Arrays in a request's config are merged positionally with the default config

Hello everyone,

first things first – thanks to all of you for devoting your valuable time to this project! I have not found a corresponding issue for this case and I am not sure whether it is a bug or a feature. If it is a feature, please read this post as a suggestion for a more explicit documentation of the behavior.

Describe the bug When I specify some request (or response) transformations in the default config and then use an empty array (or one with less transformation functions than the default has) in a request’s transformation config, some of the default transformations are still being applied.

To Reproduce

client = Axios.create({
  transformRequest: [
    deepKebabTransform,
    JSON.stringify
  ]
})

// ...

// a) 
client.put(url, data, { transformRequest: [] })

// b)
client.put(url, data, { transformRequest: [data => data] })

In both cases, JSON.stringify is still run. In case a), both functions from the default config are applied.

Expected behavior I would expect the default config entries for transformRequest and transformResponse (if not all array entries) to be either replaced completely by or merged with their counterparts in a request’s config.

Environment:

• Axios Version 0.18.0
• OS: OSX 10.15.2
• Browser: tested in Chrome 79.0.3945.88, Safari 13.0.4

Additional context Regarding the merge function in utils I came to the assumption, that the arrays would likely be treated as objects and thus their entries are effectively overridden positionally.

[chinesedfan]

@lagr Oops, I noticed that you are using 0.18. The old codes have similar bug.

Maybe you can try 0.19, but be careful with other new changes/bugs. 😪

// axios/lib/utils.js, v0.18
function merge(/* obj1, obj2, obj3, ... */) {
  var result = {};
  function assignValue(val, key) {
    if (typeof result[key] === 'object' && typeof val === 'object') {
      result[key] = merge(result[key], val);
    } else {
      result[key] = val;
    }
  }

  for (var i = 0, l = arguments.length; i < l; i++) {
    forEach(arguments[i], assignValue);
  }
  return result;
}

//
merge([1,2],[3])
// {0: 3, 1: 2}

[chinesedfan]

Duplicate of #2567. Sorry for the inconvenient.