Source: elements-sk/define.js

// Copyright 2018 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
/** @module elements-sk/define */
/**
 * Define a custom element definition. It will only define a tag name once
 * and will log an error if there's an attempt to define a tag name a second
 * time. This is useful for tests since you can't undefine a custom element.
 *
 * See also https://github.com/karma-runner/karma/issues/412
 *
 * @param tagName - The name of the tag.
 * @param cl - The class for the given tag.
 *
 * @example
 *
 * Instead of:
 *
 *     window.customElements.define('my-element', class extends HTMLElement {...});
 *
 * Use:
 *
 *     import { define } from 'elements-sk/define'
 *     define('my-element', class extends HTMLElement {...});
 *
 */
export function define(tagName, cl) {
    if (window.customElements.get(tagName) === undefined) {
        window.customElements.define(tagName, cl);
    }
    else {
        console.log(`Multiple registration attempts for ${tagName}. ` +
            'This should only happen during testing, ' +
            'it\'s probably an error outside of testing.');
    }
}
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