Mock Service Worker Post Expectation with Sinon

msw-post-expectation.ts

// msw-utils.ts
import { rest, RestHandler } from 'msw'
import { afterEach } from 'vitest'
import sinon from 'sinon'

/**
 * Creates an MSW POST handler that will throw in afterEach
 * if the actual request body doesn’t deep-equal the expected one,
 * using Sinon for comparison.
 */
export function createPostExpectation<T>(
  url: string | RegExp,
  expectedBody: T,
  responseResolver = (_req, ctx) => ctx.status(200)
): RestHandler {
  let mismatch: { actual: any } | null = null

  const handler = rest.post(url, async (req, res, ctx) => {
    const actual = await req.json()
    let equal = true
    try {
      // Throws if actual and expectedBody are not deeply equal
      sinon.assert.deepEqual(actual, expectedBody)
    } catch {
      equal = false
    }
    if (!equal) {
      mismatch = { actual }
    }
    return res(responseResolver(req, ctx))
  })

  afterEach(() => {
    if (mismatch) {
      throw new Error(
        `POST ${url} request body mismatch:\n` +
        `  expected: ${JSON.stringify(expectedBody)}\n` +
        `  actual:   ${JSON.stringify(mismatch.actual)}`
      )
    }
  })

  return handler
}