You can initialize property values for a lit-element:

Initialize a property from a lit-element constructor

To initialize a property from a lit-element constructor:

  this.myString='initial value';
  this.myObject= { prop1: 'stuff', prop2: 'blah' }
**Always initialize object and array properties in the constructor if their subproperties or items are used in an HTML template.** It's fine to initialize objects to the empty object (`{}`), and arrays to the empty array (`[]`); but they must be initialized. For example, if `myObject.prop1` and `anotherObject.myArray[x]` are used in an element template, initialize them as follows: wzxhzdk:1

Initialize a property from markup in an HTML document

To provide a value for an element's declared String or Number property when the element is used in an HTML document, include the value as a string:

<script type="module" src="/my-element.js"></script>

<my-element myString="stuff"></my-element>
<my-element myNumber="8"></my-element>

To provide a value for an element's declared Boolean property when the element is used in an HTML document:

<script type="module" src="/my-element.js"></script>

<!-- Initialize myBool to true -->
<my-element myBool></my-element>

<!-- Initialize myBool to false -->
**Only `String`, `Number` and `Boolean` properties can be initialized by supplying strings in HTML.** For performance reasons, lit-element does not support supplying Object or Array property values via strings in HTML markup. See [] for alternatives.

Initialize a property from a data binding in another lit-element template

From inside another lit-element template, you can provide values to a lit-element with data bindings using JavaScript expressions:

import {LitElement, html} from '@polymer/lit-element';
import './my-element.js';

class SomeElement extends LitElement {
  _render({prop1, prop2}){
    return html`
      <my-element myprop="${prop1}"></my-element>
customElement.define('some-element', SomeElement);
**Supplying object and array property values with JavaScript expressions is...???** wzxhzdk:5