attr gets or sets properties on the Map it's called on. Here's a tour through how all of its forms work:

var people = new Map({});

// set a property:
people.attr('a', 'Alex');

// get a property:
people.attr('a'); // 'Alex'

// set and merge multiple properties:
people.attr({
    a: 'Alice',
    b: 'Bob'
});

// get all properties:
people.attr(); // {a: 'Alice', b: 'Bob'}

// set properties while removing others:
people.attr({
    b: 'Bill',
    e: 'Eve'
}, true);

people.attr(); // {b: 'Bill', e: 'Eve'}

Deep properties

attr can also set and read deep properties. All you have to do is specify the property name as you normally would if you weren't using attr.

var people = new Map({names: {}});

// set a property:
people.attr('names.a', 'Alice');

// get a property:
people.attr('names.a'); // 'Alice'
people.names.attr('a'); // 'Alice'

// get all properties:
people.attr(); // {names: {a: 'Alice'}}

Objects that are added to Observes become Observes themselves behind the scenes, so changes to deep properties fire events at each level, and you can bind at any level. As this example shows, all the same events are fired no matter what level you call attr at:

var people = new Map({names: {}});

people.bind('change', function(ev, attr, how, newVal, oldVal) {
    console.log('people change: ' + attr + ', ' + how + ', ' + newVal + ', ' + oldVal);
});

people.names.bind('change', function(ev, attr, how, newVal, oldVal) {
    console.log('people.names change' + attr + ', ' + how + ', ' + newVal + ', ' + oldVal);
});

people.bind('names', function(ev, newVal, oldVal) {
    console.log('people names: ' + newVal + ', ' + oldVal);
});

people.names.bind('a', function(ev, newVal, oldVal) {
    console.log('people.names a: ' + newVal + ', ' + oldVal);
});

people.bind('names.a', function(ev, newVal, oldVal) {
    console.log('people names.a: ' + newVal + ', ' + oldVal);
});

people.attr('names.a', 'Alice'); // people change: names.a, add, Alice, undefined
                              // people.names change: a, add, Alice, undefined
                              // people.names a: Alice, undefined
                              // people names.a: Alice, undefined

people.names.attr('b', 'Bob');   // people change: names.b, add, Bob, undefined
                              // people.names change: b, add, Bob, undefined
                              // people.names b: Bob, undefined
                              // people names.b: Bob, undefined

Properties with dots in their name

As shown above, attr enables reading and setting deep properties so special care must be taken when property names include dots (.). When setting a property containing dots, attr looks for an existing container object in the path. If found, it will repeat the process for the child Map and the rest of the path; if not, any remaining path (dots included) becomes the property key to be set on the container.

var person = new Map({
    'first.name': 'Alice',
    'second': {
        'name': 'Amy',
        'old.name': 'Andrea'
    }
});

person.attr('first.name', 'Bob'); // 'Alice' -> 'Bob'
person.attr('second.name', 'Bob'); // 'Amy' -> 'Bob'
person.attr({'second.old.name': 'Bob'}); // 'Andrea' -> 'Bob'
person.attr({'second.better.name': 'Bob'}); // 'better.name' is set to 'Bob' on `person.second`
person.attr({'third.name': 'Bob'}); // 'third.name' is set to 'Bob' on `person`

A property foo and a property foo.bar will be in conflict with each other; when reading 'foo.bar' with attr, the full string foo.bar takes precedence, but when writing, foo takes precedence and foo.bar cannot be written to. For this reason, it is inadvisable to set properties that create these conflicts.

var person = new Map({
    'first.name': 'Alice',
    'first': {
        'name': 'Amy'
    }
});

person.attr('first.name'); // 'Alice'
person.attr('first').attr('name'); // 'Amy'

person.attr('first.name', 'Bob'); // 'Amy' -> 'Bob'
person.attr('first').attr('name'); // 'Amy' -> 'Bob'

See also

For information on the events that are fired on property changes and how to listen for those events, see [can.Map.prototype.bind bind].