Q:

axios transformRequest - how to alter JSON payload

I am using axios in my Express API and I want to transform the payload before sending it off to another API. axios has just the thing for this called transformRequest. This is where I ran into issues though. 
The code I have looks like:
const instance = axios.create({
  baseURL: 'api-url.com',
  transformRequest: [
    (data, headers) => {
      const encryptedString = encryptPayload(JSON.stringify(data));

      data = {
        SecretStuff: encryptedString,
      };

      return data;
    },
  ],  
});

// firing off my request using the instance above:
const postData = {
    id: 1,
    name: 'James',
};
instance.post('/getStuff', postData)

and ultimately, I want to post api-url.com the JSON: {"SecretStuff": "some-base64-string"} - not the postData object shown above.
From the docs, it says: "The last function in the array must return a string or an instance of Buffer, ArrayBuffer, FormData or Stream" - but of course here I am returning an object, data. Oddly enough in the axios docs it shows them returning data from transformRequest, but in their case that must be the correct data type.
How do I actually transform a payload with axios?

A:

axios.create({
    transformRequest: [(data, headers) => {
        // modify data here
        return data;
    }, ...axios.defaults.transformRequest]
});

have to append the original axios.defaults.transformRequest to the transformRequest option here.. 

A:

Wouldn't you want to JSON.stringify() your transformed post data? Like below:
const instance = axios.create({
    baseURL: 'api-url.com',
    transformRequest: [
        (data, headers) => {
            const encryptedString = encryptPayload(JSON.stringify(data));

            data = {
                SecretStuff: encryptedString,
            };

            return JSON.stringify(data);
        },
    ],  
});

