SVG file altering

input.component.ts

  onFileChange(ev: any) {
    var data_uri = new String;
    // asuming that only 1 file is uploaded
    const file = ev.target.files[0];

    // Check file size
    if (file.size > <SILE_LIMIT>) {
      // do your feedback
      return;
    }

    new Promise((resolve, reject) => {
      const reader = new FileReader();
      // onload event triggered after data from readAsXXX is available
      reader.onerror = reject;
      reader.onload = e => {
        if (file.type == "image/svg+xml") {
          const parser = new DOMParser();
          const serializer = new XMLSerializer();
          var doc = parser.parseFromString(<string>e.target.result, "image/svg+xml");
          // check if height or width contains a % sign OR do any other check here
          if ((/%/.test(doc.documentElement.getAttribute("width"))) || /%/.test(doc.documentElement.getAttribute("height"))) {
            doc.documentElement.setAttribute("width", doc.documentElement.getAttribute("viewBox").split(" ")[2]);
            doc.documentElement.setAttribute("height", doc.documentElement.getAttribute("viewBox").split(" ")[3]);
          }
          resolve("data:image/svg+xml;base64," + btoa(serializer.serializeToString(doc)))
        } else {
          resolve(<string>e.target.result)
        }
      }
 
      if (file.type == "image/svg+xml") {
        reader.readAsText(file);
      } else {
        reader.readAsDataURL(file);
      }

    }).then((data_uri) => this.parentForm.controls[<form_field_name>].setValue(data_uri))

  }