fromJS()

If a reviver is optionally provided, it will be called with every collection as a Seq (beginning with the most nested collections and proceeding to the top-level collection itself), along with the key refering to each collection and the parent JS object provided as this. For the top level, object, the key will be "". This reviver is expected to return a new Immutable Iterable, allowing for custom conversions from deep JS objects.

This example converts JSON to List and OrderedMap:

Immutable.fromJS({a: {b: [10, 20, 30]}, c: 40}, function (key, value) { var isIndexed = Immutable.Iterable.isIndexed(value); return isIndexed ? value.toList() : value.toOrderedMap(); }); // true, "b", {b: [10, 20, 30]} // false, "a", {a: {b: [10, 20, 30]}, c: 40} // false, "", {"": {a: {b: [10, 20, 30]}, c: 40}}

If reviver is not provided, the default behavior will convert Arrays into Lists and Objects into Maps.

reviver acts similarly to the same parameter in JSON.parse.

Immutable.fromJS is conservative in its conversion. It will only convert arrays which pass Array.isArray to Lists, and only raw objects (no custom prototype) to Map.

Keep in mind, when using JS objects to construct Immutable Maps, that JavaScript Object properties are always strings, even if written in a quote-less shorthand, while Immutable Maps accept keys of any type.

var obj = { 1: "one" }; Object.keys(obj); // [ "1" ] obj["1"]; // "one" obj[1]; // "one" var map = Map(obj); map.get("1"); // "one" map.get(1); // undefined

Property access for JavaScript Objects first converts the key to a string, but since Immutable Map keys can be of any type the argument to get() is not altered.