This rule raises an issue when a traditional C-style for loop is used to iterate over an array when a for-of loop would be more appropriate.

Why is this an issue?

Traditional for loops with manual index management are prone to off-by-one errors, which are among the most common bugs in software development. These errors occur when the loop condition is incorrectly specified (like using ⇐ instead of <) or when array bounds are miscalculated.

Modern JavaScript provides the for-of loop syntax, which eliminates the need for manual index management. This approach is:

• Safer: No risk of off-by-one errors since there’s no manual index arithmetic
• More readable: The intent to iterate over array elements is clearer
• Less error-prone: No need to remember array.length or worry about loop conditions

When you need both the index and the element, you can use array.entries() with destructuring assignment, which still avoids manual index management while providing both values.

What is the potential impact?

Using traditional for loops increases the risk of off-by-one errors that can lead to:

• Array index out of bounds errors
• Infinite loops or premature loop termination
• Logic errors that cause incorrect program behavior
• Reduced code maintainability due to more complex loop logic

How to fix?

Replace the traditional for loop with a for-of loop when you only need the array elements.

Non-compliant code example

for (let index = 0; index < array.length; index++) {
    const element = array[index]; // Noncompliant
    console.log(element);
}

Compliant code example

for (const element of array) {
    console.log(element);
}

Documentation

• eslint-plugin-unicorn - Rule no-for-loop
• MDN - for…​of statement - Complete documentation of the for-of loop syntax and usage
• MDN - Array.prototype.entries() - Documentation for the entries() method used to get both index and value

Standards

• ECMAScript 2015 (ES6) - Official specification for for-of loops introduced in ES6