Convert Camel Case to Snake Case

This challenge involves transforming strings from camelCase notation to snake_case notation. This is a common task in programming, especially when dealing with different naming conventions across languages or frameworks. Your goal is to write a JavaScript function that accurately performs this conversion.

Problem Description

You need to create a JavaScript function named toSnakeCase that accepts a single string argument representing a camelCase string. The function should return a new string where all uppercase letters in the input string are converted to lowercase and preceded by an underscore.

Key Requirements:

• The function must handle strings that start with an uppercase letter.
• The function must handle strings with consecutive uppercase letters (e.g., "APIResponse").
• The function should not add an underscore at the beginning of the output string if the input string begins with a lowercase letter.
• The function should not add an underscore at the end of the output string.

Expected Behavior:

• myVariableName should become my_variable_name.
• APIResponse should become api_response.
• someHTMLStuff should become some_html_stuff.
• HTTPRequest should become http_request.
• variable should remain variable.

Edge Cases to Consider:

• Empty string.
• Strings with only uppercase letters.
• Strings with no uppercase letters.
• Strings starting with an uppercase letter.

Examples

Example 1:

Input: "convertCamelToSnake"
Output: "convert_camel_to_snake"
Explanation: Each uppercase letter ('C', 'T', 'S') is converted to lowercase and preceded by an underscore.

Example 2:

Input: "APIResponseHandler"
Output: "api_response_handler"
Explanation: Consecutive uppercase letters ('API', 'R', 'H') are handled correctly. 'A', 'P', 'I' are treated as a unit, and 'R' and 'H' are also handled.

Example 3:

Input: "simple"
Output: "simple"
Explanation: A string with no uppercase letters remains unchanged.

Example 4:

Input: "XMLHTTPRequest"
Output: "xml_http_request"
Explanation: Demonstrates handling of multiple consecutive uppercase letters at the beginning and within the string.

Example 5:

Input: "FirstName"
Output: "first_name"
Explanation: Handles a string that starts with an uppercase letter.

Constraints

• The input string will consist of alphanumeric characters and may contain uppercase and lowercase letters.
• The input string length will be between 0 and 1000 characters, inclusive.
• The function should aim for efficient processing, ideally with a time complexity of O(n), where n is the length of the input string.

Notes

Consider using regular expressions for a concise and efficient solution. Pay close attention to how you handle the beginning of the string and sequences of uppercase letters. Remember that the goal is to replace uppercase letters with their lowercase equivalents preceded by an underscore, but only when a transition from a lowercase letter to an uppercase letter or a transition from an uppercase letter to another uppercase letter followed by a lowercase letter occurs.