How to get id startUser of discussion?

when I enter url : https://discuss.flarum.org/
I get easily userId

var discussion= app.store.getById("discussions", X);
var user = discussion.startUser();
var userId = discussion.startUser().id();

discussion.startUser() = false
and 
var userId = discussion.startUser().id();
TypeError: discussion.startUser(...).id is not a function(…)

Help me.

You want $start_user_id.

jordanjay29 You want $start_user_id.

var userId = discussion.start_user_id(); is the line in your code?

discussion.start_user_id() = false

TypeError: discussion.start_user_id is not a function(…)

BinhQuang remove the brackets

luceos
var userId = discussion.start_user_id;
console.log(userId);

undefined

when I enter

https://discuss.flarum.org

and click 1 discussion

var discussion= app.store.getById("discussions", app.current.discussion.id());
discussion.startUser().id()

run well
but I turned off the page https://discuss.flarum.org and enter

https://discuss.flarum.org/d/xx--xxxx

var discussion= app.store.getById("discussions", app.current.discussion.id());
discussion.startUser() = false

Is the Discussion object even loaded on the /d/ page?