polymer Polymer Cheat Sheet Extending an element


Docs: extending elements, inherited templates.

Instead of Polymer.Element, a custom element can extend a different element:

class ParentElement extends Polymer.Element {
  /* ... */
class ChildElement extends ParentElement {
  /* ... */

To change or add to the parent's template, override the template getter:

<dom-module id="child-element">
    <style> /* ... */ </style>
    var childTemplate;
    var childTemplate = Polymer.DomModule.import('child-element', 'template');
    var parentTemplate = ParentElement.template.cloneNode(true);
    // Or however you want to assemble these.
    childTemplate.content.insertBefore(parentTemplate.firstChild, parentTemplate);

    class ChildElement extends ParentElement {
      static get is() { return 'child-element'; }
      // Note: the more work you do here, the slower your element is to
      // boot up. You should probably do the template assembling once, in a
      // static method outside your class (like above).
      static get template() {
        return childTemplate;
    customElements.define(ChildElement.is, ChildElement);

If you don't know the parent class, you can also use:

class ChildElement extends customElements.get('parent-element') {
  /* ... */