How do you determine that the code is written with quality?

High-quality code is concise, readable, and understandable by other developers. It follows generally accepted conventions and coding standards.

Main criteria for code quality:

• Readability and maintainability: Easy to read, understand, and modify.
• Performance: Efficiently uses resources and executes quickly.
• Testability: Written in a way that makes it easy to cover with tests.
• Reliability: Works predictably and contains no critical errors.
• Scalability: Easy to extend and improve functionality.

Tools and practices to ensure quality:

1. Linters and formatters: eslint, prettier for automatic code checking and formatting.
2. Writing tests: Unit, integration, E2E tests.
3. Code review: Peer review of code to identify potential issues.
4. Following design patterns and SOLID principles.
5. Good documentation: Explanation of complex code sections and APIs.

Example of poorly readable code:

// Bad code
function procData(d) {
  let res = [];
  for (let i = 0; i < d.length; i++) {
    if (d[i].val > 0) {
      res.push(d[i].val * 2);
    }
  }
  return res;
}

Example of quality code with the same functions:

// Good code. Using clear names and array methods
function processPositiveValuesDoubled(data) {
  return data
    .filter(item => item.value > 0) // Filter elements with positive value
    .map(item => item.value * 2); // Double the value
}