Portal is like your own personal ArcGIS Online. You can create layers that are accessible using their defined REST endpoint. The 'JSON web map specification' is accessible by entering the feature layer's URL, with our without the ArcGIS Javascript API. Such as https://services5.arcgis.com/4LKAHwqnBooVDUlX/arcgis/rest/services/PARKS_Historic_Old_Growth_Trees/F... . The only difference between this (ArcGIS Online) and Portal would be the server information such as https://services5.arcgis.com would be something else like https://local.server.com.
ArcGIS Javascript API is a Javascript library located through a URL that provides functionality (through Javascript) to access ArcGIS Online and Portal REST endpoints (and other things). But the 'JSON' representation of the data being access is available through the layers URL (REST URL). The only thing needed to write a publicly accessible ArcGIS Javascript API web application that access Portal data is to have the web app housed on a publicly exposed server running IIS or Apache, and the Portal publicly (outside the agency's firewall) available with data layers that are shared to everyone.
Hope this makes sense and is answering your question.
