@polymer/polymer/lib/legacy/polymer-fn.html

The Polymer library makes it easy to create your own web components. Give your element some markup and properties, and then use it on a site. Polymer provides features like dynamic templates and data binding to reduce the amount of boilerplate you need to

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Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
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<link rel="import" href="class.html">

<script>

  (function() {
    'use strict';

    /**
     * Legacy class factory and registration helper for defining Polymer
     * elements.
     *
     * This method is equivalent to
     * `customElements.define(info.is, Polymer.Class(info));`
     *
     * See `Polymer.Class` for details on valid legacy metadata format for `info`.
     *
     * @global
     * @override
     * @function Polymer
     * @param {!PolymerInit} info Object containing Polymer metadata and functions
     *   to become class methods.
     * @return {function(new: HTMLElement)} Generated class
     * @suppress {duplicate, invalidCasts, checkTypes}
     */
    window.Polymer._polymerFn = function(info) {
      // if input is a `class` (aka a function with a prototype), use the prototype
      // remember that the `constructor` will never be called
      let klass;
      if (typeof info === 'function') {
        klass = info;
      } else {
        klass = Polymer.Class(info);
      }
      customElements.define(klass.is, /** @type {!HTMLElement} */(klass));
      return klass;
    };

  })();

</script>