ZDK getField doesn't work

ZDK getField doesn't work

I'm trying to get the value of a field on a canvas in a client script. It keeps returning null, though I'm sure some of the fields I've tested aren't null.

Running the code below shows only getApiName to be available. The getValue method is unavailable it seems. 

The $Page.record.Datum_voorstel shortcut also doesn't work.

Any ideas?

const test = ZDK.Page.getField("Datum_voorstel");
for (const key of Object.keys(test)) {
    log(key);
}