JavaScript tutorial:
for...in statement

The for...in statement is used to execute one or more statements for each property of an object, or each element of an array.

Syntax

for (variable in [object | array])
    statement

The for statement syntax has these parts:

Part	Description
variable	A variable that can be any property of object or any element of array.
object, array	An object or array over which to iterate.
statement	The statement or statements to be executed for each property of object or each element of array. Can be a compound statement.

Example

Before each iteration of a loop, variable is assigned the next property of object or the next element of array. You can then use it in any of the statements inside the loop, exactly as if you were using the property of object or the element of array.

When iterating over an object, there is no way to determine or control the order in which the members of the object are assigned to variable. Iterating through an array will be performed in element order, that is, 0, 1, 2, ...

The following example illustrates the use of the for ... in statement with an object used as an associative array:

function ForInDemo() { // Create some variables. var a, key, s = ""; // Initialize object. a = {"a" : "Athens" , "b" : "Belgrade", "c" : "Cairo"} // Iterate the properties. for (key in a) { s += a[key] + "&ltBR>"; } return(s); } document.write(ForInDemo());

  To run the code, paste it into JavaScript Editor, and click the Execute button.

Note  Use the enumerator object to iterate members of a collection.

See also: for Statement, while Statement