Uncaught DOMException: Failed to execute 'define' on 'CustomElementRegistry': this name has already been used with this registry

Question

Uncaught DOMException: Failed to execute 'define' on 'CustomElementRegistry': this name has already been used with this registry at http://127.0.0.1:8000/components/@polymer/polymer/lib/elements/dom-module.js:175:16

Tried deleting node-modules and package-lock and reinstalling did not work.

Answer 1

this error is due to a custom element tag-name being registered which is already registered; to fix simply check that an element by this name hasn't already been registered. This example solution checks to see if something is already registered using the existing API and if not, registers the given Class (inheriting from/extending HTMLElement--at some point):

customElements.get('the-element') || customElements.define('the-element', HTMLTheElement);

For more on the API see https://developer.mozilla.org/docs/Web/API/CustomElementRegistry

most/mature libraries address this problem and those that don't, or are mangled by package and build process complexities can have it pop up; in most cases either updating to a current version, migrating to Lit (https://lit.dev) or patching the problem somehow provides a path to a solution; note the simpler solutions are far easier to maintain--as can be seen in the conflation of npm, polymer over the actual error in the original question; the Polymer project became lit-html and LitElement, and recently rebranded as "Lit" (and still includes these lit-things). Professionally I'm migrating away from npm and Nodejs to Deno with the aim of generally resolve the many problems related to npm and tooling insecurity and complexity, however this answer provides a more direct solution (understand the problem and fix directly, or update to the relevant latest solution which includes this somehow).

Answer 2

Well, this worked for me, with no Typescript warnings,

if (!customElements.get('the-element')) { customElements.define('the-element', HTMLTheElement); }

Hope someone will find this useful.

Cheers.

Answer 3

It is unwise to use the answers above. You want it to fail! The reason being is that your NPM should be deduping duplicate packages, so the fact that you see a certain component being defined on the custom elements registry more than once is a crucial error that you need to debug why the same component is registered more than once.

How to debug, in short, go to your browser, inspect element, network tab, refresh, figure out which files are both registering the same element. Then check in the initiator to see which files are loading those files. Then you get a way better idea of why your app is not resolving the same import to a single place (your deduped dependency).

One reason why you might face this problem is due to semver. If you have multiple different major versions of the same dependency, NPM cannot just dedupe all of the installations to your root node_modules. How you solve this is up to you. Some people use npm-aliases for their different majors of a dependency, some people implement a plugin in their build-tool to resolve paths to a single installation, etc.