chore: updated deps and used non-sync fs

.gitignore

@@ -59,3 +59,5 @@ typings/
 
 # next.js build output
 .next
+
+.ignored

index.js

@@ -7,26 +7,55 @@ const loaderName = 'app-settings-loader';
 const schema = { env: 'string' };
 const defaultOptions = { env: "development" };
 
-const parseFileContent = (content, filePath) => {
+function parseFileContent(content, filePath) {
     try {
         return JSON.parse(content);
     } catch (e) {
         throw new Error(`Unable to parse the file ${filePath}; ${loaderName} can only be used to load and transform well-formed json files.`);
     }
 }
 
+/**
+ * Merges source with envOverride.
+ * @param {(err: Error| null, content?: string) => void} source content
+ * @param {string} source content
+ * @param {string} sourcePath path to the source configuration file
+ * @param {string|undefined|null} envOverride content
+ * @param {string|undefined|null} envOverridePath path to the environment-specific configuration file
+ */
+function merge(callback, source, sourcePath, envOverride, envOverridePath) {
+    try {
+        const sourceConfig = parseFileContent(source, sourcePath);
+        const envConfig = envOverridePath != null ? parseFileContent(envOverride, envOverridePath) : {};
+        callback(null, JSON.stringify(sealedMerge(sourceConfig, envConfig)));
+    } catch (e) {
+        callback(e);
+    }
+}
+
+
 module.exports = function (source) {
+    const callback = this.async();
+
     const options = Object.assign(defaultOptions, this.getOptions());
-    validate(schema, options, loaderName);
+    validate(schema, options, loaderName);    
 
     const ext = path.extname(this.resourcePath);
     const envFile = path.join(path.dirname(this.resourcePath), `${path.basename(this.resourcePath, ext)}.${options.env}${ext}`);
-    let envConfig = {};
+    fs.access(envFile, fs.constants.R_OK, (err) => {
+        if (err) {
+            merge(callback, source, this.sourcePath);
+            return;
+        }
 
-    if (fs.existsSync(envFile)) {
-        envConfig = parseFileContent(fs.readFileSync(envFile), envFile);
-    }
-    const sourceConfig = parseFileContent(source, this.resourcePath);
+        this.addDependency(envFile);
+        fs.readFile(envFile, (err, content) => {
+            if (err) {
+                callback(err);
+                return;
+            }
 
-    return JSON.stringify(sealedMerge(sourceConfig, envConfig));
+            merge(callback, source, this.sourcePath, content, envFile);
+        });
+    });
 };