You can use TextDecoder:

try {
  parsedJson = JSON.parse(new TextDecoder().decode(response as ArrayBuffer));
} catch (e) {
  parsedJson = {};
}

In my case the request is waiting for an ArrayBuffer response, but if an error occours on the backend, I get a JSON object containing the error. I check if the response is an error with this code.

The try-catch block is needed because if the ArrayBuffer is not a json, then the JSON.parse() method will throw an error.

As @wOxxOm said, convert it into a string and then parse it.

Note: This will not work for IE (and possibly Edge as well).

// Obviosuly replace this with whatever your array buffer is
const arrayBuffer = new TextEncoder().encode(JSON.stringify({crawlerInfo: "crawler info"}))

const json = JSON.parse(new TextDecoder().decode(arrayBuffer))
console.log(json)

Use the syntax below to switch back to JSON.

let arrayBufferConverted = JSON.parse(
 String.fromCharCode.apply(null, new Uint8Array(res.data))
   )

Edit: As @Paul LeBeau points out, my answer assumes that the response is ASCII encoded.

Basically you just need to decode the ArrayBuffer into a string and use JSON.parse().

var decodedString = String.fromCharCode.apply(null, new Uint8Array(data));
var obj = JSON.parse(decodedString);
var message = obj['message'];

I ran tests in IE11 & Chrome and this works just fine.