How to build json dynamically from formidable

Am trying to build a json object from request's form fields dynamically, the fields are recevied okey, but I can't get the json to be built as I want.

I tried the below approach

let userJson = {}

var form = new formidable.IncomingForm();

form.parse(req);

form.on('field', function (name, value) {
   userJson[name]=value; //didn't work
   [userJson.name]=value]; // didn't work
});

I did it manually in two steps ...

var userJson = []

    var form = new formidable.IncomingForm();

    form.parse(req);

    form.on('field', function (name, value) {
        userJson.push('"' + name + '"' + ':' + '"' + value + '"')
    });

and then I did

form.on('end', function (name, file) {
        userJson = '{' + userJson.toString() + '}'
        userJson = JSON.parse(userJson)
        console.log(userJson)
    });