fix: Fix the function for restricting to direct dependencies

Add and improve documentation. Unfortunately, the
read-installed-packages module returns undocumented attributes with
unclear names. This has now been documented and improved locally.

lib/index.js

@@ -371,24 +371,28 @@ const recursivelyCollectAllDependencies = (options) => {
  *  the json result.
  */
 // TODO: Add tests for this function
-const deleteNonDirectDependencies = (json, options) => {
-    const allDependencies = Object.keys(json.dependencies);
-    let wantedDependencies = [];
+const deleteNonDirectDependenciesFromAllDependencies = (
+    { _dependencies: directDependencies, dependencies: allDependencies, devDependencies },
+    options,
+) => {
+    const allDependenciesArray = Object.keys(allDependencies);
+    const directDependenciesArray = Object.keys(directDependencies);
+    const devDependenciesArray = Object.keys(devDependencies);
+    let wantedDependenciesArray = [];
 
     if (options.production && !options.development) {
-        const devDependencies = Object.keys(json.devDependencies);
-        wantedDependencies = Object.keys(json.dependencies).filter(
-            (directDependency) => !devDependencies.includes(directDependency),
+        wantedDependenciesArray = directDependenciesArray.filter(
+            (directDependency) => !devDependenciesArray.includes(directDependency),
         );
     } else if (!options.production && options.development) {
-        wantedDependencies = Object.keys(json.devDependencies);
+        wantedDependenciesArray = devDependenciesArray;
     } else {
-        wantedDependencies = Object.keys(json.dependencies);
+        wantedDependenciesArray = directDependenciesArray;
     }
 
-    allDependencies.forEach((currentDependency) => {
-        if (!wantedDependencies.includes(currentDependency)) {
-            delete json.dependencies[currentDependency];
+    allDependenciesArray.forEach((currentDependency) => {
+        if (!wantedDependenciesArray.includes(currentDependency)) {
+            delete allDependencies[currentDependency];
         }
     });
 };
@@ -446,8 +450,12 @@ exports.init = function init(args, callback) {
     }
 
     readInstalledPackages(args.start, optionsForReadingInstalledPackages, (err, json) => {
+        // Good to know:
+        // The json object returned by readInstalledPackages stores all direct (prod and dev) dependencies from
+        // the package.json file in the property '_dependencies'. The property 'dependencies' contains all dependencies,
+        // including the ones that are only required by other dependencies.
         if (optionsForReadingInstalledPackages.depth === 0) {
-            deleteNonDirectDependencies(json, args);
+            deleteNonDirectDependenciesFromAllDependencies(json, args);
         }
 
         const data = recursivelyCollectAllDependencies({