Implement privatize, private fields on prototypes

CHANGELOG.md

@@ -8,6 +8,8 @@ and this project adheres to
 
 ## [Unreleased][unreleased]
 
+- Implement privatize, private fields on prototypes: `privatize(instance)`
+
 ## [2.2.0][] - 2020-07-10
 
 ### Added

README.md

@@ -188,6 +188,7 @@ $ npm install @metarhia/common
 - [parseHost](#parsehosthost)
 - [override](#overrideobj-fn)
 - [mixin](#mixintarget-source)
+- [privatize](#privatizeinstance)
 - [Pool](#class-pool)
   - [Pool.prototype.constructor](#poolprototypeconstructorfactory--null)
   - [Pool.prototype.get](#poolprototypeget)
@@ -1334,6 +1335,20 @@ Previous function will be accessible by obj.fnName.inherited
 
 Mixin for ES6 classes without overriding existing methods
 
+### privatize(instance)
+
+- `instance`: [`<Object>`][object] source instance
+
+_Returns:_ [`<Object>`][object] - destination instance
+
+Convert instance with public fields to instance with private fields
+
+_Example:_
+
+```js
+common.privatize({ private: 5, f() { return this.private; } });
+```
+
 ### class Pool
 
 #### Pool.prototype.constructor(factory = null)

common.mjs

@@ -90,6 +90,7 @@ export const localIPs = common.localIPs;
 export const parseHost = common.parseHost;
 export const override = common.override;
 export const mixin = common.mixin;
+export const privatize = common.privatize;
 export const Pool = common.Pool;
 export const sortComparePriority = common.sortComparePriority;
 export const sortCompareDirectories = common.sortCompareDirectories;

lib/oop.js

@@ -23,7 +23,36 @@ const mixin = (target, source) => {
   Object.assign(target, mix);
 };
 
+const ASCII_A = 65;
+
+// Convert instance with public fields to instance with private fields
+//   instance - <Object>, source instance
+// Returns: <Object> - destination instance
+//
+// Example: common.privatize({ private: 5, f() { return this.private; } });
+const privatize = instance => {
+  const iface = {};
+  const fields = Object.keys(instance);
+  for (const fieldName of fields) {
+    const field = instance[fieldName];
+    if (typeof field === 'function') {
+      const boundMethod = field.bind(instance);
+      iface[fieldName] = boundMethod;
+    } else if (fieldName === fieldName.toUpperCase()) {
+      const first = fieldName.charCodeAt(0);
+      if (first >= ASCII_A) {
+        Object.defineProperty(iface, fieldName, {
+          enumerable: true,
+          get: () => field,
+        });
+      }
+    }
+  }
+  return Object.freeze(iface);
+};
+
 module.exports = {
   override,
   mixin,
+  privatize,
 };

test/oop.js

@@ -62,3 +62,43 @@ metatests.test('multiple inheritance with mixin', test => {
   test.strictSame(obj.property5, 'from Child.method3');
   test.end();
 });
+
+metatests.test('privatize', test => {
+  const source = {
+    CONSTANT: 'constant value',
+    field: 'field value',
+    '123': 'number field value',
+    counter: 0,
+    method() {
+      return [this.CONSTANT, this.field];
+    },
+    inc(n = 1) {
+      this.counter += n;
+      return this.counter;
+    },
+  };
+  const destination = common.privatize(source);
+  try {
+    destination.CONSTANT = 'can not change freezed';
+  } catch (err) {
+    test.strictSame(err.constructor.name, 'TypeError');
+    test.strictSame(destination.CONSTANT, 'constant value');
+  }
+  test.strictSame(destination.CONSTANT, 'constant value');
+  try {
+    destination.field = 'can not change freezed';
+  } catch (err) {
+    test.strictSame(err.constructor.name, 'TypeError');
+    test.strictSame(destination.field, undefined);
+  }
+  test.strictSame(destination['123'], undefined);
+  test.strictSame(destination.method(), ['constant value', 'field value']);
+  test.strictSame(destination.inc(), 1);
+  try {
+    destination.inc = () => 0;
+  } catch (err) {
+    test.strictSame(err.constructor.name, 'TypeError');
+    test.strictSame(destination.inc(), 2);
+  }
+  test.end();
+});