[Term Entry] JavaScript Map: keys()

content/javascript/concepts/map/terms/keys/keys.md

@@ -0,0 +1,101 @@
+---
+Title: '.keys()'
+Description: 'Returns a new map iterator object that contains the keys of each element in the map.'
+Subjects:
+  - 'Computer Science'
+  - 'Web Development'
+Tags:
+  - 'Map'
+  - 'Object'
+CatalogContent:
+  - 'introduction-to-javascript'
+  - 'paths/front-end-engineer-career-path'
+---
+
+The **`.keys()`** method returns a new map [iterator](https://www.codecademy.com/resources/docs/javascript/iterators) object containing the keys of each element in the map, respecting the insertion order. This method enables explicit iteration using a `for...of` loop or the iterator's `.next()` method.
+
+> **Note:** The iterable returned by `.keys()` is _not reusable_. Once it has been fully consumed (i.e., all elements have been iterated over), it becomes exhausted. To iterate again, a new iterator must be created by calling `.keys()` again on the map.
+
+## Syntax
+
+```pseudo
+map.keys();
+```
+
+**Parameters:**
+
+The `.keys()` method does not take any parameters.
+
+**Return value:**
+
+Returns a new map iterator object containing the keys of each element in the map, in order.
+
+## Example 1: Printing Each Key
+
+This example uses a `for...of` loop to iterate over the iterable object returned by `.keys()` and prints the keys of the `ratings` map:
+
+```js
+const ratings = new Map();
+
+ratings.set(1, 'Bad');
+ratings.set(5, 'Medium');
+ratings.set(10, 'Excellent');
+
+const iterator = ratings.keys();
+
+for (const key of iterator) {
+  console.log(key);
+}
+```
+
+The code will produce this output:
+
+```shell
+1
+5
+10
+```
+
+## Example 2: Getting the Values
+
+In this example, the keys are saved in the `numsKeys` array, and the map's [`.get()`](https://www.codecademy.com/resources/docs/javascript/map/get) method is used to retrieve and print each value:
+
+```js
+const nums = new Map([
+  [2, 'Two'],
+  [4, 'Four'],
+  [6, 'Six'],
+  [8, 'Eight'],
+]);
+
+const numsKeys = Array.from(nums.keys());
+
+numsKeys.forEach((key) => console.log(nums.get(key)));
+```
+
+The code will produce this output:
+
+```shell
+Two
+Four
+Six
+Eight
+```
+
+## Codebyte Example
+
+This example uses the `.next()` method to manually iterate through the keys ​​and print them to the console:
+
+```codebyte/js
+const map = new Map();
+
+map.set(1, 'One');
+map.set(true, 'True');
+map.set({ id: 1 }, 'Object');
+
+const iterator = map.keys();
+
+console.log(iterator.next().value);
+console.log(iterator.next().value);
+console.log(iterator.next().value);
+```