precondition

Enforces that unsafe requests include a conditional header (If-Match or If-Unmodified-Since) before proceeding. Prevents lost-update problems by requiring clients to prove they have a current representation.

Pipeline stage: Negotiation (Fast Fail)

Import

import { precondition } from "@centralping/ergo";

Options

Option	Type	Default	Description
methods	string[] | Set<string>	—	Methods to enforce on; omit to enforce unconditionally

Return Value

Returns undefined on success — the middleware is a gate.

Error Responses

Status	Condition
428 Precondition Required	Neither If-Match nor If-Unmodified-Since header is present

Usage

Standalone

import { compose, precondition } from "@centralping/ergo";

// Enforce on all methods
const pipeline = compose(
  precondition(),
);

// Enforce only on PUT and PATCH
const pipeline = compose(
  precondition({ methods: ["PUT", "PATCH"] }),
);

ergo-router

router.put("/users/:id", {
  preconditionRequired: true,
  execute: (req, res, acc) => ({
    response: { body: { updated: true } },
  }),
});

RFC References

• RFC 6585 §3 — 428 Precondition Required

API Reference

See the auto-generated precondition API docs.