JavaScript for…in loop not working – Object property is not defined Error

A for…in loop is a special loop in JavaScript that enumerates all the keys (object property names) of an object. With each new iteration, a new property string is assigned to the loop variable.

Error:

Uncaught ReferenceError: Object property is not defined

If we try to compare this loop variable with a non-string variable a ReferenceError is generated which can be solved simply by using a variable in string format while comparing it with the for…in loop variable. This error can happen in two ways discussed below:

Example 1: Comparing the for…in loop variable with the non-string variable 

Here we have created an object ‘GeeksforGeeks’. We used for…in the loop to look for a property named ‘about’  and log it to the console. But here the about is not in string format, Hence we will get a Reference Error in this code.

Javascript

<script>

    // A GeeksforGeeks object
    const GeeksforGeeks = {
        about: "Computer Science portal for geeks",
        problems_count: 2690,
        used_by: ['Professionals', 'Students'],
        used_for: ['DSA Practice', ' Articles & Editorials']
    }

    for (key in GeeksforGeeks) {
        if (key === about) {

            // ReferenceError:'about' is not defined
            console.log(`GeeksforGeeks is a
                ${GeeksforGeeks[key]}.`)
            break;
        }
    }
</script>

Output:

ReferenceError: about is not defined

Solution: In the above example, simply converting ‘about‘ to the string will remove the Reference Error from the code.

Javascript

<script>

    // A GeeksforGeeks object
    const GeeksforGeeks = {
        about: "Computer Science portal for geeks",
        problems_count: 2690,
        used_by: ['Professionals', 'Students'],
        used_for: ['DSA Practice', ' Articles & Editorials']
    }

    for (key in GeeksforGeeks) {
        if (key === "about") {

            // Used string to compare with
            // loop variable
            console.log(`GeeksforGeeks is
                a ${GeeksforGeeks[key]}.`)
            break;
        }
    }
</script>

Output:

Example 2: Non-string arguments to the function

This error can also happen if we passed non-string arguments to the function, which later will be used in comparison with the loop variable. In the below code, we used the non-string argument ‘used_for’ while calling the function ‘fun’. 

Javascript

<script>

    // A GeeksforGeeks object
    const GeeksforGeeks = {
        Desc: "Computer Science portal for geeks",
        Problems_count: 2690,
        used_by: ['Professionals', 'Students'],
        used_for: ['DSA Practice', ' Articles & Editorials']
    }

    // Call the gfg method
    fun(GeeksforGeeks, used_for)

    // A method to console info about GeeksforGeeks
    function fun(obj, uses) {
        for (key in obj) {
            if (key === uses) {
                console.log(`GeeksforGeeks is
                    used for ${obj[uses]}.`)
            }
        }
    }
</script>

Output:

ReferenceError: used_for is nor defined

Solution: Passing the argument ‘used_for’ in string format while calling the function ‘fun’ will solve the Error.

Javascript

<script>

    // A GeeksforGeeks object
    const GeeksforGeeks = {
        Desc: "Computer Science portal for geeks",
        Problems_count: 2690,
        used_by: ['Professionals', 'Students'],
        used_for: ['DSA Practice', ' Articles & Editorials']
    }

    // Call the gfg method
    fun(GeeksforGeeks, "used_for")


    // A method to console info about GeeksforGeeks
    function fun(obj, uses) {
        for (key in obj) {
            if (key === uses) {
                console.log(`GeeksforGeeks is
                    used for ${obj[uses]}.`)
            }
        }
    }
</script>

Output: