Add documentation for JSON.OBJLEN command

Adding a new page for the JSON.OBJLEN command, showing usage and
possible error messages.

Also fixed a few typos in the JSON.OBJKEYS page.

DiceDB#1282

docs/src/content/docs/commands/JSON.OBJKEYS.md

@@ -1,6 +1,6 @@
 ---
 title: JSON.OBJKEYS
-description: The `JSON.OBJKEYS` command command in DiceDB retrieves the keys of a JSON object located at a specified path within the document stored under the given key. This command is useful when you want to list the fields within a JSON object stored in a database.
+description: The `JSON.OBJKEYS` command in DiceDB retrieves the keys of a JSON object located at a specified path within the document stored under the given key. This command is useful when you want to list the fields within a JSON object stored in a database.
 ---
 
 The `JSON.OBJKEYS` command in DiceDB allows users to access the keys of a JSON object stored at a specific path within a document identified by a given key. By executing this command, users can easily retrieve a list of the fields present in the JSON object, making it a valuable tool for exploring and managing the structure of JSON data stored in the database.
@@ -26,7 +26,7 @@ JSON.OBJKEYS key [path]
 | ------------------------------------------------------ | ------------------------------------------------------------------------------------------------------- |
 | Success                                                | ([]String) `Array of strings containing the keys present within the JSON object at the specified path.` |
 | Key does not exist                                     | Error: `(error) ERR could not perform this operation on a key that doesn't exist`                       |
-| Wrong number of arguments                              | Error: `(error) ERR wrong number of arguments for JSON.ARRTRIM command`                                 |
+| Wrong number of arguments                              | Error: `(error) ERR wrong number of arguments for JSON.OBJKEYS command`                                 |
 | Key has wrong type                                     | Error: `(error) ERR Existing key has wrong Dice type`                                                   |
 | Operation attempted on a key with an incompatible type | Error: `(error) ERR WRONGTYPE Operation against a key holding the wrong kind of value`                  |
 
@@ -41,7 +41,7 @@ JSON.OBJKEYS key [path]
 
 1. `Wrong number of arguments`:
 
-   - Error Message: `(error) ERR wrong number of arguments for JSON.ARRINSERT command`
+   - Error Message: `(error) ERR wrong number of arguments for JSON.OBJKEYS command`
    - Raised if the number of arguments are less or more than expected.
 
 2. `Key doesn't exist`:
@@ -52,7 +52,7 @@ JSON.OBJKEYS key [path]
 3. `Key has wrong Dice type`:
 
    - Error Message: `(error) ERR Existing key has wrong Dice type`
-   - Raised if thevalue of the specified key doesn't match the specified value in DIceDb
+   - Raised if thevalue of the specified key doesn't match the specified value in DiceDb
 
 4. `Path doesn't exist`:
 

