1.1.1 • Public • Published

fetch-element & slotted-element

are covering the typical UI tasks:

  1. fetch data via fetch() api
  2. populate data into UI via custom render callback or provided content-type sensitive renderers: JSON/XML data to table, inline for HTML/SVG (aka html import); exposes customize-able transformation pipeline.
  3. control UI parts(slots) depending on fetch state.

git GitHub | Demo: slotted-element , fetch-element JSON as table | tests project

NPM version Published on coverage

As slots without shadow DOM primarily would be used for displaying remotely fetched data, the slotted-element is derived from fetch-element.



npm i -P slotted-element

or use binary bundle from CDN

    <script type="module" scr=""></script>

The size of binary distribution bundle along with its dependency CssChain is 11Kb. Bundle has export of SlottedElement along with CssChain.

  1. if JS served by CDN as in demo, skip this step. Otherwise, add slotted-element dependency into project via package manager like npm, yarn, bower, Or simply clone fetch-element.js and slotted-element.js into project tree
  2. Import into page/module either by ES6 import or simple SCRIPT tag
  3. In page body add
<fetch-element src="url/to/some.html"></fetch-element>
<slotted-element src="url/to/some.json">
    <i slot="loading"> Loading... please wait. </i>
    <i slot="errror"> System error, please try again.  </i>
        <legend>Object or array from JSON</legend>
        <div slot="done"></div>



Gives ability to use slots without Shadow DOM and exposes API to work with slots programmatically by adding and removing slots by slot name.

Coupled with fetch-element, it provides UI management for each stage of data fetch and UI transformation.

The slots concept is described in using slots in MDN

Originally slots are designed to work in conjunction with template and shadowDOM when slot values are defined in content of element and referenced in rendered shadowDOM by name. I.e. template DOM defines which slot and where will be displayed inside of web component.

Template vs inline HTML

If the template is defined as getter method, property, or attribute then

slotted-element simulates the usual ShadowDOM slots handling by template cloning into local DOM and 
placing the slots from inner DOM into template clone. Unlike in ShadowDOM this is less efficient as template DOM 
is not reused and inner DOM has to be re-build.

Without template property defined the inner content is uses as template:

inner DOM is shown except of elements with slot attribute. It is up to application code to trigger the visibility of particular slots. embed-page activates slots for fetch-element handling when src attribute is set.

Using inline HTML is handy in CMS or publishing systems when content is authored by editors rather than developers. It is more performant than separate template as there is no content cloning involved.


  • slotsInit() read slots from internal DOM to be cloned later by
  • slotClone( name ) returns node (clone of slot subtree) to be modified before insertion by
  • slotAdd( node ) adds slot clone node to internal content immediately after original slot
  • slotAdd( name ) clones slot node and inserts immediately after original slot
  • slotsClear() removes cloned slots from internal DOM

Overrides for fetched-element to support following fetch() lifecycle slots loading, error, loaded;

  • fetch, onResponse, onResult, onError overrides of fetch-element to switch slots according to state change



  1. exposes interruptible fetch() api as web component.
  2. Input parameters and result are exposed via element attributes
  3. Provides default rendering for JSON and XML as table, inline SVG and HTML
  4. Exposes overriding methods for fetch() and render life cycle

The rendering code is loading on demand according to content-type reducing initial JS size to 4.5kb uncompressed / 1.7 gzipped.


  • get headers() override to set headers programmatically is no matching attribute is given
  • async fetch( url, options ) invokes native fetch() API. return Promise resolved upon data rendering.
  • abort() interrupts ongoing http request. fetch lifecycle overrides:
  • async onResponse( response ) sets status, contentType, responseHeaders and resolves the method for data conversion according to content type. Returns data promise from response.json() or response.text()
  • async onResult( result ) - called when data available. Invokes render( data, contentType, code ) callback and if it does not return anything renders content either from JSON as table or text as HTML Sets state="loaded"


  • render( data, contentType, code, responseHeaders ) callback allows to apply data which could be used for inner DOM changing. Returns either
    • html string to be populated as internal content or
    • true to state that internally provided rendering should be omitted.
  • onError( error )
  • json2table( data ) - default render JSON object or array to table. Override for custom render. Return html string.
  • getKeys( obj ) - override to limit or define the order of keys on json object to be rendered in table.


all attributes reflected as component properties

  • src - url for data retrieval ( data: as url TBD )
  • method - http method
  • headers - JS expression to be evaluated as string key-value object
  • state - '' or one of loading( fetch started ), rendering, loaded, error
  • status - http code response.status
  • skiprender - set to 'true' to omit response rendering. Used when binding to fetch-element via events.

NOTE: for defining the payload in http request leave src undefined and call fetch(url, options) with needed parameters

rendering by CSS

fetch-element could be self-sufficient without using a slots pattern: the state attribute is available to trigger visibility of internal dom subtree branches by [state="xxx"] ... selector.


The fetch-element is inspired by ideas of iron-ajax in regards of using properties for declarative programming. Unlike iron-ajax in fetch-element the primary use is not in data share via binding (which requires framework like PolymerJS) but in response rendering as table. The data and content rendering customization is done by callbacks via inheritance or runtime methods override.

test and demo

reside in separate repository to avoid unnecessary dependency in source repo and npm.


None, just a browser with Web Components support.

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npm i slotted-element

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  • sasha-firsov