Writing Maintainable Code: Alternatives to continue in ESLint

What is the no-continue rule?

The no-continue rule in ESLint is a linting rule that discourages the use of the continue statement within loops. The continue statement instructs the loop to skip the remaining code in the current iteration and immediately jump to the next iteration.

Why is no-continue discouraged?

While continue can be used in certain scenarios, it often leads to code that is:

Less testable
Complex logic involving continue can make it trickier to write effective unit tests.
Less maintainable
Code that relies heavily on continue can become more difficult to modify and reason about over time.
Less readable
Excessive use of continue can make the control flow of your loops harder to understand.

Alternatives to no-continue

In many cases, you can achieve the same functionality as continue using alternative approaches:

Refactoring
Sometimes, restructuring your code to avoid nested loops or complex conditions can eliminate the need for continue.
Early return
If the loop is part of a function, consider returning a value or throwing an exception from within the loop to signal early termination.
break statement
If you need to exit the loop entirely based on a condition, use break.

Configuration options

While no-continue is generally recommended, you can configure its behavior in your ESLint configuration file:

'warn'
Reports continue statements as warnings.
'error'
Reports continue statements as errors (default).
'off'
Disables the rule completely.

Example

Consider this code with continue:

for (let i = 0; i < 10; i++) {
  if (i % 2 === 0) {
    continue;
  }
  console.log(i); // Only odd numbers will be logged
}

An alternative using break:

for (let i = 0; i < 10; i++) {
  if (i % 2 === 0) {
    break;
  }
  console.log(i); // Only odd numbers will be logged
}

Code with continue (discouraged)

// This loop skips even numbers and prints only odd numbers
for (let i = 0; i < 10; i++) {
  if (i % 2 === 0) {
    continue;
  }
  console.log(i);
}

Alternative using break (preferred)

// This loop exits the loop after finding the first even number
for (let i = 0; i < 10; i++) {
  if (i % 2 === 0) {
    break;
  }
  console.log(i); // Only prints up to the first even number
}

Code with potentially unnecessary continue

// This code checks if a number is prime and then continues
// However, the loop only iterates once because the number is changed
// inside the loop
for (let num = 2; num <= 10; num++) {
  let isPrime = true;
  for (let divisor = 2; divisor < num; divisor++) {
    if (num % divisor === 0) {
      isPrime = false;
      continue; // Potentially unnecessary here
    }
  }
  if (isPrime) {
    console.log(num + " is prime");
  }
}

// This code avoids `continue` by breaking out of the inner loop
// if a divisor is found
for (let num = 2; num <= 10; num++) {
  let isPrime = true;
  for (let divisor = 2; divisor < num; divisor++) {
    if (num % divisor === 0) {
      isPrime = false;
      break; // Exit the inner loop if not prime
    }
  }
  if (isPrime) {
    console.log(num + " is prime");
  }
}

break statement
This is the most common alternative. Use break when you need to completely exit the loop based on a condition. It jumps out of the entire loop and continues execution after the loop.
```
for (let i = 0; i < 10; i++) {
  if (i === 5) {
    break; // Exit the loop when i reaches 5
  }
  console.log(i);
}
```
Early return (within functions)
If the loop is part of a function, you can consider returning a value or throwing an exception from within the loop to signal early termination. This approach is particularly useful when the loop's purpose is to calculate or determine something specific within the function.
```
function findEvenNumber(numbers) {
  for (const num of numbers) {
    if (num % 2 === 0) {
      return num; // Return the first even number
    }
  }
  return null; // Or throw an exception if no even number found
}
```
Refactoring
Sometimes, restructuring your code to avoid nested loops or complex conditions can eliminate the need for continue. This can involve breaking down complex logic into separate functions, simplifying conditions, or using different loop structures (e.g., while loops).
Example
Instead of using continue to skip even numbers, you could create a separate loop to iterate only over odd numbers.