I have been trying to code the following exercise on freecodecamp and I am getting the same string as is shown in desired output, but the test fails on the site.

https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/convert-html-entities

Code:

function convertHTML(str) {
    res = "";
    
    for (let i in str) {
        if (str[i] == '&') {
            res= res.concat("&");
        }
        else if (str[i] == '<') {
            res = res.concat("&lt;");
            
        }
        else if (str[i] == '>') {
            res = res.concat("&gt;");
            
        }
        else if (str[i] == '\"') {
            res = res.concat("&quot;");
        }
        else if (str[i] == "\'") {
            res = res.concat("&apos;");
        }
        else {
            res = res.concat(str[i]);
        }
    }
    return res;
}

What could be the source of error