Map

In general, a map is an object that associates keys and values.

var gifts = { 'first': 'partridge', 'second': 'turtledoves', 'fifth': 'golden rings' }; 
var nobleGases = { 2: 'helium', 10: 'neon', 18: 'argon', };

var gifts = Map();
gifts['first'] = 'partridge';
gifts['second'] = 'turtledoves';
gifts['fifth'] = 'golden rings';
var nobleGases = Map();
nobleGases[2] = 'helium';
nobleGases[10] = 'neon';
nobleGases[18] = 'argon';

• Both keys and values can be any type of object.
• Each key occurs only once, but you can use the same value multiple times.
• If you look for a key that isn’t in a map, you get a null in return.

  var gifts = {'first': 'partridge'};
  assert(gifts['fifth'] == null);

• Use .length to get the number of key-value pairs in the map.

  var gifts = {'first': 'partridge'};
  gifts['fourth'] = 'calling birds';
  assert(gifts.length == 2);

• To create a map that’s a compile-time constant, add const before the map literal.

  final constantMap = const { 2: 'helium', 10: 'neon', 18: 'argon', }; 
  // constantMap[2] = 'Helium'; // Uncommenting this causes an error