For-In Loop In JavaScript - JavaScript Tutorials

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For-in loop

A for-in loop iterates through the properties of an object and executes the loop's body once for each enumerable property of the object.

Example:

var student = { name:"Slightbook", age: 19, degree: "Bachelors" };
for (var item in student)
{
	alert(student[item]);     // => "Slightbook", then 19, then "Bachelors"
}

With each iteration, JavaScript assigns the name of the property (a string value) to the variable item. In the example above these are name, age, and degree.

var Student = function(name)
{
	this.name = name;
}
Student.prototype.age = 26;
var student = new Student("Slightbook-2");
for (var item in student)
{
	if (student.hasOwnProperty(item))
	{
		alert (student[item]);         // => Slightbook-2. age is not displayed
	}
}

We have not discussed objects yet, but the student object has a name property on the object itself. Its prototype object has an age property. The for-in loop iterates overall properties, but the hasOwnProperty ensures that the age property on the prototype does not get displayed because it is not the student's own property.