JavaScript for-in Loop

For-in loop in JavaScript is used to iterate over properties of an object. It can be a great debugging tool if we want to show the contents of an object. The for-in loop iterates only over those keys of an object which have their enumerable property set to “true”. The key values in an object have four attributes (value, writable, enumerable, configurable). Enumerable when set to “true” means that we can iterate over that property. You can read about the four key attributes in the property attributes section of Objects in JavaScript. Read more on enumerable Enumerable Properties in JavaScript.

Important points:

  • Use the for-in loop to iterate over non-array objects. Even though we can use for-in loop for array, it is generally not recommended. Instead, use a for loop for looping over an array.
  • The properties iterated with for-in loop also includes the properties of the objects higher in the Prototype chain.
  • The order in which properties are iterated may not match with the properties that are defined in the object.

Syntax:

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for (let i in obj1) {
  
    // Prints all the keys in
    // obj1 on the console
    console.log(i);
}

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Example:

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<script>
    const courses = {
  
        // Declaring a courses object
        firstCourse: "C++ STL",
        secondCourse: "DSA Self Paced",
        thirdCourse: "CS Core Subjects"
    };
  
    // Creating a new empty object with
    // prototype set to courses object
    const student1 = Object.create(courses);
  
    // Defining student1 properties and methods
    student1.id = 123;
    student1.firstName = "Prakhar";
    student1.showEnrolledCourses = function () {
        console.log(courses);
    }
  
    // Iterating over all properties of
    // student1 object
    for (let prop in student1) {
        console.log(prop + " -> " 
            + student1[prop]);
    }
</script>

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Console Output:



For in loop

For-in loop iterates over properties of an object and it’s prototype chain’s properties.
If we want to display only properties of the “student1” object which belongs to that object only and not on the prototype chain, then we can perform an “if” check with hasOwnProperty() method.

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<script>
  
    // Iterating over only those properties
    // of student 1 object which is not on
    // its prototype chain
    for (let prop in student1) {
        if (student1.hasOwnProperty(prop)) {
            console.log(prop + " -> " 
                    + student1[prop]);
        }
    }
  
</script>

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Console Output:
For in loop - hasOwnProperty

For-in loop with hasOwnProperty check, iterates over properties of the object.

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