fix: stop dropping command and runtime headers when the client has none

[maxholman]

The regression

Reported downstream: 5 tests in services/listing/test/app.test.ts fail against @block65/rest-client@14.0.0 — every one that sends a body.

RestServiceClient.#resolveHeaders bails before merging anything:

async #resolveHeaders(command: Command, runtimeOptions?: RuntimeOptions) {
  if (!this.#headers) {
    return {};   // drops command.headers AND runtimeOptions.headers
  }

A client constructed without config.headers sends no headers at all — including the accept and content-type that json() sets on every call. That is why only body-sending requests fail: they arrive without content-type: application/json and the server never parses them.

How it happened

0f03f56 inlined the old resolveHeaders() helper from lib/common.ts. That helper's if (!headers) return {} guard was correct there, because it received the already-merged object:

headers: await resolveHeaders({
  ...this.#headers,
  ...command.headers,
  ...runtimeOptions?.headers,
}),

The refactor carried the guard along but re-pointed it at this.#headers alone, so it now short-circuits the merge rather than an empty result.

The same refactor also flipped precedence — client headers were spread over command headers instead of under them, so a command could no longer override a client default.

The fix

Resolve this.#headers ?? {} and merge unconditionally, restoring the pre-14.0.0 order: client < command < runtime.

Why the suite missed it

Every test in describe("Client") shares one client constructed with headers, so this.#headers was never falsy and the early return never ran. No test constructed a bare client, and no command carried a header colliding with a client one, so the precedence flip was invisible too.

Four tests added: a bare-client block (json() defaults; command + runtime headers) and two precedence tests. Three fail without the fix — the fourth (runtime over command) passed either way, since the runtime spread was last in both versions.

Checks

tsc clean, oxfmt --check clean, oxlint unchanged (same 2 pre-existing warnings), 41/41 tests pass.

Note

14.0.0 is published, so this needs a 14.0.1 once merged.

🤖 Generated with Claude Code