Add JS property enumerability

blog_posts/javascript-property-enumerability.md

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+---
+title: JavaScript property enumerability
+shortTitle: Property enumerability
+type: story
+tags: javascript,object
+expertise: intermediate
+author: chalarangelo
+cover: blog_images/old-consoles.jpg
+excerpt: Property enumerability dictates how JavaScript object properties behave in different scenarios.
+firstSeen: 2022-09-11T05:00:00-04:00
+---
+
+In most cases, object properties are enumerable by default, unless they are Symbols. This means that you can use the `for...in` loop to iterate over the properties of an object. Similarly, enumerable properties appear in object methods that enumerate the properties of an object. An example of this is the `Object.keys()` method, which will omit properties that are not enumerable. Finally, when using the object spread operator (`...`), only enumerable properties are copied to the new object.
+
+```js
+const person = {
+  name: 'John',
+  surname: 'Doe',
+  age: 30,
+  socialSecurityNumber: '123-45-6789',
+};
+
+Object.defineProperty(person, 'socialSecurityNumber', {
+  enumerable: false,
+});
+
+person.hasOwnProperty('socialSecurityNumber'); // true
+person.propertyIsEnumerable('socialSecurityNumber'); // false
+
+Object.keys(person); // ['name', 'surname', 'age']
+Object.getOwnPropertyNames(person);
+// ['name', 'surname', 'age', 'socialSecurityNumber']
+
+const clone = { ...person };
+clone.socialSecurityNumber; // undefined
+```
+
+To create a non-enumerable property, you can use `Object.defineProperty()` with the appropriate descriptor. You can check for the property's existence, using `Object.prototype.hasOwnProperty()` and for its enumerability, using `Object.prototype.propertyIsEnumerable()`. Additionally, in contrast to Symbols, non-enumerable properties will show up when using `Object.getOwnPropertyNames()`.