View Source JavaScript interoperability

To enable LiveView client/server interaction, we instantiate a LiveSocket. For example:

import {Socket} from "phoenix"
import {LiveSocket} from "phoenix_live_view"

let csrfToken = document.querySelector("meta[name='csrf-token']").getAttribute("content")
let liveSocket = new LiveSocket("/live", Socket, {params: {_csrf_token: csrfToken}})
liveSocket.connect()

All options are passed directly to the Phoenix.Socket constructor, except for the following LiveView specific options:

bindingPrefix - the prefix to use for phoenix bindings. Defaults "phx-"
params - the connect_params to pass to the view's mount callback. May be a literal object or closure returning an object. When a closure is provided, the function receives the view's element.
hooks – a reference to a user-defined hooks namespace, containing client callbacks for server/client interop. See the Client hooks section below for details.
uploaders – a reference to a user-defined uploaders namespace, containing client callbacks for client-side direct-to-cloud uploads. See the External uploads guide for details.

Debugging client events

To aid debugging on the client when troubleshooting issues, the enableDebug() and disableDebug() functions are exposed on the LiveSocket JavaScript instance. Calling enableDebug() turns on debug logging which includes LiveView life-cycle and payload events as they come and go from client to server. In practice, you can expose your instance on window for quick access in the browser's web console, for example:

// app.js
let liveSocket = new LiveSocket(...)
liveSocket.connect()
window.liveSocket = liveSocket

// in the browser's web console
>> liveSocket.enableDebug()

The debug state uses the browser's built-in sessionStorage, so it will remain in effect for as long as your browser session lasts.

Simulating Latency

Proper handling of latency is critical for good UX. LiveView's CSS loading states allow the client to provide user feedback while awaiting a server response. In development, near zero latency on localhost does not allow latency to be easily represented or tested, so LiveView includes a latency simulator with the JavaScript client to ensure your application provides a pleasant experience. Like the enableDebug() function above, the LiveSocket instance includes enableLatencySim(milliseconds) and disableLatencySim() functions which apply throughout the current browser session. The enableLatencySim function accepts an integer in milliseconds for the round-trip-time to the server. For example:

// app.js
let liveSocket = new LiveSocket(...)
liveSocket.connect()
window.liveSocket = liveSocket

// in the browser's web console
>> liveSocket.enableLatencySim(1000)
[Log] latency simulator enabled for the duration of this browser session.
      Call disableLatencySim() to disable

Handling server-pushed events

When the server uses Phoenix.LiveView.push_event/3, the event name will be dispatched in the browser with the phx: prefix. For example, imagine the following template where you want to highlight an existing element from the server to draw the user's attention:

<div id={"item-#{item.id}"} class="item">
  <%= item.title %>
</div>

Next, the server can issue a highlight using the standard push_event:

def handle_info({:item_updated, item}, socket) do
  {:noreply, push_event(socket, "highlight", %{id: "item-#{item.id}"})}
end

Finally, a window event listener can listen for the event and conditionally execute the highlight command if the element matches:

let liveSocket = new LiveSocket(...)
window.addEventListener("phx:highlight", (e) => {
  let el = document.getElementById(e.detail.id)
  if(el) {
    // logic for highlighting
  }
})

If you desire, you can also integrate this functionality with Phoenix' JS commands, executing JS commands for the given element whenever highlight is triggered. First, update the element to embed the JS command into a data attribute:

<div id={"item-#{item.id}"} class="item" data-highlight={JS.transition("highlight")}>
  <%= item.title %>
</div>

Now, in the event listener, use LiveSocket.execJS to trigger all JS commands in the new attribute:

let liveSocket = new LiveSocket(...)
window.addEventListener("phx:highlight", (e) => {
  document.querySelectorAll(`[data-highlight]`).forEach(el => {
    if(el.id == e.detail.id){
      liveSocket.execJS(el, el.getAttribute("data-highlight"))
    }
  })
})

Client hooks via `phx-hook`

To handle custom client-side JavaScript when an element is added, updated, or removed by the server, a hook object may be provided via phx-hook. phx-hook must point to an object with the following life-cycle callbacks:

mounted - the element has been added to the DOM and its server LiveView has finished mounting
beforeUpdate - the element is about to be updated in the DOM. Note: any call here must be synchronous as the operation cannot be deferred or cancelled.
updated - the element has been updated in the DOM by the server
destroyed - the element has been removed from the page, either by a parent update, or by the parent being removed entirely
disconnected - the element's parent LiveView has disconnected from the server
reconnected - the element's parent LiveView has reconnected to the server

Note: When using hooks outside the context of a LiveView, mounted is the only callback invoked, and only those elements on the page at DOM ready will be tracked. For dynamic tracking of the DOM as elements are added, removed, and updated, a LiveView should be used.

