freeCodeCamp/guide/english/javascript/loops/for-in-loop/index.md

---
title: For...In Loop
---
The `for...in` statement iterates over the enumerable properties of an object, in an arbitrary order. For each distinct property, statements can be executed.
```js
for (variable in object) {
  ...
}
```

| Required/Optional | Parameter | Description |
|-------------------|-----------|----------------------------------------------------------------------|
| Required | Variable | A different property name is assigned to variable on each iteration. |
| Optional | Object | Object whose enumerable properties are iterated. |



## Examples
```js
// Initialize object.
const a = { "a": "Athens", "b": "Belgrade", "c": "Cairo" };

// Iterate over the properties.
let s = "";
for (let key in a) {
  s += key + ": " + a[key];
  s += "<br />";
}

document.write (s);

// Output:
// a: Athens
// b: Belgrade
// c: Cairo

// Initialize the array.
let arr = new Array("zero", "one", "two");

// Add a few expando properties to the array.
arr["orange"] = "fruit";
arr["carrot"] = "vegetable";

// Iterate over the properties and elements.
let s = "";
for (let key in arr) {
    s += key + ": " + arr[key];
    s += "<br />";
}

document.write (s);

// Output:
//   0: zero
//   1: one
//   2: two
//   orange: fruit
//   carrot: vegetable

// Efficient way of getting an object's keys using an expression within the for-in loop's conditions
var myObj = {a: 1, b: 2, c:3}, myKeys = [], i=0;
for (myKeys[i++] in myObj);

document.write(myKeys);

//Output:
//   a
//   b
//   c
```
## Other Resources:
* [MDN link](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in)
* [MSDN link](https://msdn.microsoft.com/library/55wb2d34.aspx)