fix: Support diffing single-line strings

Author: thymikee

__tests__/__snapshots__/snapshotDiff.test.js.snap

@@ -134,3 +134,13 @@ exports[`failed optional deps throws with sensible message on missing react-test
 "Failed to load optional module \\"react-test-renderer\\". If you need to compare React elements, please add \\"react-test-renderer\\" to your project's dependencies.
 Cannot find module 'non-existent-module-for-testing' from 'snapshotDiff.test.js'"
 `;
+
+exports[`supports diffing single-line strings 1`] = `
+"Snapshot Diff:
+- First value
++ Second value
+
+- foo
++ bar
+  "
+`;

__tests__/snapshotDiff.test.js

@@ -56,6 +56,10 @@ class Component extends React.Component<Props> {
   }
 }
 
+test('supports diffing single-line strings', () => {
+  expect(snapshotDiff('foo', 'bar')).toMatchSnapshot();
+});
+
 test('collapses diffs and strips ansi by default', () => {
   expect(snapshotDiff(a, b)).toMatchSnapshot();
 });

src/index.js

@@ -8,6 +8,7 @@ const prettyFormat = require('pretty-format');
 
 const { ReactElement } = prettyFormat.plugins;
 const reactElement = Symbol.for('react.element');
+const MULTILINE_REGEXP = /[\r\n]/;
 
 type Options = {
   expand?: boolean,
@@ -50,6 +51,15 @@ const isReactComponent = (value: any) =>
   value && value.$$typeof === reactElement;
 
 function diffStrings(valueA: any, valueB: any, options: Options) {
+  const multiline =
+    MULTILINE_REGEXP.test(valueA) && MULTILINE_REGEXP.test(valueB);
+
+  // jest-diff doesn't support single-line values, but we want to
+  if (typeof valueA === 'string' && typeof valueB === 'string' && !multiline) {
+    valueA += '\n'; // eslint-disable-line no-param-reassign
+    valueB += '\n'; // eslint-disable-line no-param-reassign
+  }
+
   return diff(valueA, valueB, {
     expand: options.expand,
     contextLines: options.contextLines,