convex-functions

Convex functions

Every exported function in convex/ uses the object form with args and returns validators. Pick the type by what the handler touches: queries read, mutations write, actions call out.

Pick the function type

Type	Database	External calls	Callable by	Use for
query	Read	No	Clients, other functions	Reads. Cached and reactive.
mutation	Read and write	No	Clients, other functions	Writes. One transaction.
action	Only via runQuery and runMutation	Yes	Clients, scheduler, other actions	fetch, third party SDKs, Node APIs
internalQuery, internalMutation, internalAction	Same as the public form	Same	Only other Convex functions	Scheduled work, crons, privileged writes
httpAction	Only via runQuery and runMutation	Yes	HTTP requests in convex/http.ts	Webhooks, REST endpoints

Default to query or mutation. Reach for an action only when the handler must talk to something outside Convex.

The object form

Declare args and returns on every function. A function that returns nothing declares returns: v.null() and returns null. Hoist a shared document validator when several functions return the same shape.

// convex/tasks.ts
import { query, mutation } from "./_generated/server";
import { v } from "convex/values";

const taskValidator = v.object({
  _id: v.id("tasks"),
  _creationTime: v.number(),
  userId: v.id("users"),
  title: v.string(),
  completed: v.boolean(),
});

export const get = query({
  args: { taskId: v.id("tasks") },
  returns: v.union(taskValidator, v.null()),
  handler: async (ctx, args) => {
    return await ctx.db.get(args.taskId);
  },
});

export const remove = mutation({
  args: { taskId: v.id("tasks") },
  returns: v.null(),
  handler: async (ctx, args) => {
    await ctx.db.delete(args.taskId);
    return null;
  },
});

Reading data

Use ctx.db.get(id) for one document by id. For everything else use withIndex against an index defined in convex/schema.ts. Never call .filter() on a table query; it scans the whole table.

export const listByUser = query({
  args: { userId: v.id("users") },
  returns: v.array(taskValidator),
  handler: async (ctx, args) => {
    return await ctx.db
      .query("tasks")
      .withIndex("by_user", (q) => q.eq("userId", args.userId))
      .order("desc")
      .take(50);
  },
});

Pick the terminal method by how many documents you expect:

Method	Returns	Use when
.unique()	One doc or null, throws on more than one	The index guarantees at most one match
.first()	First doc or null	You want the newest or oldest match
.take(n)	Up to n docs	A bounded list such as a recent feed
.collect()	Every match	The result set is small and stays small
.paginate(opts)	A page plus cursor	The table is unbounded

Paginated queries take paginationOpts: paginationOptsValidator (from convex/server) as an argument.

Writing data

Method	What it does
ctx.db.insert("tasks", doc)	Inserts and returns the new id
ctx.db.patch(id, fields)	Shallow merges fields. Throws if the doc is missing
ctx.db.replace(id, doc)	Replaces the whole doc. Throws if missing
ctx.db.delete(id)	Deletes the doc

Patch directly when you do not need the old value. Reading first widens the window for write conflicts. Make mutations safe to retry.

export const rename = mutation({
  args: { taskId: v.id("tasks"), title: v.string() },
  returns: v.null(),
  handler: async (ctx, args) => {
    await ctx.db.patch(args.taskId, { title: args.title });
    return null;
  },
});

Internal functions and references

query, mutation, and action are public. Anyone with the deployment URL can call them. Use internalQuery, internalMutation, and internalAction for code that should only run from other Convex code: scheduled jobs, crons, webhook handlers, privileged writes.

Reference functions through the generated objects in ./_generated/api:

• api.tasks.get points at a public function in convex/tasks.ts
• internal.tasks.markPaid points at an internal function in the same file
• Folders map to paths: convex/billing/invoices.ts gives api.billing.invoices.list

Always schedule internal.*. Scheduled functions and crons run without a client, so a public reference there skips the auth checks a client call would hit.

// convex/messages.ts
import { mutation, internalMutation } from "./_generated/server";
import { internal } from "./_generated/api";
import { v } from "convex/values";

export const send = mutation({
  args: { channelId: v.id("channels"), content: v.string() },
  returns: v.id("messages"),
  handler: async (ctx, args) => {
    const messageId = await ctx.db.insert("messages", args);
    await ctx.scheduler.runAfter(0, internal.messages.notifySubscribers, {
      channelId: args.channelId,
      messageId,
    });
    return messageId;
  },
});

export const notifySubscribers = internalMutation({
  args: { channelId: v.id("channels"), messageId: v.id("messages") },
  returns: v.null(),
  handler: async (ctx, args) => {
    const subs = await ctx.db
      .query("subscriptions")
      .withIndex("by_channel", (q) => q.eq("channelId", args.channelId))
      .collect();
    await Promise.all(
      subs.map((sub) =>
        ctx.db.insert("notifications", {
          userId: sub.userId,
          messageId: args.messageId,
          read: false,
        }),
      ),
    );
    return null;
  },
});

