add examples for export/import

example/nodejs-esm/document-export-import/README.md

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+```bash
+npm install
+```
+
+```bash
+node index.js
+```

example/nodejs-esm/document-export-import/data.json

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+[
+  {
+    "tconst": "tt0000001",
+    "titleType": "short",
+    "primaryTitle": "Carmencita",
+    "originalTitle": "Carmencita",
+    "isAdult": 0,
+    "startYear": "1894",
+    "endYear": "",
+    "runtimeMinutes": "1",
+    "genres": [
+      "Documentary",
+      "Short"
+    ]
+  },
+  {
+    "tconst": "tt0000002",
+    "titleType": "short",
+    "primaryTitle": "Le clown et ses chiens",
+    "originalTitle": "Le clown et ses chiens",
+    "isAdult": 0,
+    "startYear": "1892",
+    "endYear": "",
+    "runtimeMinutes": "5",
+    "genres": [
+      "Animation",
+      "Short"
+    ]
+  },
+  {
+    "tconst": "tt0000003",
+    "titleType": "short",
+    "primaryTitle": "Pauvre Pierrot",
+    "originalTitle": "Pauvre Pierrot",
+    "isAdult": 0,
+    "startYear": "1892",
+    "endYear": "",
+    "runtimeMinutes": "4",
+    "genres": [
+      "Animation",
+      "Comedy",
+      "Romance"
+    ]
+  },
+  {
+    "tconst": "tt0000004",
+    "titleType": "short",
+    "primaryTitle": "Un bon bock",
+    "originalTitle": "Un bon bock",
+    "isAdult": 0,
+    "startYear": "1892",
+    "endYear": "",
+    "runtimeMinutes": "12",
+    "genres": [
+      "Animation",
+      "Short"
+    ]
+  },
+  {
+    "tconst": "tt0000005",
+    "titleType": "short",
+    "primaryTitle": "Blacksmith Scene",
+    "originalTitle": "Blacksmith Scene",
+    "isAdult": 0,
+    "startYear": "1893",
+    "endYear": "",
+    "runtimeMinutes": "1",
+    "genres": [
+      "Comedy",
+      "Short"
+    ]
+  },
+  {
+    "tconst": "tt0000006",
+    "titleType": "short",
+    "primaryTitle": "Chinese Opium Den",
+    "originalTitle": "Chinese Opium Den",
+    "isAdult": 0,
+    "startYear": "1894",
+    "endYear": "",
+    "runtimeMinutes": "1",
+    "genres": [
+      "Short"
+    ]
+  },
+  {
+    "tconst": "tt0000007",
+    "titleType": "short",
+    "primaryTitle": "Corbett and Courtney Before the Kinetograph",
+    "originalTitle": "Corbett and Courtney Before the Kinetograph",
+    "isAdult": 0,
+    "startYear": "1894",
+    "endYear": "",
+    "runtimeMinutes": "1",
+    "genres": [
+      "Short",
+      "Sport"
+    ]
+  },
+  {
+    "tconst": "tt0000008",
+    "titleType": "short",
+    "primaryTitle": "Edison Kinetoscopic Record of a Sneeze",
+    "originalTitle": "Edison Kinetoscopic Record of a Sneeze",
+    "isAdult": 0,
+    "startYear": "1894",
+    "endYear": "",
+    "runtimeMinutes": "1",
+    "genres": [
+      "Documentary",
+      "Short"
+    ]
+  },
+  {
+    "tconst": "tt0000009",
+    "titleType": "movie",
+    "primaryTitle": "Miss Jerry",
+    "originalTitle": "Miss Jerry",
+    "isAdult": 0,
+    "startYear": "1894",
+    "endYear": "",
+    "runtimeMinutes": "45",
+    "genres": [
+      "Romance"
+    ]
+  },
+  {
+    "tconst": "tt0000010",
+    "titleType": "short",
+    "primaryTitle": "Leaving the Factory",
+    "originalTitle": "La sortie de l'usine Lumière à Lyon",
+    "isAdult": 0,
+    "startYear": "1895",
+    "endYear": "",
+    "runtimeMinutes": "1",
+    "genres": [
+      "Documentary",
+      "Short"
+    ]
+  }
+]

example/nodejs-esm/document-export-import/index.js

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+import { Document, Charset } from "flexsearch/esm";
+import { promises as fs } from "fs";
+
+const dirname = import.meta.dirname;
+// loading test data
+const data = JSON.parse(await fs.readFile(dirname + "/data.json", "utf8"));
+
+// you will need to keep the index configuration
+// they will not export, also every change to the
+// configuration requires a full re-index
+const config = {
+    document: {
+        id: "tconst",
+        store: true,
+        index: [{
+            field: "primaryTitle",
+            tokenize: "forward",
+            encoder: Charset.LatinBalance
+        },{
+            field: "originalTitle",
+            tokenize: "forward",
+            encoder: Charset.LatinBalance
+        }],
+        tag: [{
+            field: "startYear"
+        },{
+            field: "genres"
+        }]
+    }
+};
+
+// create the document index
+let document = new Document(config);
+
+// add test data
+for(let i = 0; i < data.length; i++){
+    document.add(data[i]);
+}
+
+// perform a query
+let result = document.search({
+    query: "karmen",
+    tag: {
+        "startYear": "1894",
+        "genres": [
+            "Documentary",
+            "Short"
+        ]
+    },
+    suggest: true,
+    enrich: true,
+    merge: true
+});
+
+// output results
+console.log(result);
+
+// EXPORT
+// -----------------------
+
+await fs.mkdir("./export/").catch(e => {});
+await document.export(async function(key, data){
+    await fs.writeFile("./export/" + key, data, "utf8");
+});
+
+// IMPORT
+// -----------------------
+
+// create the same type of index you have used by .export()
+// along with the same configuration
+document = new Document(config);
+
+// load them in parallel
+const files = await fs.readdir("./export/");
+await Promise.all(files.map(async file => {
+    const data = await fs.readFile("./export/" + file, "utf8");
+    await document.import(file, data);
+}))
+
+// perform query
+result = document.search({
+    query: "karmen",
+    tag: {
+        "startYear": "1894",
+        "genres": [
+            "Documentary",
+            "Short"
+        ]
+    },
+    suggest: true,
+    enrich: true,
+    merge: true
+});
+
+// output results
+console.log("-------------------------------------");
+console.log(result);

example/nodejs-esm/document-export-import/package.json

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+{
+  "name": "nodejs-esm-document-export-import",
+  "type": "module",
+  "dependencies": {
+    "flexsearch": "github:nextapps-de/flexsearch#v0.8-preview"
+  }
+}