Fix: Response object mutation fails in ESM environments with node-fetch v3 #1429
Description
Problem
The generated Fetch client attempts to directly mutate the Response object by adding data and error properties. This fails in ESM environments using node-fetch v3 because Response objects are read-only.
Error
TypeError: Cannot set property data of #<Body> which has only a getter
at /node_modules/.pnpm/@[email protected]/node_modules/@oars/v1.ts:1237:13
Environment
- Node.js with ESM modules (
"type": "module"in package.json) - node-fetch v3.x (ESM-only version)
- swagger-typescript-api v13.0.23 (also confirmed in v13.2.13)
Root Cause
In templates/base/http-clients/fetch-http-client.ejs (lines 187-189), the code attempts to directly mutate the Response object:
const r = response as HttpResponse<T, E>;
r.data = (null as unknown) as T;
r.error = (null as unknown) as E;This works in CommonJS environments but fails in ESM with node-fetch v3 where Response objects are truly read-only and cannot have new properties added.
Impact
Anyone migrating to ESM modules or using node-fetch v3 cannot use the generated Fetch client without workarounds.
Current Workaround
Users must wrap fetch with a Proxy to make Response properties writable:
const wrappedFetch = async (...args) => {
const response = await fetch(...args);
return new Proxy(response, {
set(target, prop, value) {
if (prop === 'data' || prop === 'error') {
target[prop] = value;
return true;
}
return Reflect.set(target, prop, value);
},
get(target, prop) {
if (prop === 'data' || prop === 'error') {
return target[prop];
}
const value = target[prop];
if (typeof value === 'function') {
return value.bind(target);
}
return value;
}
});
};Proposed Solution
Create a wrapper object instead of mutating the Response directly. This maintains backward compatibility while supporting modern ESM environments.
I'll be submitting a PR with a fix shortly.