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clientAction

A client-side mutation function that runs in the browser, allowing you to handle form submissions entirely on the client or augment server action behavior.

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Signature

export function clientAction(args: ClientActionFunctionArgs): Promise<Data> | Data
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args
ClientActionFunctionArgs
required
Arguments passed to the clientAction function
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return
Data | Promise<Data>
Data returned to the UI (accessible via useActionData())

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Basic Example

import { Form, useActionData } from "react-router";

export async function clientAction({ request }: Route.ClientActionArgs) {
  const formData = await request.formData();
  const email = formData.get("email");
  
  // Validate on client
  if (!email?.includes("@")) {
    return { error: "Invalid email" };
  }
  
  // Save to localStorage
  localStorage.setItem("newsletter", email);
  
  return { success: true };
}

export default function Newsletter() {
  const actionData = useActionData<typeof clientAction>();
  
  return (
    <Form method="post">
      <input name="email" type="email" />
      {actionData?.error && <span>{actionData.error}</span>}
      {actionData?.success && <span>Subscribed!</span>}
      <button type="submit">Subscribe</button>
    </Form>
  );
}

​
Calling Server Action

export async function action({ request }: Route.ActionArgs) {
  const formData = await request.formData();
  const comment = await db.comment.create({
    data: { text: formData.get("text") }
  });
  return { comment };
}

export async function clientAction({ 
  request, 
  serverAction 
}: Route.ClientActionArgs) {
  // Optimistic update
  const formData = await request.formData();
  const optimisticComment = {
    id: "temp-" + Date.now(),
    text: formData.get("text"),
    status: "pending",
  };
  
  // Show immediately in UI
  updateUIWithOptimisticComment(optimisticComment);
  
  // Call server
  const { comment } = await serverAction<typeof action>();
  
  // Replace optimistic with real
  replaceOptimisticComment(optimisticComment.id, comment);
  
  return { comment };
}

​
Client-Only Mutations

// No server action needed
export async function clientAction({ request }: Route.ClientActionArgs) {
  const formData = await request.formData();
  const theme = formData.get("theme");
  
  // Update localStorage
  localStorage.setItem("theme", theme);
  
  // Update document
  document.documentElement.setAttribute("data-theme", theme);
  
  return { theme };
}

export default function ThemeToggle() {
  return (
    <Form method="post">
      <select name="theme">
        <option value="light">Light</option>
        <option value="dark">Dark</option>
      </select>
      <button type="submit">Change Theme</button>
    </Form>
  );
}

​
Validation Before Server

export async function clientAction({ 
  request, 
  serverAction 
}: Route.ClientActionArgs) {
  const formData = await request.formData();
  
  // Client-side validation
  const errors: Record<string, string> = {};
  
  const email = formData.get("email");
  if (!email?.includes("@")) {
    errors.email = "Invalid email";
  }
  
  const password = formData.get("password");
  if (password.length < 8) {
    errors.password = "Password must be at least 8 characters";
  }
  
  // Return early if validation fails
  if (Object.keys(errors).length > 0) {
    return { errors };
  }
  
  // Call server action if valid
  return await serverAction();
}

​
Optimistic UI Updates

import { useFetcher } from "react-router";

export async function clientAction({ 
  request, 
  serverAction 
}: Route.ClientActionArgs) {
  const formData = await request.formData();
  const text = formData.get("text");
  
  // Create optimistic item
  const optimisticItem = {
    id: crypto.randomUUID(),
    text,
    createdAt: new Date().toISOString(),
    optimistic: true,
  };
  
  // Dispatch custom event for optimistic update
  window.dispatchEvent(
    new CustomEvent("optimistic-add", { detail: optimisticItem })
  );
  
  try {
    // Call server
    const result = await serverAction<typeof action>();
    
    // Replace optimistic with real
    window.dispatchEvent(
      new CustomEvent("optimistic-replace", {
        detail: { optimisticId: optimisticItem.id, real: result }
      })
    );
    
    return result;
  } catch (error) {
    // Remove optimistic on error
    window.dispatchEvent(
      new CustomEvent("optimistic-remove", { detail: optimisticItem.id })
    );
    throw error;
  }
}

​
Offline Support

export async function clientAction({ 
  request, 
  serverAction 
}: Route.ClientActionArgs) {
  const formData = await request.formData();
  
  // Queue action if offline
  if (!navigator.onLine) {
    await queueOfflineAction(formData);
    return { 
      queued: true, 
      message: "Will sync when online" 
    };
  }
  
  try {
    return await serverAction();
  } catch (error) {
    // Queue if server is unreachable
    await queueOfflineAction(formData);
    return { 
      queued: true, 
      message: "Queued for retry" 
    };
  }
}

