Originally, we wanted to update custom attributes of my entity.

We tried the following:

entity.custom_attributes.push({ key, value: value instanceof Date ? value.toISOString() : value, });

Only to realize that it doesn't look like this is supported.

We then stumbled upon an old forum post: https://forum.ftrack.com/topic/1293-update-custom-attribute-via-javascript-api/

It still contains the solution it seems. But I think we should officially support attributes, or at least document how these are updated.