calling instance when promise resolves works one way but not another

I'm trying to load a PDF URL using Apryse's webViewer method. When I use the method as specified in their docs it works as intended (as shown below). But I don't want to initialize a new webViewer every function call so I tried to initialize webViewer once and call loadDocument() on the instance but that doesn't work. I don't get any errors or anything. What am I doing wrong?

Function calling:

loadDocViewer('https://pdfobject.com/pdf/sample.pdf')

Doesn't work:

let webViewerInstance;

WebViewer({
    path: '/@pdftron/webviewer/public',
}, document.getElementById('doc-viewer'))
    .then(instance => {
        webViewerInstance = instance
    })
    .catch(error => {
        console.error('Error initializing WebViewer:', error);
    });

function loadDocViewer(url) {
    if (!webViewerInstance) {
        console.error('Doc Viewer is not initialized! Try again in a bit.');
        return;
    }

    webViewerInstance.UI.loadDocument(url);
}

Works:

function loadDocViewer(url) {
    WebViewer({
        path: '/@pdftron/webviewer/public',
    }, document.getElementById('doc-viewer'))
        .then(instance => {
            instance.UI.loadDocument(url);
        })
        .catch(error => {
            console.error('Error initializing WebViewer:', error);
        });
}

But I don't want to initialize a new webViewer every function

In that case, i recommend you save the promise to a variable and reuse it:

let promise;
function loadDocViewer(url) {
  if (!promise) {
    promise = WebViewer(
      { path: "/@pdftron/webviewer/public" },
      document.getElementById("doc-viewer"),
    );
  }

  return promise
    .then((instance) => {
      return instance.UI.loadDocument(url);
    })
    .catch((error) => {
      console.error("Error initializing WebViewer:", error);
    });
}

The first time loadDocViewer runs it will create the promise and then have to wait a little for it to resolve. Subsequent times will not have to wait since the promise is already resolved.