for...in

Learn to iterate over object keys with for...in.

The for...in loop

To walk an object's keys in order you use for...in. The form is for (const key in object) { ... }. On each iteration key takes a key name (as a string).

For example const user = { name: "Alex", age: 25 }; for (const key in user) { console.log(key); } prints name and age in order. To reach the value use user[key]. for...in is designed for objects; using it on arrays is technically possible but discouraged because it can mix indices and properties.

for...in vs for...of

for...in walks object keys; for...of walks iterable elements (array, string, Map, Set). They are designed for different purposes; mixing them leads to errors.

If you try for...of on a plain object you get a TypeError because objects are not iterable. If you try for...in on an array you get the indices ("0", "1", ...) which is usually unwanted. The rule is simple: objects use for...in, arrays use for...of.

Exercise Print the object keys

The user variable holds { name: "Alex", age: 25 }. Write a for...in loop: walk each key with const key in user and print it with console.log(key). The output must be two lines: name, age.

Show example solution

const user = { name: "Alex", age: 25 };
for (const key in user) {
    console.log(key);
}

Test yourself

What is the output of for (const k in {a: 1, b: 2}) { console.log(k); }?
```
for (const k in { a: 1, b: 2 }) { console.log(k); }
```
- a b
- a 1
- 1 2 a b
- 1 2
Show answer

a b — for...in gives keys; "a" and "b" are printed.
Match each construct to its use

for...of

Array elements
for...in

Object keys
for (let i...)

Counted index
while

Conditional loop

Write the for...in header

const user = { name: "Alex", age: 25 };
____ (const key in user) {
    console.log(key);
}

Show answer

const user = { name: "Alex", age: 25 };
for (const key in user) {
    console.log(key);
}

Next lesson: break and continue

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