What is the difference between for...in and for...of loops?

The for...in loop iterates over the keys (properties) of enumerable object properties, including properties in its prototype chain.

The for...of loop iterates over the values of iterable objects (Array, Map, Set, String, arguments, etc.). It does not iterate over prototype properties.

	for...in	for...of
Iterates over	Object keys (properties)	Values of iterable objects
Used with	Objects	Arrays, strings, Map, Set, and other iterable objects
Prototype	Iterates over prototype properties	Does not iterate over prototype properties
Order of traversal	Not guaranteed for non-integer keys	Guaranteed for arrays (by index)

Example of for...in:

const obj = { a: 1, b: 2 };
for (const key in obj) {
  console.log(key, obj[key]); // Outputs: "a 1", "b 2"
}

Example of for...of:

const arr = [1, 2, 3];
for (const value of arr) {
  console.log(value); // Outputs: 1, 2, 3
}

In general, for iterating over arrays and similar structures, for...of is preferred, while for iterating over object properties, for...in is used (often in conjunction with hasOwnProperty to exclude prototype properties).