Trigger.newMap in APEX Trigger

This variable returns a map of ids to the new versions of sObject records.

Note:

This map is only available in before update, after insert, after update and after undelete triggers.

    
 trigger ApexTrigger on Account(before update
 {
    Map < Id, Account > nMap = new Map < Id, Account > ();
    nMap = Trigger.newMap;
    List < Contact > cList = [SELECT LastName FROM Contact WHERE AccountId  IN: nMap.keySet()];
    for (Contact c: cList) {
        Account a = nMap.get(c.AccountId);
        c.MailingCity = a.BillingState;
    }
    update cList;
}