What is updateValueAndValidity

You can subscribe to value changes of a control or the whole form.

updateValueAndValidity allows you to modify the value of one or more form controls and the flag allows you to specify if you want this to emit the value to valueChanges subscribers.

The sourcecode can be helpful to clear up exactly what it's doing:

https://github.com/angular/angular/blob/master/packages/forms/src/model.ts

Currently it seems to be doing the following (this list is based on method names):

• 'Set initial status' - which makes .status 'VALID' except if ALL controls are disabled, in which case it makes it 'DISABLED'
• 'Updates value' - this seems to set .value if the control is enabled, or clear it if disabled.
• 'Runs validator' - this updates the whole error object. So custom errors would be cleared if you'd set any.
• 'Cancel subscriptions' - stops any async validators running at the time
• 'Emit' event - (if emitEvent != false in options). This is just the value and status normal form events.
• Updates parent with same rules - unless onlySelf is set.

Note: it doesn't go down the tree, only up.

I wish they'd put something like this in the docs. They currently say 'Recalculates the value and validation status of the control.' which isn't particularly helpful.