JSON, or Javascript Object Notation, has long been a common format for modern web applications using AJAX methodologies to dynamically display data. Recent years have seen an extension of this format into geography, called GeoJSON, providing web developers with a lightweight, open format that is easily parsed by Javascript and shown on simple maps. The format proved so popular that in 2014 Google Maps began supporting it, as did Esri in ArcGIS Online (AGOL) REST endpoints. As of version 10.4, you can request GeoJSON as a format from Esri services authored in ArcGIS Server. If you're working with an Esri-flavored endpoint that is older than that, you can leverage open Javascript libraries such as this one from Terraformer to convert their original version of JSON-with-geometry to GeoJSON.
Below we have a simple map of the Detroit area, and using just jQuery $.getJSON, we add this libraries layer via REST, setting the output format to GeoJSON. (You'll only find this format in the HTML query form if it's available.) Want a more challenging dataset? Try grabbing geojson from the US Wind Turbine Database, which has an ArcGIS REST endpoint here, which we can't quite get to work. But maybe you can! Or try their officially documented API, which just uses JSON (but includes coordinates!)...
All this is overlain on the Esri imagery web map tile service.