Explain what is the custom element lifecycle in Polymer.js?

Custom elements provide a component model for the web. The custom element specifications offer a set of callbacks, which are known as “custom element reactions”. These allow you to run user code in response to certain lifecycle changes. Some of the reactions, which allow making changes in elements and their descriptions are listed as follows:

• Constructor: This is called when the element is upgraded (that is, when an element is created, or when a previously-created element becomes defined).
• ConnectedCallback: This is called when the element is added to a document.
• DisconnectedCallback: This is called when the element is removed from a document.
• AttributeChangedCallback: This is called when any of the element’s attributes are changed, appended, removed, or replaced.