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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.

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

// 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

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