Tell about the main principles of code refactoring. How would you conduct refactoring?

Main principles of code refactoring:

• Readability: improving the structure and clarity of the code without changing its external behavior.
• Simplicity: simplifying complex sections, reducing duplication.
• Modularity: isolating logical blocks, functions, components.
• Testability: the code should remain covered by tests to guarantee no regressions.

How to perform refactoring:

1. Cover the code with tests — to ensure behavior does not change.
2. Identify problematic areas — find duplication, large functions, tangled logic.
3. Make small changes — refactor small sections, checking tests.
4. Use techniques: renaming variables, extracting functions, removing dead code, simplifying conditions.

Example:

// Was
function calc(a, b) {
  if (a > 0 && b > 0) {
    return a + b;
  } else {
    return 0;
  }
}

// Became
function isPositive(num) {
  return num > 0;
}

function calc(a, b) {
  if (isPositive(a) && isPositive(b)) {
    return a + b;
  }
  return 0;
}

This approach improves readability and makes maintenance easier.