VueInjectScript-partial.js

methods: {
  /*
    * Inject a script tag into the page
    * Parameters:
    *   path: Path of JS file
    *   target (default = head): Element that script should be injected into
    *     Pass a querySelector or a node here
    *   isAsync (default= true): Apply the 'async' property to the output tag
    *   isDefer (default = true): Apply the 'defer' property to the output tag.
    */
  
  injectScript(path, target = 'head', isAsync = true, defer = true, id = path) {
    const scriptTag = document.createElement("script");
    scriptTag.setAttribute('src', path);
    scriptTag.async = isAsync;
    scriptTag.defer = isDefer;
    scriptTag.id = id;
    
    if (target === 'head') {
      document.getElementsByTagName('head')[0].appendChild(scriptTag);
    } else {
      target.appendChild(scriptTag);
    }
  }
},
mounted() {
  this.injectScript('https://bootswatch.com/_vendor/jquery/dist/jquery.min.js');
  this.injectScript('https://bootswatch.com/_vendor/popper.js/dist/umd/popper.min.js');
  this.injectScript ('https://bootswatch.com/_vendor/bootstrap/dist/js/bootstrap.min.js');
}

I'm getting this error:
Error in mounted hook: "ReferenceError: injectScript is not defined"