[Term Entry] JavaScript Arrays: .keys()

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

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+---
+Title: '.keys()'
+Description: 'Returns a new array iterator object containing the keys for each index in the array.'
+Subjects:
+  - 'Computer Science'
+  - 'Web Development'
+Tags:
+  - 'Arrays'
+  - 'Iterators'
+  - 'JavaScript'
+  - 'Methods'
+CatalogContent:
+  - 'introduction-to-javascript'
+  - 'paths/front-end-engineer-career-path'
+---
+
+The **`.keys()`** method returns a new array iterator object that contains the keys (indices) for each index in the array.
+
+## Syntax
+
+```pseudo
+array.keys();
+```
+
+**Parameters:**
+
+- This method does not take any parameters.
+
+**Return value:**
+
+- A new array iterator object containing the keys (indices) of the array.
+
+## Example 1: Using `.keys()` to get array indices
+
+In this example, the `.keys()` method creates an iterator over the indices of the array:
+
+```js
+const cats = ['Sundae', 'Gandalf', 'Campanita'];
+const catsIterator = cats.keys();
+
+for (const cat of catsIterator) {
+  console.log(cat);
+}
+```
+
+This produces the following output:
+
+```shell
+0
+1
+2
+```
+
+Example 2: Converting the `.keys()` iterator to an array
+
+In this example, the spread operator (`...`) with `.keys()` is used to convert the index iterator into a full array of indices:
+
+```js
+const colors = ['red', 'black', 'white'];
+const indices = [...colors.keys()];
+
+console.log('Array indices: ', indices);
+console.log('First index: ', indices[0]);
+```
+
+This produces the following output:
+
+```shell
+Array indices:  [0, 1, 2]
+First index:  0
+```
+
+## Codebyte Example: Using `.keys()` with sparse arrays
+
+In this example, `.keys()` is used on a sparse array. Even though some elements are missing, `.keys()` still returns all valid indices:
+
+```codebyte/javascript
+const sparseArray = ['Lia', , 'ny', , '💖'];
+const keys = [...sparseArray.keys()];
+
+console.log('Keys:', keys);
+console.log('Length:', sparseArray.length);
+```