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Browser-javascript

Reflex compiles your frontend code, defined as python functions, into a Javascript web application that runs in the user's browser. There are instances where you may need to supply custom javascript code to interop with Web APIs, use certain third-party libraries, or wrap low-level functionality that is not exposed via Reflex's Python API.

There are four ways to execute custom Javascript code into your Reflex app:

  • rx.script - Injects the script via next/script for efficient loading of inline and external Javascript code. Described further in the component library .
    • These components can be directly included in the body of a page, or they may be passed to rx.App(head_components=[rx.script(...)]) to be included in the <Head> tag of all pages.
  • rx.call_script - An event handler that evaluates arbitrary Javascript code, and optionally returns the result to another event handler.

These previous two methods can work in tandem to load external scripts and then call functions defined within them in response to user events.

The following two methods are geared towards wrapping components and are described with examples in the Wrapping React section.

  • _get_hooks and _get_custom_code in an rx.Component subclass
  • Var.create with _var_is_local=False

The rx.script component is the recommended way to load inline Javascript for greater control over frontend behavior.

The functions and variables in the script can be accessed from backend event handlers or frontend event triggers via the rx.call_script interface.

class SoundEffectState(rx.State):
    @rx.background
    async def delayed_play(self):
        await asyncio.sleep(1)
        return rx.call_script("playFromStart(button_sfx)")


def sound_effect_demo():
    return rx.hstack(
        rx.script(
            """
            var button_sfx = new Audio("/vintage-button-sound-effect.mp3")
            function playFromStart (sfx) {sfx.load(); sfx.play()}"""
        ),
        rx.button(
            "Play Immediately",
            on_click=rx.call_script(
                "playFromStart(button_sfx)"
            ),
        ),
        rx.button(
            "Play Later",
            on_click=SoundEffectState.delayed_play,
        ),
    )

External scripts can be loaded either from the assets directory, or from CDN URL, and then controlled via rx.call_script.

rx.vstack(
    rx.script(
        src="https://cdn.jsdelivr.net/gh/scottschiller/snowstorm@snowstorm_20131208/snowstorm-min.js",
        on_ready=rx.call_script(
            "snowStorm.autoStart = false; snowStorm.snowColor = '#111'"
        ),
    ),
    rx.button(
        "Start Duststorm",
        on_click=rx.call_script("snowStorm.start()"),
    ),
    rx.button(
        "Toggle Duststorm",
        on_click=rx.call_script("snowStorm.toggleSnow()"),
    ),
)

The rx.call_script function accepts a callback parameter that expects an Event Handler with one argument which will receive the result of evaluating the Javascript code. This can be used to access client-side values such as the window.location or current scroll location, or any previously defined value.

Scroll Position: {}

window.location:

class WindowState(rx.State):
    location: dict[str, str] = {}
    scroll_position: dict[str, int] = {}

    def update_location(self, location):
        self.location = location

    def update_scroll_position(self, scroll_position):
        self.scroll_position = {
            "x": scroll_position[0],
            "y": scroll_position[1],
        }

    def get_client_values(self):
        return [
            rx.call_script(
                "window.location",
                callback=WindowState.update_location,
            ),
            rx.call_script(
                "[window.scrollX, window.scrollY]",
                callback=WindowState.update_scroll_position,
            ),
        ]


def window_state_demo():
    return rx.vstack(
        rx.button(
            "Update Values",
            on_click=WindowState.get_client_values,
        ),
        rx.text(
            f"Scroll Position: {WindowState.scroll_position.to_string()}"
        ),
        rx.text("window.location:"),
        rx.text_area(
            value=WindowState.location.to_string(),
            is_read_only=True,
        ),
        on_mount=WindowState.get_client_values,
    )

To use React Hooks directly in a Reflex app, you must subclass rx.Component, typically rx.Fragment is used when the hook functionality has no visual element. The hook code is returned by the _get_hooks method, which is expected to return a str containing Javascript code which will be inserted into the page component (i.e the render function itself).

For supporting code that must be defined outside of the component render function, use _get_custom_code.

The following example uses useEffect to register global hotkeys on the document object, and then triggers an event when a specific key is pressed.

Press a, s, d or w to trigger an event

Last watched key pressed:

class GlobalKeyState(rx.State):
    key: str = ""

    def update_key(self, key):
        self.key = key


class GlobalKeyWatcher(rx.Fragment):
    # List of keys to trigger on
    keys: rx.Var[list[str]] = []

    # The event handler that will be called
    on_key_down: rx.EventHandler[lambda ev: [ev.key]]

    def _get_imports(self) -> rx.utils.imports.ImportDict:
        return rx.utils.imports.merge_imports(
            super()._get_imports(),
            {
                "react": {
                    rx.utils.imports.ImportVar(
                        tag="useEffect"
                    )
                }
            },
        )

    def _get_hooks(self) -> str | None:
        return """
            useEffect(() => {
                const handle_key = (_ev) => {
                    if (%s.includes(_ev.key))
                        %s
                }
                document.addEventListener("keydown", handle_key, false);
                return () => {
                    document.removeEventListener("keydown", handle_key, false);
                }
            })
            """ % (
            self.keys,
            rx.utils.format.format_event_chain(
                self.event_triggers["on_key_down"]
            ),
        )

    def render(self):
        return ""  # No visual element, hooks only


def global_key_demo():
    return rx.vstack(
        GlobalKeyWatcher.create(
            keys=["a", "s", "d", "w"],
            on_key_down=GlobalKeyState.update_key,
        ),
        rx.text("Press a, s, d or w to trigger an event"),
        rx.heading(
            f"Last watched key pressed: {GlobalKeyState.key}"
        ),
    )

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