Jest Mutation Operators: Testing for Side Effects

Mutation testing is a powerful technique to assess the quality of your unit tests. It works by introducing small, deliberate changes (mutations) to your code and verifying that your tests fail when these mutations are present. This challenge focuses on creating Jest mutation operators that simulate common code modifications, allowing you to evaluate how well your tests cover your codebase.

Problem Description

You are tasked with creating a set of mutation operators for Jest. These operators will take a Jest code snippet (represented as a string) and apply a specific mutation to it. The goal is to create operators that simulate realistic code changes that would break tests if the tests are not comprehensive. You'll need to implement several operators, each targeting a different aspect of the code. The operators should return the mutated code snippet as a string.

Key Requirements:

• Operator Functions: Each operator should be a function that accepts a string (the code snippet) and returns a modified string (the mutated code snippet).
• Mutation Types: Implement at least the following mutation operators:
  • negateBoolean: Changes a boolean literal (true or false) to its opposite.
  • replaceConstant: Replaces a numeric constant with another numeric constant.
  • removeReturnStatement: Removes the first return statement in a function.
  • changeOperator: Changes a binary operator (+, -, *, /) to another.
• Error Handling: Operators should gracefully handle cases where the mutation cannot be applied (e.g., negateBoolean called on a string). Return the original code snippet unchanged in such cases.
• String Manipulation: You'll primarily be working with string manipulation to apply the mutations.

Expected Behavior:

Each operator should modify the input code snippet according to its defined mutation type. The returned string should be a valid (though potentially broken) JavaScript code snippet. If the mutation is not applicable, the original code snippet should be returned.

Edge Cases to Consider:

• Multiple instances of the target element (e.g., multiple true values in a snippet). The operators should only modify the first instance.
• Code snippets that don't contain the target element.
• Invalid JavaScript syntax resulting from the mutation. The focus is on applying the mutation, not ensuring the resulting code is valid.

Examples

Example 1:

Input: "const isValid = true; return isValid;"
Output: "const isValid = false; return isValid;"
Explanation: The `negateBoolean` operator changed `true` to `false`.

Example 2:

Input: "let result = 5 + 10; return result;"
Output: "let result = 5 - 10; return result;"
Explanation: The `changeOperator` operator changed `+` to `-`.

Example 3:

Input: "const message = 'Hello';"
Output: "const message = 'Hello';"
Explanation: The `negateBoolean` operator was called on a string, so the original code is returned.

Example 4:

Input: "function add(a: number, b: number): number { return a + b; }"
Output: "function add(a: number, b: number): number { return a - b; }"
Explanation: The `changeOperator` operator changed `+` to `-`.

Constraints

• Input String Length: The input code snippet string will be no longer than 500 characters.
• Mutation Scope: Each operator should only modify the first instance of the target element.
• Operator Complexity: Keep the operators relatively simple and focused on their specific mutation type.
• Return Type: All operators must return a string.

Notes

• Consider using regular expressions for pattern matching and replacement.
• Focus on the core logic of the mutation. You don't need to handle all possible JavaScript syntax variations.
• Think about how to make your operators robust to different code styles and formatting.
• This challenge is about creating the mutation operators themselves, not about running a full mutation testing suite. You don't need to write tests for the operators themselves (although you certainly could!).
• The goal is to simulate realistic code changes that would break tests if the tests are not comprehensive.