MongoDB update operators

MongoDB update operators (https://docs.mongodb.com/manual/reference/operator/update/#update-operators) are needed in order to effect changes when performing an update. Update operators are categorized according to what element they affect: fields, arrays, modifiers, and bitwise. 

The following table summarizes the more important update operators:

Affects ...	Operator	Description
Fields	$rename	Allows you to rename a field
	$set	Overwrites the current value of the field with the new, updated, value
	$unset	Deletes the current value of a field
Arrays	$	Placeholder for first element that matches the filter query criteria
	$[]	Placeholder for all elements that match the filter query criteria
	$addToSet	Adds an element to the array only if it doesn't already exist
	$push	Adds an item to the array
Modifiers	$each	Works with $push to append multiple items to the array
	$sort	Works with $push to re-order the array
Bitwise	$bit	Used to...