// Holds the scale state. doesn't have to be in a global var,
// just for the clarity of the explanation
let allowScale = true;

// Just to show the state
// You do not need this function
function showViewportState() {
    const viewportState = document.getElementById('viewport-state');
    viewportState.innerHTML = allowScale? 'Can Scale': 'Can not Scale';
}
showViewportState();

const btn = document.getElementById('toggle');
btn.addEventListener('click', toggleViewport);

// The toggle view port updates the
function toggleViewport(){
    let viewport = document.querySelector("meta[name=viewport]");
    if (!viewport){
        // in case there is no view port meta tag creates one and add it to the head
        viewport=document.createElement('meta');
        viewport.name = "viewport";
        document.getElementsByTagName('head')[0].appendChild(viewport);
    }
    
    const content = allowScale?
        'width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no':'width=device-width, initial-scale=1.0, user-scalable=yes';
    // this is where the magic happens by changing the vewport meta tag
    viewport.setAttribute('content', content);
    allowScale = !allowScale;
    showViewportState();
}

        body{
            margin: 0;
            padding: 0;
        }
        .main{
            display: flex;
            min-height: 100vh;
            justify-content: center;
            align-items: center;
            text-align: center;
        }
        #viewport-state{
            margin-top: 20px;
            font-weight: bold;
            display: block;
        }

    

        Try to toggle on mobile, and then zoom in

        Toggle Scale