Getting object properties dynamically

[faveoled]

@JsType(isNative=true, namespace= JsPackage.GLOBAL)
public class JSON {

    public static native Object parse(String json);
}

Object parsed = JSON.parse("{\"a\":\"b\"}");

Hi. How can I use the parsed object to retrieve its properties in Java code?

[niloc132]

The https://github.com/google/jsinterop-base/ project provides some tools that can help here - the simplest would be Js.asPropertyMap(parsed), which would return a jsinterop.base.JsPropertyMap<Object> instance. This doesn't directly provide a way to read all keys, but you can query known keys as Object or as jsinterop.base.Any instances.

Object parsed = JSON.parse("{\"a\":\"b\"}");
JsPropertyMap<Object> contents = Js.asPropertyMap(parsed);
// Using Any lets you put in runtime checks for the property type
String value = contents.getAsAny("a").asString();
// You can also directly read the value and cast
String checked = (String) contents.get("a");
String checked2 = Js.cast(contents.get("a"));
String unchecked = Js.uncheckedCast(contents.get("a"));

Rule of thumb: Java's cast is for when Java will accept this as correct and you want the type checked, Js.cast() is for when Java won't understand (casting String to JsString for example) but it is still correct and can be type checked, and Js.uncheckedCast() is for when you want let the runtime accept what might actually fail a type check.

JsPropertyMap also provides access to iterate keys:

contents.forEach(key -> {
  // do something with contents.get(key)
});

In https://github.com/google/elemental2/ then we also have elemental2.core.JsObject, to use various methods on the JS Object type to read properties (consider Object.keys, Object.prototype.hasOwnProperty). Likely not applicable here, but worth knowing about.

[faveoled]

Thanks for the detailed answer