[Term Entry] JavaScript Objects: seal() (#7475)

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Author: StuartMosquera

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

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+---
+Title: '.seal()'
+Description: 'Prevents new properties from being added to an object and marks all existing properties as non-configurable'
+Subjects:
+  - 'Computer Science'
+  - 'Web Development'
+Tags:
+  - 'Methods'
+  - 'Objects'
+  - 'Properties'
+CatalogContent:
+  - 'introduction-to-javascript'
+  - 'paths/front-end-engineer-career-path'
+---
+
+The **`Object.seal()`** static method seals an object, preventing new properties from being added and making all existing properties non-configurable. In other words:
+
+- New properties cannot be added.
+- Existing properties cannot be removed.
+- Property enumerability and configurability cannot be changed.
+
+Unlike [`Object.freeze()`](https://www.codecademy.com/resources/docs/javascript/objects/freeze), objects sealed with `Object.seal()` may still have their existing properties updated if they are writable (`writable: true`).
+
+> **Note:** For debugging purposes, it is good practice to use [`strict mode`](https://www.codecademy.com/resources/docs/javascript/strict-mode) when working with sealed objects, as it helps detect unauthorized modifications and prevents silent errors.
+
+## Syntax
+
+```pseudo
+Object.seal(obj)
+```
+
+**Parameters:**
+
+`obj`: The object to seal.
+
+**Return value:**
+
+The `Object.seal()` method returns the sealed object.
+
+## Example 1: Basic Object Sealing
+
+In this example, the `person` object is sealed. In non-strict mode, attempts to add or remove properties will fail silently (no error is thrown), while modifying writable properties will still work:
+
+```js
+const person = {
+  name: 'Joe',
+  age: 30,
+};
+
+// Sealing the person object
+Object.seal(person);
+
+// Modifying the age property will work
+person.age = 31;
+
+// Attempting to add or delete a property will not work
+person.city = 'Madrid';
+delete person.name;
+
+console.log(person);
+```
+
+The code will produce this output:
+
+```shell
+{ name: 'Joe', age: 31 }
+```
+
+## Example 2: Check Sealed Object
+
+The following example uses the [`Object.isSealed()`](https://www.codecademy.com/resources/docs/javascript/objects/isSealed) to check if the `car` object is sealed. This method returns `true` if the object is sealed, regardless of how it was sealed (e.g., via `Object.seal()` or `Object.freeze()`:
+
+```js
+const car = {
+  brand: 'Audi',
+  model: 'Sedan',
+};
+
+// Sealing the car object
+Object.seal(car);
+
+// This will print 'true' because the car object is sealed
+console.log(Object.isSealed(car));
+
+car.brand = 'Honda';
+car.color = 'red';
+
+console.log(car);
+```
+
+The code will produce this output:
+
+```shell
+true
+{ brand: 'Honda', model: 'Sedan' }
+```
+
+## Codebyte Example: Strict Mode Behavior
+
+This example demonstrates the core functionality of `Object.seal()` by showing what operations are allowed versus prohibited on a sealed object. The sealed object allows modification of existing properties but prevents structural changes like adding new properties or deleting existing ones:
+
+```codebyte/javascript
+'use strict';
+
+const obj = { name: 'John', age: 25 };
+
+Object.seal(obj);
+
+obj.age = 26;
+console.log(obj.age); // 26
+
+try {
+  obj.city = 'NYC'; // Can't add new properties
+} catch (e) {
+  console.log('Cannot add property:', e.message);
+}
+
+try {
+  delete obj.name; // Can't delete existing properties
+} catch (e) {
+  console.log('Cannot delete property:', e.message);
+}
+```