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Lock screen orientation on mobile website

Acothley

I've searched this question on StackOverflow but none of the answers works. I simply want to lock screen orientation to vertical (portrait) when user opens my website on mobile phone (I dedicate my website to mobile phones only). I tried these solutions but none of them works. In html file I have:

<body onload="lockOrientation()">

In js file I tried these solutions, but none of them works:

function lockOrientation() {
    // GO INTO FULL SCREEN FIRST
    let de = document.documentElement;  //returns html element
    if (de.requestFullscreen) { de.requestFullscreen(); }
    else if (de.mozRequestFullScreen) { de.mozRequestFullScreen(); }
    else if (de.webkitRequestFullscreen) { de.webkitRequestFullscreen(); }
    else if (de.msRequestFullscreen) { de.msRequestFullscreen(); }
    screen.orientation.lock("portrait-primary");
}

function lockOrientation() {
    if(document.querySelector("body").requestFullscreen) {
        document.querySelector("body").requestFullscreen();
    }
    else if(document.querySelector("body").webkitRequestFullScreen) {
        document.querySelector("body").webkitRequestFullScreen(); 
    }
    else if(document.querySelector("body").mozRequestFullScreen) {
        document.querySelector("body").mozRequestFullScreen(); 
    }
    else if(document.querySelector("body").msRequestFullscreen) {
        document.querySelector("body").msRequestFullscreen(); 
    }    
    screen.orientation.lock("portrait-primary");
}