One of the sources for assign has an enumerable key on the prototype chain

[jamesone]

I'm getting the following error when trying to use this package on react-native (0.55.4):

ExceptionsManager.js:71 Unhandled JS Exception: One of the sources for assign has an enumerable key on the prototype chain. Are you trying to assign a prototype property? We don't allow it, as this is an edge case that we do not support. This error is a performance optimization and not spec compliant.

The simulator is throwing the following error:

You can see the simulator is referencing fetchWithMiddleware.js (line: 41-13)

Any ideas why this is happening? I've looked everywhere to try and find a solution, none of them work.

I believe it has something todo with a middleware I added, however, commenting this out doesn't fix the error:

let headers = {
x: 'y',
z: 'x',
};

...otherMiddlewares
next =>
        req => {
          Object.assign(req.headers, headers);
          return next(req);
        },

[nodkz]

Try to do it in such way:

req.headers = Object.assign({}, req.headers, headers);

[jamesone]

This didn't seem to fix the error. I don't think it's related to the code snippet above ^ 🤔 I commented it out & the error remains.

[jamesone]

After investigating this further, I found that removing Object.assign on line 7 in fetchWithMiddleware fixes the error.

var _extends = function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; };

I think this is an issue with react-native not allowing the use of Object.assign in certain scenarios.

---

How can I go about creating a patch for this?

[jamesone]

I had to add the following to fetchWithMiddleware.js:

function extend(target) {
  for (var i = 1; i < arguments.length; i++) {
    var source = arguments[i];
    for (var key in source) {
      if (Object.prototype.hasOwnProperty.call(source, key)) {
        target[key] = source[key];
      }
    }
  }
  return target;
};

I then had to use it here:

return extend({}, res, { payload: payload }); Inside the runFetch method