
+  @some
+  @some
+  @some
+  @not
++ @so
+  @very
+  @long
+  @script
+  "
+`;
+
 exports[`collapses diffs and strips ansi by default 1`] = `
 "Snapshot Diff:
 - First value

__tests__/snapshotDiff.test.js

@@ -30,6 +30,51 @@ const b = `
     long
     script
 `;
+const noIndentA = `
+@some
+@some
+@some
+@some
+@some
+@foo
+@some
+@some
+@some
+@some
+@some
+@some
+@some
+@some
+@some
+@some
+@not
+@very
+@long
+@script
+`;
+const noIndentB = `
+@some
+@some
+@some
+@some
+@some
+@bar
+@some
+@some
+@some
+@some
+@some
+@some
+@some
+@some
+@some
+@some
+@not
+@so
+@very
+@long
+@script
+`;
 
 type Props = {
   test: string,
@@ -154,6 +199,10 @@ test('can use contextLines with React components', () => {
   ).toMatchSnapshot();
 });
 
+test('can use stablePatchmarks on diff', () => {
+  expect(snapshotDiff(noIndentA, noIndentB, { stablePatchmarks: true })).toMatchSnapshot();
+});
+
 describe('failed optional deps', () => {
   beforeEach(() => {
     jest.mock('react-test-renderer', () => {

src/index.js

@@ -21,6 +21,7 @@ const defaultOptions = {
   expand: false,
   colors: false,
   contextLines: -1, // Forces to use default from Jest
+  stablePatchmarks: false,
   aAnnotation: 'First value',
   bAnnotation: 'Second value',
 };
@@ -43,6 +44,10 @@ const snapshotDiff = (valueA: any, valueB: any, options?: Options): string => {
     difference = stripAnsi(difference);
   }
 
+  if (mergedOptions.stablePatchmarks && !mergedOptions.expand) {
+    difference = difference.replace(/^@@ -[0-9]+,[0-9]+ \+[0-9]+,[0-9]+ @@$/gm, '@@ --- --- @@')
+  }
+
   return SNAPSHOT_TITLE + difference;
 };