feat: add import function, closes #8

src/import.ts

@@ -0,0 +1,54 @@
+import Thing from './Thing'
+import * as types from './types'
+
+/**
+ * Import a `Thing`.
+ *
+ * @param thing The thing to be imported
+ * @param format The current format of the thing as a MIME type e.g. `text/markdown`
+ * @returns An instance of a class derived from `Thing`
+ */
+export default function import_ (thing: string | object | Thing, format: string= 'application/ld+json'): Thing {
+  if (thing instanceof Thing) {
+    return thing
+  } else if (typeof thing === 'object') {
+    return importObject(thing)
+  } else {
+    switch (format) {
+      case 'application/ld+json':
+        return importJsonLd(thing)
+      default:
+        throw Error(`Unhandled import format: ${format}`)
+    }
+  }
+}
+
+/**
+ * Import an `Object` to a `Thing`
+ *
+ * This function demarshalls a plain JavaScript object into an
+ * instance of a class derived from `Thing` based on the `type`
+ * property of the object.
+ *
+ * @param object A plain JavaScript object with a `type` property
+ * @returns An instance of a class derived from `Thing`
+ */
+export function importObject (object: any): Thing {
+  const type = object.type
+  if (!type) throw new Error('Object is missing required "type" property')
+  // @ts-ignore
+  const Type = types[type]
+  if (!Type) throw new Error(`Unknown type "${type}"`)
+  return new Type(object)
+}
+
+/**
+ * Import a JSON-LD document to a `Thing`
+ *
+ * @param jsonld A JSON-LD document with a `type` property
+ * @returns An instance of a class derived from `Thing`
+ */
+export function importJsonLd (jsonld: string): Thing {
+  const object = JSON.parse(jsonld)
+  return importObject(object)
+}

tests/import.test.ts

@@ -0,0 +1,17 @@
+import {default as import_, importJsonLd} from '../src/import'
+import Thing from '../src/Thing'
+import Person from '../src/Person'
+
+test('import:Thing', () => {
+  const thing = new Thing()
+  expect(import_('{"type": "Thing"}')).toEqual(thing)
+  expect(import_({type: 'Thing'})).toEqual(thing)
+  expect(import_(thing)).toEqual(thing)
+
+  expect(import_('{"type": "Thing"}', 'application/ld+json')).toEqual(thing)
+  expect(() => import_("", 'foo/bar')).toThrow(/^Unhandled import format: foo\/bar/)
+})
+
+test('import:Person', () => {
+  expect(import_('{"type": "Person", "name": "Jane"}')).toEqual(new Person({name: "Jane"}))
+})