When I try to create an iframe dynamically with parameters in the URL, javascript encodes them as & instead of leaving them.

Here is my code:

    let myIframe = document.getElementById("myiframe");
    let src = 'https://mywebsite.com/?first_name='+firstName+'&last_name='+lastName+'&email='+email;
    myIframe.src =src;

However, when I view the URL in the src of the rendered HTML, I get the following: https://mywebsite.com/?first_name=firstName`&last_name=lastName&`email=email

MY question is why is the javascript rendering the & as &? It's not playing well with my ability to get the parameters from within the iFrame.