arrayBuffer() is not a function?

I currently trying to download a file via a documenturl using the function below. The download function returns a promise using arrayBuffer(). When run from the front-end I receive no errors, and the arrayBuffer contains the file-data as required. However once I move the function to a .jsw file in the backend, I then receive an error saying that arrayBuffer is not a function. Where am I going wrong? The code is literally the same and the only difference is where it is stored.

import {fetch} from ‘wix-fetch’;
export function downloaddocument(Documenturl) {

//return new promise
return new Promise((resolve,reject)=>{

fetch(Documenturl, {method:'Get'}) 
.then((data)=>{resolve(data.arrayBuffer())}) 
. **catch** (error=>{reject(error)}); 

})
}

Could it be that arrayBuffer is a client-side (browser) concept and backend modules run, by def, on the server? Try Buffer (node.js concept) instead.