Before we cover the basics of ArcGIS Online, let's discuss your access to ArcGIS Online. Some readers will have employers with organizational accounts. From Esri:
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If you are not a member of an ArcGIS Online organization, you can create a public account to access ArcGIS Online. A public account is available for individuals to create and share content with limits on usage. If you already have an Esri Account, it is automatically a public account and you can use it to sign in.
That is one potential path. Another is to take advantage of Esri's free developer ArcGIS Online account. This is a limited usage account, which comes with 50 credits per month. It is an excellent way to take your first steps with ArcGIS Online.
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Find out more about the ArcGIS Online developer account at: https://developers.arcgis.com/en/
There are three key ArcGIS Online concepts which are important to cover: Named Users, web maps, and OAuth authentication.
The concept of a Named User is important to ArcGIS Online. To quote Esri:
"Named User credentials give you a unique, secure identity in the ArcGIS platform. You can use the platform through permissions given to you by your administrator, join groups, and access resources that you own or have been shared with you. Basically, a Named User is a person licensed to use the software".
Notice the use of the term platform and not specifically ArcGIS Online. You will need to be a Named User to access ArcGIS Online but, most importantly, having an identity provides access to the overall ArcGIS platform.
ArcGIS Online administrators have important controls over Named Users, which include access to data, grouping and much more.
Webmaps are at the heart of ArcGIS Online. Webmaps can be thought of as map mashups; the combination of a base map with layer overlays. Geodata published as layers in ArcGIS Online or ArcGIS Server can be combined and made available through a webmap. Other data sources include comma-separated values (CSV) files, GeoJSON files or zipped shapefiles, feature collections, ArcMap documents, ArcGIS Pro maps, or zipped file geodatabases. Esri provides a rich set of free base maps with ArcGIS Online.
ArcGIS Online administrators and publishers can create web maps. The process entails choosing a base map, adding appropriate layers, setting extent and access rights. Each web map, as we will see later, has a unique id.
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Read more about webmaps in the Esri documentation:
https://developers.arcgis.com/javascript/jshelp/intro_webmap.html
A key part of the code we will develop in the next section uses OAuth authentication. From Esri's documentation:
"OAuth 2.0 is an open standard for authorization over HTTP that allows apps to access server resources on behalf of an authorized user. It provides a mechanism that allows your user to authenticate without having to give their username and password directly to the your app. Instead, a successful login on a remote server, such as ArcGIS Online, will respond with an access token that your app can use to access protected resources on behalf of your user".
Building authentication into your mobile ArcGIS applications is important moving forward. This controls who you provide access to your data. As we will see, access to ArcGIS Online is controlled by OAuth authentication.
Again from Esri:
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OAuth user logins allow app users to authorize themselves on an ArcGIS portal and access secured resources for which they have permission, without the app handling the user's credentials. User logins can be implemented in all types of apps: browser-based Web apps, server-based Web apps, device or tablet-based apps, and desktop apps. A Web app redirects the user to the login page on the remote server, while device, tablet, and desktop-based apps use client side browser controls to integrate this login experience into the app. Your app receives a unique alphanumeric string known as an authorization code in response to a successful login. An authorization code provides proof to the server that the user has authenticated successfully and allows access to resources for which the user has permission.
Once authenticated, Named User can access webmaps, ArcGIS services and other ArcGIS Platform resources.
Let's now show the relevance of these core concepts by jumping into code.