docs/src/content/docs/commands/JSON.OBJLEN.md

@@ -0,0 +1,123 @@
+---
+title: JSON.OBJLEN
+description: The `JSON.OBJLEN` command in DiceDB retrieves the number of keys stored in the JSON object located at key.
+---
+
+The `JSON.OBJLEN` command in DiceDB retrieves the number of keys stored in the JSON object located at key. By default, it counts the keys in the whole JSON object, but you can optionally specify a JSONPath to narrow the operation to a subset of the JSON object.
+
+This functionality is particularly useful for developers working with complex JSON structures who need to quickly gauge the size of those structures.
+
+## Syntax
+
+```bash
+JSON.OBJLEN key [path]
+```
+
+## Parameters
+
+| Parameter | Description                                             | Type   | Required |
+| --------- | ------------------------------------------------------- | ------ | -------- |
+| `key`     | The name of the key holding the JSON document.          | String | Yes      |
+| `path`    | JSONPath pointing to an array within the JSON document. | String | No       |
+
+## Return values
+
+| Condition                       | Return Value                                                                                |
+| ------------------------------- | ------------------------------------------------------------------------------------------- |
+| Command is successful           | `Integer` denoting the number of keys length of the list at the specified key.              |
+| Wrong number of arguments       | Error: `(error) ERR wrong number of arguments for JSON.OBJLEN command`                      |
+| Key does not exist              | Error: `(error) ERR could not perform this operation on a key that doesn't exist`           |
+| Key is not for a JSON object    | Error: `(error) ERR WRONGTYPE Operation against a key holding the wrong kind of value`      |
+| Path malformed or doesn't exist | Error: `(error) ERR Path 'foo' does not exist`                                              |
+
+## Behaviour
+
+- Root Path (Default): If no path is provided, JSON.OBJLEN retrieves keys from the root object of the JSON document.
+- Path Validation: If the specified path does not point to an object (e.g., if it points to a scalar, array, or does not exist), the command returns `(nil)` for that path.
+- Non-existing Key: If the specified key does not exist in the database, an error is returned.
+- Invalid JSON Path: If the provided JSONPath expression is invalid, an error message with the details of the parse error is returned.
+
+## Errors
+
+1. `Wrong number of arguments`:
+
+   - Error Message: `(error) ERR wrong number of arguments for JSON.OBJLEN command`
+   - Happens if the number of arguments is less or more than required. (It must have at least one argument, or at most two arguments).
+
+2. `Key doesn't exist`:
+
+   - Error Message: `(error) ERR could not perform this operation on a key that doesn't exist`
+   - Happens if the specified key does not exist in the DiceDB database.
+
+3. `Key has wrong Dice type`:
+
+   - Error Message: `(error) ERR WRONGTYPE Operation against a key holding the wrong kind of value`
+   - Happens if an operation attempted on a key with an incompatible type.
+
+4. `ERR Path 'foo' does not exist`:
+   - Error Message: `(error) ERR Path 'foo' does not exist`
+   - Happens if the path string provided (ie. 'foo') could not be parsed into a valid JSONPath, or if the JSONPath does not exist in the object.
+
+## Example Usage
+
+### Basic usage
+
+Get number of keys in the Root JSON Object. You can specify the JSON root using the symbol `$`.
+
+```bash
+127.0.0.1:7379> JSON.SET a $ '{"name": "Alice", "age": 30, "address": {"city": "Wonderland", "zipcode": "12345"}}'
+"OK"
+127.0.0.1:7379> JSON.OBJLEN a $
+1) 3
+```
+It returns 3, because there are three root keys in the root JSON object: `name`, `age`, and `address`.
+
+Or, if you don't want to specify a JSON path, it may be omitted. The path defaults to the root, and the result is given as a scalar:
+```bash
+127.0.0.1:7379> JSON.OBJLEN a
+3
+```
+
+
+### Keys inside nested object
+
+To count the number of keys inside a nested object, specify a JSON Path. The root of the JSON object is referred to by the `$` symbol.
+
+```bash
+127.0.0.1:7379> JSON.SET b $ '{"name": "Alice", "address": {"city": "Wonderland", "state": "Fantasy", "zipcode": "12345"}}'
+"OK"
+127.0.0.1:7379> JSON.OBJLEN b $.address
+1) 3
+```
+Here, it returns 3 because it's counting the three keys inside the `$.address` JSON object: `city`, `state`, and `zipcode`.
+
+### When path is not a JSON object
+
+When `path` points to an existing element in a JSON object, but that element is not itself a JSON object, the result is `(nil)`.
+
+```bash
+127.0.0.1:7379> JSON.SET c $ '{"name": "Alice", "age": 30}'
+"OK"
+127.0.0.1:7379> JSON.OBJLEN c $.age
+1) (nil)
+```
+
+### When path doesn't exist
+
+When `path` does not exist, the result is an empty list or set.
+
+```bash
+127.0.0.1:7379> JSON.SET d $ '{"name": "Alice", "address": {"city": "Wonderland"}}'
+"OK"
+127.0.0.1:7379> JSON.OBJLEN d $.nonexistentPath
+(empty list or set)
+```
+
+### Invalid Usage: When key doesn't exist
+
+When `key` does not exist, the result is an error.
+
+```bash
+127.0.0.1:7379> JSON.OBJLEN nonexistent_key $
+(error) ERR could not perform this operation on a key that doesn't exist
+```