JavaScript can be used to extend model driven apps.

It is possible to configure model driven apps to call a JavaScript function:

When a form has loaded (OnLoad)

When a user make a change to a control (OnChange)

When a user saves a record (OnSave); and

When a user clicks a button

The JavaScript function called might:

Validate user input

Show or Hide part of the form

Update a Business Process Flow

Display a Notification; or

Create a new Record

See also: Power Apps Component Framework

Xrm.Page.data.process.moveNext() and Xrm.Page.data.process.movePrevious() can be used to update a Business Process Flow Stage.

JavaScript to Show or Hide a Tab

Xrm.Page.data.ui.tabs.get("<tab name>").setVisible(false) and Xrm.Page.data.ui.tabs.get("<tab name>").setVisible(true) can be used to hide or show a tab.

JavaScript to Read and Update Field/Control/Attribute Values

Xrm.Page.getAttribute('nh_businessneed').getValue() and Xrm.Page.getAttribute('nh_businessneed').setValue('Hello') can be used to read or update a field/column/attribute value.

JavaScript to clone a record

Configuring a model driven app to call a JavaScript function

I added a Clone button to a model driven app's form command bar and used JavaScript to implement the record copying functionality

Notice that in the Modern Command Designer the execution context is not passed to a JavaScript handler function as a parameter.