d3.map()

d3.map([object[, key]])

Constructs a new map. If object is specified, copies all enumerable properties from the specified object into this map. The specified object may also be an array or another map. An optional key function may be specified to compute the key for each value in the array. For example:

var map = d3.map([{name: "foo"}, {name: "bar"}], function(d) { return d.name; });
map.get("foo"); // {"name": "foo"}
map.get("bar"); // {"name": "bar"}
map.get("baz"); // undefined

See also nests.