Manipulate SVG with javascript

HTML

<svg id="mySVG" width="10000" height="1000" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
    <line x1="0" 
        x2="0" 
        y1="0"
        y2="120"
        stroke="red"
        stroke-width="1"
    ></line>
    <line x1="120" 
        x2="120" 
        y1="0"
        y2="120"
        stroke="red"
        stroke-width="1"
    ></line>
    <line x1="0" 
        x2="120" 
        y1="0"
        y2="0"
        stroke="red"
        stroke-width="1"
    ></line>
        <line x1="0" 
        x2="120" 
        y1="120"
        y2="120"
        stroke="red"
        stroke-width="1"
    ></line>
        <line x1="0" 
        x2="120" 
        y1="60"
        y2="60"
        stroke="green"
        stroke-width="1"
    ></line>
      <line x1="0" 
        x2="120" 
        y1="30"
        y2="30"
        stroke="green"
        stroke-width="1"
    ></line>
      <line x1="0" 
        x2="120" 
        y1="90"
        y2="90"
        stroke="green"
        stroke-width="1"
    ></line>
        <line x1="30" 
        x2="30" 
        y1="0"
        y2="120"
        stroke="green"
        stroke-width="1"
    ></line>
            <line x1="60" 
        x2="60" 
        y1="0"
        y2="120"
        stroke="green"
        stroke-width="1"
    ></line>
            <line x1="90" 
        x2="90" 
        y1="0"
        y2="120"
        stroke="green"
        stroke-width="1"
    ></line>
</svg>

JavaScript

function saveSvg(svgEl, name) {
    svgEl.setAttribute("xmlns", "http://www.w3.org/2000/svg");
    var svgData = svgEl.outerHTML;
    var preface = '<?xml version="1.0" standalone="no"?>\r\n';
    var svgBlob = new Blob([preface, svgData], {type:"image/svg+xml;charset=utf-8"});
    var svgUrl = URL.createObjectURL(svgBlob);
    var downloadLink = document.createElement("a");
    downloadLink.href = svgUrl;
    downloadLink.download = name;
    document.body.appendChild(downloadLink);
    downloadLink.click();
    document.body.removeChild(downloadLink);
}

saveSvg(document.getElementById("mySVG"), 'test.svg')