JS For In Loop

JavaScript For..In Loop

The for..in statement iterates over the enumerable properties of an object, in arbitrary order. For each distinct property, statements can be executed.

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.

MDN link | MSDN link

Examples

// Initialize object.
a = {"a" : "Athens" , "b" : "Belgrade", "c" : "Cairo"}

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

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

// Initialize the array.
var 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.
var s = "";
for (var key in arr) {
    s += key + ": " + arr[key];
    s += "<br />";
}

document.write (s);

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