request报错

let url = $request.url;
let headers = $request.headers;
let body = $request.body;

body = body.replace(/free/,"paid");
$done({
url: url,
headers: headers,
body: body
});

TypeError: undefined is not an object (evaluating 'body.replace'). Line number 5, column: 13
16:27:39.709383 [Script Aborted]

但是报这个错，这个要怎么修改，
就是要把
http://a.com/json?platform=ios&user=free
改成
http://a.com/json?platform=ios&user=paid
再去request

求教

url = url.replace

把headers和body去掉

改成了

let url = $request.url;
url = url.replace(/free/,"paid");
$done({url});