What is the difference between for...of and for...in loops?
for...in loop is used to traverse the keys of an object, while for...of is used to traverse the values of an iterable object. 1. for...in is suitable for ordinary objects, traversing all their enumerable attribute key names, including array indexes (returned as strings); 2. for...of is suitable for iterable objects such as arrays, strings, Map, Set, etc., and directly access element values; 3. Selection basis: If you need to process key names or check the object structure, use for...in. If you pay attention to the value itself and want the code to be concise, use for...of.
The main difference between for...of and for...in loops lies in what they iterate over. for...in loops are used to iterate over enumerable properties of an object, such as the keys (property names) in an object or array. On the other hand, for...of loops are used to iterate over iterable objects , like arrays, strings, maps, sets, and more — essentially anything that implements the iterable protocol.
for...in Loops: Iterate Over Object Keys
When you use a for...in loop, it goes through all the enumerable string-keyed properties of an object. This makes it useful for inspecting the structure of an object — not necessarily its values.
const obj = { a: 1, b: 2, c: 3 };
for (let key in obj) {
console.log(key); // Outputs: a, b, c
}- It works on any object, including arrays.
- The iteration order is not guaranteed to be numeric, especially with non-array objects.
- Be cautious when using it with arrays — it iterates over the index positions as strings , not the actual values.
Common mistake:
const arr = [10, 20, 30];
for (let i in arr) {
console.log(i); // Outputs "0", "1", "2" — not numbers, but strings!
}Use this loop when you need to access property names or keys, especially in plain objects.
for...of Loops: Iterate Over Values Directly
This loop is designed specifically for iterable objects — things like arrays, strings, maps, sets, NodeLists, etc.
const arr = [10, 20, 30];
for (let value of arr) {
console.log(value); // Outputs: 10, 20, 30
}- It gives you direct access to the elements, not their indexes or keys.
- Works great with data structures that implement the iterable protocol.
- Doesn't work directly on regular objects unless you wrap them or define your own iterator.
Example with a string:
for (let char of "hello") {
console.log(char); // h, e, l, l, o
}Use this loop when you just want to process each item in a collection without worrying about keys or indexes.
When to Use Which?
Here's a quick guide to help you decide which loop to use based on your needs:
-
✅ Use
for...inwhen:- You're working with a plain object and need to access keys or property names.
- You need to inspect an object's structure (like debugging or dynamic property access).
-
✅ Use
for...ofwhen:- You're dealing with an iterable like arrays, strings, maps, or sets.
- You care about the values themselves, not their keys or indexes.
- You want clean, readable code that focuses on the data, not the mechanics.
Also note:
-
for...incan include inherited enumerable properties if the object has a prototype chain. -
for...ofcan be broken out of usingbreak, just like a normal loop.
Basically, choose based on what you're looping through and what you need from it. One isn't better than the other — they're just mean for different purposes.
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