[Term Entry] JavaScript Objects: preventExtensions()

content/javascript/concepts/objects/terms/preventExtensions/preventExtensions.md

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+---
+Title: '.preventExtensions()'
+Description: 'Prevents new properties from being added to an object while allowing existing properties to be modified or deleted.'
+Subjects:
+  - 'Code Foundations'
+  - 'Computer Science'
+  - 'Web Development'
+Tags:
+  - 'Objects'
+  - 'Properties'
+  - 'Methods'
+CatalogContent:
+  - 'introduction-to-javascript'
+  - 'paths/front-end-engineer-career-path'
+---
+
+The **`Object.preventExtensions()`** method prevents new properties from being added to an object. Unlike [`Object.freeze()`](https://www.codecademy.com/resources/docs/javascript/objects/freeze) or [`Object.seal()`](https://www.codecademy.com/resources/docs/javascript/objects/seal), this method still allows existing properties to be modified or deleted. It returns the same object that was passed to it.
+
+> **Note:** Once an object is made non-extensible, it cannot be made extensible again. This operation is irreversible.
+
+## Syntax
+
+```pseudo
+Object.preventExtensions(obj)
+```
+
+**Parameters:**
+
+- `obj`: The object to make non-extensible.
+
+**Return value:**
+
+Returns the same object that was passed in, now marked as non-extensible.
+
+## Example: Basic Usage of `Object.preventExtensions()`
+
+The following code demonstrates how `Object.preventExtensions()` works:
+
+```js
+const user = {
+  name: 'Arthur',
+  age: 42,
+};
+
+// Show object is extensible
+console.log(Object.isExtensible(user));
+
+// Prevent extensions
+Object.preventExtensions(user);
+
+// Show object is no longer extensible after using Object.preventExtensions()
+console.log(Object.isExtensible(user));
+
+// New properties cannot be added
+user.email = '[email protected]';
+console.log(user.email);
+
+// Existing properties can be modified
+user.age = 31;
+console.log(user.age);
+
+// Existing properties can be deleted
+delete user.name;
+console.log(user.name);
+```
+
+The output generated by this code is:
+
+```shell
+true
+false
+undefined
+31
+undefined
+```
+
+## Codebyte Example
+
+This example shows the effect of `Object.preventExtensions()` in non-strict mode, and notes how strict mode would throw a `TypeError`:
+
+```codebyte/javascript
+const config = {
+  theme: 'dark',
+  language: 'en'
+};
+
+console.log('Before preventExtensions:', Object.isExtensible(config));
+
+Object.preventExtensions(config);
+
+console.log('After preventExtensions:', Object.isExtensible(config));
+
+// Non-strict mode: fails silently
+config.debug = true;
+console.log('Attempted to add debug property:', config.debug); // undefined
+
+// Strict mode: will throw TypeError if uncommented
+// 'use strict';
+// config.debug = true;
+
+// Modifying existing properties works
+config.theme = 'light';
+console.log('Modified theme:', config.theme); // light
+
+// Deleting existing properties works
+delete config.language;
+console.log('After deleting language:', config); // { theme: 'light' }
+```