map.update(props)

var MyMap = DefineMap.extend({
    list: DefineList,
    name: 'string'
});
var obj = new MyMap({
    list: ['1', '2', '3'],
    foo: 'bar'
});
obj.update({
    list: ['first']
});

obj.list //-> ['first', '2', '3']
obj.foo //-> 'undefined'

Assigns each value in props to a property on this map instance named after the corresponding key in props, effectively merging props into the Map. Properties not in props will be set to undefined.