Implement Snapshot Diffing in Jest

Jest's snapshot testing is a powerful tool for capturing the UI of your components. However, when snapshots fail, the default diff output can be overwhelming, especially for large snapshots. This challenge asks you to implement a custom snapshot diffing function that provides a more concise and actionable diff, highlighting only the changed lines.

Problem Description

Your task is to create a Jest custom snapshot serializer that replaces Jest's default diffing mechanism with a custom one. This custom diff should focus on presenting the changed lines in a clear and readable format, making it easier to understand the differences between the expected and actual snapshot output.

Key Requirements:

• Custom Diff Function: Implement a function that takes the expected and actual snapshot strings and returns a formatted string representing the differences.
• Highlighting Changes: The custom diff should clearly indicate added, removed, and modified lines.
• Conciseness: Avoid displaying unchanged lines to make the diff more readable.
• Integration with Jest: The solution should be integrated as a Jest custom serializer.

Expected Behavior:

When a snapshot test fails, instead of the default Jest diff, your custom diff should be displayed. This diff should be similar to what you might see in a Git diff tool, focusing only on the lines that have changed.

Edge Cases:

• Snapshots with no differences.
• Snapshots with only additions or only deletions.
• Large snapshots with numerous changes.
• Snapshots containing special characters or multi-line strings.

Examples

Example 1:

Input:

• Expected Snapshot:

  {
    "name": "Button",
    "props": {
      "text": "Click Me",
      "color": "blue"
    },
    "children": []
  }
  

• Actual Snapshot:

  {
    "name": "Button",
    "props": {
      "text": "Submit",
      "color": "blue"
    },
    "children": []
  }
  

Output (Custom Diff):

 {
   "name": "Button",
   "props": {
-    "text": "Click Me",
+    "text": "Submit",
     "color": "blue"
   },
   "children": []
 }

Explanation: The diff clearly shows that the text property within props has changed from "Click Me" to "Submit". Unchanged lines are omitted.

Example 2:

Input:

• Expected Snapshot:

  {
    "user": {
      "id": 1,
      "username": "alice",
      "isActive": true
    }
  }
  

• Actual Snapshot:

  {
    "user": {
      "id": 1,
      "username": "alice"
    }
  }
  

Output (Custom Diff):

 {
   "user": {
     "id": 1,
     "username": "alice"
-    "isActive": true
   }
 }

Explanation: The isActive property was removed from the user object. The diff indicates this deletion.

Example 3: (Multiple Changes)

Input:

• Expected Snapshot:

  [
    "item1",
    "item2",
    "item3",
    "item4"
  ]
  

• Actual Snapshot:

  [
    "item1",
    "new_item",
    "item3",
    "item5"
  ]
  

Output (Custom Diff):

 [
   "item1",
-  "item2",
+  "new_item",
   "item3",
-  "item4"
+  "item5"
 ]

Explanation: Line 2 changed from "item2" to "new_item", and line 4 changed from "item4" to "item5". The diff highlights these specific changes.

Constraints

• The input snapshots will be valid JSON strings.
• The diffing algorithm should be reasonably efficient, aiming to avoid excessive computation time for large snapshots.
• The output diff format should use standard diff markers (e.g., - for removed, + for added, potentially for context lines if needed, though the goal is conciseness).

Notes

Consider using a well-established diffing algorithm or library for comparing the two string inputs. You will need to implement a custom Jest serializer that tells Jest how to handle your specific data type (likely objects that can be serialized to JSON) and how to generate the diff when a mismatch occurs. Remember that Jest's toMatchSnapshot() and toEqual() expect a specific structure for custom serializers. You might need to explore jest.addSerializer() and the structure of objects returned by serializer functions.