perspective-dev/perspective · error

Custom element " " is already registered; skipping…

Error message

Custom element "${name}" is already registered; skipping duplicate registration (Perspective was loaded more than once on this page).

What it means

Generic duplicate-registration guard for perspective-viewer custom elements: before defining a custom element named ${name}, the code checks the registry and, if already defined, warns instead of letting customElements.define throw 'already defined'. It indicates the perspective-viewer module was evaluated more than once on the page.

Solutions

  1. Ensure the perspective-viewer bundle is imported/evaluated only once per page
  2. Check bundler config for duplicate chunks or both ESM and UMD builds being loaded
  3. If multiple perspective copies are unavoidable, treat the warning as safe — registration is skipped
  4. Align all perspective package versions to a single build
Defensive patterns

Strategy: fallback

When it happens

Trigger: Thrown at rust/perspective-viewer/src/rust/utils/custom_element.rs:20 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of perspective-dev/perspective@11c8238c0c (2026-09-09). Data as JSON: /api/errors/067e2b47bdca6f3f. Report an issue: GitHub.

Appendix: source

Thrown at rust/perspective-viewer/src/rust/utils/custom_element.rs:20

// ┃ ██████ ██████ ██████       █      █      █      █      █ █▄  ▀███ █       ┃
// ┃ ▄▄▄▄▄█ █▄▄▄▄▄ ▄▄▄▄▄█  ▀▀▀▀▀█▀▀▀▀▀ █ ▀▀▀▀▀█ ████████▌▐███ ███▄  ▀█ █ ▀▀▀▀▀ ┃
// ┃ █▀▀▀▀▀ █▀▀▀▀▀ █▀██▀▀ ▄▄▄▄▄ █ ▄▄▄▄▄█ ▄▄▄▄▄█ ████████▌▐███ █████▄   █ ▄▄▄▄▄ ┃
// ┃ █      ██████ █  ▀█▄       █ ██████      █      ███▌▐███ ███████▄ █       ┃
// ┣━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┫
// ┃ Copyright (c) 2017, the Perspective Authors.                              ┃
// ┃ ╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌ ┃
// ┃ This file is part of the Perspective library, distributed under the terms ┃
// ┃ of the [Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0). ┃
// ┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛

//! Utilities for building JavaScript Custom Elements with Rust.

use wasm_bindgen::prelude::*;

#[wasm_bindgen(inline_js = r#"
    export function bootstrap(psp, name, clsname, statics) {
        if (customElements.get(name)) {
            console.warn(`Custom element "${name}" is already registered; skipping duplicate registration (Perspective was loaded more than once on this page).`);
            return;
        }

        const cls = psp[clsname];
        const proto = cls.prototype;
        class x extends HTMLElement {
            constructor() {
                super();
                this._instance = new cls(this);
            }
        }

        const names = Object.getOwnPropertyNames(proto);
        for (const key of names) {
            if ('get' in Object.getOwnPropertyDescriptor(proto, key)) {
                Object.defineProperty(x.prototype, key, {
                    get: function() {
                        return this._instance[key];

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