Actions and runtime boundaries

Actions have no ctx.db. They read through ctx.runQuery and write through ctx.runMutation. Each call is its own transaction, so keep the count low and do related reads and writes inside one mutation.

fetch works in the default runtime. Add "use node"; as the first line of a file only when an action needs Node built ins or a Node only SDK. A "use node" file can export actions only; queries and mutations go in a separate file.

// convex/orders.ts (default runtime)
import { action } from "./_generated/server";
import { internal } from "./_generated/api";
import { v, ConvexError } from "convex/values";
import { Doc } from "./_generated/dataModel";

export const charge = action({
  args: { orderId: v.id("orders") },
  returns: v.null(),
  handler: async (ctx, args) => {
    // Same file call: annotate the result so TypeScript does not hit a circular type
    const order: Doc<"orders"> | null = await ctx.runQuery(
      internal.orders.getForCharge,
      { orderId: args.orderId },
    );
    if (!order) {
      throw new ConvexError("Order not found");
    }
    const res = await fetch("https://api.payments.example/charge", {
      method: "POST",
      body: JSON.stringify({ amount: order.total }),
    });
    await ctx.runMutation(internal.orders.setStatus, {
      orderId: args.orderId,
      status: res.ok ? "paid" : "failed",
    });
    return null;
  },
});

Doc and Id come from ./_generated/dataModel. The annotation is only needed when the called function lives in the same file.

Errors

Throw ConvexError from convex/values for anything a client should read. Its data reaches the client; a plain Error message is redacted in production. Return null for expected absences such as a lookup that finds nothing. Throw for real failures: not authenticated, not authorized, invalid input.

import { ConvexError } from "convex/values";

throw new ConvexError({ code: "NOT_FOUND", message: "Task not found" });

Thin wrappers

Keep handlers short. Put auth lookups, validation, and business logic in plain async functions that take ctx first, then call them from the wrapper. Plain helpers are testable and shared between queries and mutations without a ctx.runQuery hop.

import { QueryCtx, MutationCtx } from "./_generated/server";
import { ConvexError } from "convex/values";

export async function getCurrentUser(ctx: QueryCtx | MutationCtx) {
  const identity = await ctx.auth.getUserIdentity();
  if (!identity) {
    throw new ConvexError("Not authenticated");
  }
  const user = await ctx.db
    .query("users")
    .withIndex("by_token", (q) =>
      q.eq("tokenIdentifier", identity.tokenIdentifier),
    )
    .unique();
  if (!user) {
    throw new ConvexError("User not found");
  }
  return user;
}

From a query or mutation, call the helper directly. ctx.runQuery and ctx.runMutation are for actions and component boundaries.

Common mistakes

Mistake	Why it breaks	Do instead
No returns validator	Return shape drifts and client types lie	Declare returns, use v.null() for nothing
.filter() on a table query	Full table scan	Add an index, use withIndex
ctx.db inside an action	Actions have no database handle	ctx.runQuery and ctx.runMutation
fetch inside a query or mutation	Transactions must be deterministic	Move it to an action
Scheduling api.*	Runs public code without a client, skips auth	Schedule internal.*
"use node" in a file with queries	Bundler rejects the file	Split actions into their own file
Date.now() in a query	Breaks caching and reactivity	Pass time as an arg or store a status field
Many runQuery calls from one action	Each is a separate transaction, races appear	One mutation that does the related work
Plain Error for user messages	Message is hidden in production	ConvexError
Missing await on ctx.db or scheduler	Write may not commit	Await every ctx call

Checklist

• Object form with args and returns on every exported function
• returns: v.null() and return null when there is nothing to return
• Reads use ctx.db.get(id) or withIndex, never .filter()
• Unbounded tables use .paginate() or .take(n), not .collect()
• Mutations patch directly and are safe to retry
• Scheduled and cron targets are internal.*
• Actions never touch ctx.db
• "use node" only in files that export actions and need Node
• Same file runQuery and runMutation results have a type annotation
• Client visible errors are ConvexError
• Every ctx.* promise is awaited

Docs

• https://docs.convex.dev/llms.txt
• https://docs.convex.dev/functions
• https://docs.convex.dev/functions/validation
• https://docs.convex.dev/functions/actions
• https://docs.convex.dev/functions/error-handling