What are the differences between for and for-in loops when iterating over elements in JavaScript?

The for loop in JavaScript is typically used for iterating over array elements by index and looks like:

for (let i = 0; i < array.length; i++) {
  console.log(array[i]);
}

The for-in loop is designed for iterating over enumerable properties of an object, including array indices as strings:

for (let key in object) {
  console.log(key, object[key]);
}

Main differences:

• for iterates over array elements by numeric indices from 0 to array length.
• for-in iterates over object keys (including prototype properties if hasOwnProperty is not used), and for arrays, keys are strings with indices, which can lead to surprises.

For array iteration, it is recommended to use the classic for or for-of, while for-in is suitable for objects.