ecmascript 6 - How to capture the json text inside fetch's then() -

this how call fetch

fetchopts = {...} return fetch(url, fetchopts)     .then(checkstatus) 

the url call return json data, in error condition:

{error_message:"encountered issue update" status:"error", status_code:400} 

i hold of error_message field value , pass error constructor.

inside checkstatus, handles exception way:

function checkstatus(response) {   let error = null;    if (!response.headers.get('content-type').includes('application/json')) {     error = new error(response.statustext);     error.response = response;     throw error;   }    if (response.status >= 200 && response.status < 300) {     return response;   }    if (response.ok) {     return response;   }   const jsondata = response.json() ; // <- `jsondata` promise   error = new error(/* how extract error_message json data? */);   error.response = response;   throw error; } 

i have checked question not address need fetch: reject promise json error object. accepted answer seems, based on comments, not resolve op's question entirely.

i may misunderstanding problem, error_message member exposed property of data object promise returns, right? seems this:

response.json() .then(jsondata => {   if (jsondata.error_message) {     error = new error(jsondata.error_message)   } }