// Process queue when online
window.addEventListener("online", async () => {
  const queued = await getQueuedActions();
  for (const action of queued) {
    await processQueuedAction(action);
  }
});

​
Client-Side Caching

const cache = new Map();

export async function clientAction({ 
  request, 
  serverAction 
}: Route.ClientActionArgs) {
  const formData = await request.formData();
  const intent = formData.get("intent");
  
  if (intent === "delete") {
    const id = formData.get("id");
    
    // Remove from cache immediately
    cache.delete(id);
    
    // Update server
    await serverAction();
    
    return { deleted: id };
  }
  
  // Other intents...
}

​
File Upload with Progress

import { useState } from "react";
import { useFetcher } from "react-router";

export async function clientAction({ request }: Route.ClientActionArgs) {
  const formData = await request.formData();
  const file = formData.get("file") as File;
  
  // Upload with progress tracking
  const result = await uploadWithProgress(file, (progress) => {
    // Emit progress events
    window.dispatchEvent(
      new CustomEvent("upload-progress", { detail: progress })
    );
  });
  
  return { url: result.url };
}

export default function FileUpload() {
  const [progress, setProgress] = useState(0);
  const fetcher = useFetcher();
  
  useEffect(() => {
    const handler = (e: CustomEvent) => setProgress(e.detail);
    window.addEventListener("upload-progress", handler);
    return () => window.removeEventListener("upload-progress", handler);
  }, []);
  
  return (
    <fetcher.Form method="post" encType="multipart/form-data">
      <input type="file" name="file" />
      <button type="submit">Upload</button>
      {progress > 0 && <progress value={progress} max={100} />}
    </fetcher.Form>
  );
}

​
Analytics and Tracking

export async function clientAction({ 
  request, 
  serverAction 
}: Route.ClientActionArgs) {
  const formData = await request.formData();
  
  // Track event
  analytics.track("form_submitted", {
    form: "contact",
    fields: Array.from(formData.keys()),
  });
  
  try {
    const result = await serverAction();
    
    // Track success
    analytics.track("form_success", { form: "contact" });
    
    return result;
  } catch (error) {
    // Track error
    analytics.track("form_error", {
      form: "contact",
      error: error.message,
    });
    
    throw error;
  }
}

​
Multi-Step Forms

export async function clientAction({ 
  request, 
  serverAction 
}: Route.ClientActionArgs) {
  const formData = await request.formData();
  const step = parseInt(formData.get("step") || "1");
  
  // Save to sessionStorage
  const existing = JSON.parse(sessionStorage.getItem("formData") || "{}");
  const updated = { ...existing, ...Object.fromEntries(formData) };
  sessionStorage.setItem("formData", JSON.stringify(updated));
  
  if (step < 3) {
    // Not final step - just return progress
    return { step: step + 1, data: updated };
  }
  
  // Final step - submit to server
  const result = await serverAction();
  sessionStorage.removeItem("formData");
  
  return result;
}

​
Best Practices

Perfect for operations that don’t need server involvement:
// ✅ Good - client-only state
export async function clientAction({ request }) {
  const formData = await request.formData();
  localStorage.setItem("preferences", JSON.stringify(
    Object.fromEntries(formData)
  ));
  return { updated: true };
}

// ❌ Bad - should use server action
export async function clientAction({ request }) {
  const formData = await request.formData();
  // This needs to be in database, use serverAction
  return { saved: true };
}
Avoid unnecessary server requests with client-side validation:
export async function clientAction({ request, serverAction }) {
  const formData = await request.formData();
  
  // Quick client validations
  if (!formData.get("email")?.includes("@")) {
    return { error: "Invalid email" };
  }
  
  // Only call server if valid
  return await serverAction();
}
Provide feedback when server is unreachable:
export async function clientAction({ request, serverAction }) {
  if (!navigator.onLine) {
    return { error: "You're offline. Please try again." };
  }
  
  try {
    return await serverAction();
  } catch (error) {
    return { error: "Server unavailable. Try again later." };
  }
}
Only use optimistic UI when failures are rare:
// ✅ Good - liking/bookmarking (low-risk)
export async function clientAction({ serverAction }) {
  showOptimisticUpdate();
  try {
    return await serverAction();
  } catch {
    revertOptimisticUpdate();
    throw error;
  }
}

// ❌ Risky - payments (high-stakes)
export async function clientAction({ serverAction }) {
  // Don't show success before server confirms!
  return await serverAction();
}

​
See Also

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