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... |
