Subscription Update Token API fails due to _baseRequest expecting JSON response

[darshanpansuriya]

When calling the Subscription Update Token API using the _baseRequest method, the request sometimes fails with the following error:

Error updating subscription token: FetchError: invalid json response body at 
https://subscriptions.solidgate.com/api/v1/subscription/update-token 
reason: Unexpected end of JSON input

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Observed Behavior

• The API call successfully updates the card token on the subscription.
• However, the _baseRequest function calls res.json() in a .then() chain directly.
• Since the update-token API returns an empty response body, .json() throws the Unexpected end of JSON input error.
• As a result, the API call appears to fail, even though the token is updated correctly.

---

Root Cause

The _baseRequest method assumes all responses contain JSON.
For APIs that return no body, parsing as JSON fails.

Example of problematic code:

fetch(this.baseSolidGateUri + path, {
    method: 'POST',
    headers: {
        'Merchant': this.publicKey,
        'Signature': this._generateSignature(data),
        'Content-Type': 'application/json'
    },
    body: data
})
  .then(res => resolve(res.json())) // fails if response is empty
  .catch(err => reject(err));

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Steps to Reproduce

1. Call the update-token API via _baseRequest.
2. Notice that the card token updates successfully.
3. Observe that the promise rejects due to .json() failing on empty response.

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Expected Behavior

• _baseRequest should handle empty response bodies gracefully.
• The API call should be considered successful when the card token updates, even if the response is empty.

---

Suggested Fix / Workaround

Modify _baseRequest to always read the response as text and then try parsing JSON, falling back to the raw text if parsing fails. For example:

fetch(this.baseSolidGateUri + path, {
    method: 'POST',
    headers: {
        'Merchant': this.publicKey,
        'Signature': this._generateSignature(data),
        'Content-Type': 'application/json'
    },
    body: data
})
  .then(res => res.text()) // always safe
  .then(text => {
      try {
          return resolve(text ? JSON.parse(text) : {}); 
      } catch {
          return resolve(text); // fallback if not JSON
      }
  })
  .catch(err => reject(err));

Benefits:

• Works with empty responses.
• Preserves JSON responses when they exist.
• Compatible with .then()-based code without introducing async/await.

---

[darshanpansuriya]

@vladpistun @dgpankratov