The above life-cycle callbacks have in-scope access to the following attributes:

el - attribute referencing the bound DOM node
liveSocket - the reference to the underlying LiveSocket instance
pushEvent(event, payload, (reply, ref) => ...) - method to push an event from the client to the LiveView server
pushEventTo(selectorOrTarget, event, payload, (reply, ref) => ...) - method to push targeted events from the client to LiveViews and LiveComponents. It sends the event to the LiveComponent or LiveView the selectorOrTarget is defined in, where its value can be either a query selector or an actual DOM element. If the query selector returns more than one element it will send the event to all of them, even if all the elements are in the same LiveComponent or LiveView. pushEventTo supports passing the node element e.g. this.el instead of selector e.g. "#" + this.el.id as the first parameter for target.
handleEvent(event, (payload) => ...) - method to handle an event pushed from the server
upload(name, files) - method to inject a list of file-like objects into an uploader.
uploadTo(selectorOrTarget, name, files) - method to inject a list of file-like objects into an uploader. The hook will send the files to the uploader with name defined by allow_upload/3 on the server-side. Dispatching new uploads triggers an input change event which will be sent to the LiveComponent or LiveView the selectorOrTarget is defined in, where its value can be either a query selector or an actual DOM element. If the query selector returns more than one live file input, an error will be logged.

For example, the markup for a controlled input for phone-number formatting could be written like this:

<input type="text" name="user[phone_number]" id="user-phone-number" phx-hook="PhoneNumber" />

Then a hook callback object could be defined and passed to the socket:

/**
 * @type {Object.<string, import("phoenix_live_view").ViewHook>}
 */
let Hooks = {}
Hooks.PhoneNumber = {
  mounted() {
    this.el.addEventListener("input", e => {
      let match = this.el.value.replace(/\D/g, "").match(/^(\d{3})(\d{3})(\d{4})$/)
      if(match) {
        this.el.value = `${match[1]}-${match[2]}-${match[3]}`
      }
    })
  }
}

let liveSocket = new LiveSocket("/live", Socket, {hooks: Hooks, ...})
...

Note: when using phx-hook, a unique DOM ID must always be set.

For integration with client-side libraries which require a broader access to full DOM management, the LiveSocket constructor accepts a dom option with an onBeforeElUpdated callback. The fromEl and toEl DOM nodes are passed to the function just before the DOM patch operations occurs in LiveView. This allows external libraries to (re)initialize DOM elements or copy attributes as necessary as LiveView performs its own patch operations. The update operation cannot be cancelled or deferred, and the return value is ignored.

For example, the following option could be used to guarantee that some attributes set on the client-side are kept intact:

...
let liveSocket = new LiveSocket("/live", Socket, {
  params: {_csrf_token: csrfToken},
  hooks: Hooks,
  dom: {
    onBeforeElUpdated(from, to) {
      for (const attr of from.attributes) {
        if (attr.name.startsWith("data-js-")) {
          to.setAttribute(attr.name, attr.value);
        }
      }
    }
  }
}

In the example above, all attributes starting with data-js- won't be replaced when the DOM is patched by LiveView.

Client-server communication

A hook can push events to the LiveView by using the pushEvent function and receive a reply from the server via a {:reply, map, socket} return value. The reply payload will be passed to the optional pushEvent response callback.

Communication with the hook from the server can be done by reading data attributes on the hook element or by using Phoenix.LiveView.push_event/3 on the server and handleEvent on the client.

For example, to implement infinite scrolling, one can pass the current page using data attributes:

<div id="infinite-scroll" phx-hook="InfiniteScroll" data-page={@page}>

And then in the client:

/**
 * @type {import("phoenix_live_view").ViewHook}
 */
Hooks.InfiniteScroll = {
  page() { return this.el.dataset.page },
  mounted(){
    this.pending = this.page()
    window.addEventListener("scroll", e => {
      if(this.pending == this.page() && scrollAt() > 90){
        this.pending = this.page() + 1
        this.pushEvent("load-more", {})
      }
    })
  },
  updated(){ this.pending = this.page() }
}

However, the data attribute approach is not a good approach if you need to frequently push data to the client. To push out-of-band events to the client, for example to render charting points, one could do:

<div id="chart" phx-hook="Chart">

And then on the client:

/**
 * @type {import("phoenix_live_view").ViewHook}
 */
Hooks.Chart = {
  mounted(){
    this.handleEvent("points", ({points}) => MyChartLib.addPoints(points))
  }
}

And then you can push events as:

{:noreply, push_event(socket, "points", %{points: new_points})}

Events pushed from the server via push_event are global and will be dispatched to all active hooks on the client who are handling that event. If you need to scope events (for example when pushing from a live component that has siblings on the current live view), then this must be done by namespacing them:

def update(%{id: id, points: points} = assigns, socket) do
  socket =
    socket
    |> assign(assigns)
    |> push_event("points-#{id}", points)

  {:ok, socket}
end

And then on the client:

Hooks.Chart = {
  mounted(){
    this.handleEvent(`points-${this.el.id}`, (points) => MyChartLib.addPoints(points));
  }
}

Note: In case a LiveView pushes events and renders content, handleEvent callbacks are invoked after the page is updated. Therefore, if the LiveView redirects at the same time it pushes events, callbacks won't be invoked on the old page's elements. Callbacks would be invoked on the redirected page's newly mounted hook elements.

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