Length of Last Word solution using TypeScript

Below is my TypeScript solution to the LeetCode "Length of Last Word" question.

Time Complexity: Because each character in the string will be visited, at most, only one time, the time complexity is O(n), where n represents the number of characters in the string.

Space Complexity: This approach will also use a constant amount of space, making the space complexity O(1).

/**
 * Given a string s consisting of upper/lower-case alphabets and 
 * empty space characters ' ', return the length of last 
 * word in the string.
 *
 * Time Complexity: O(n)
 * Space Complexity: O(1)
 *
 * Input: "Test input"
 * Output: 5
 * 
 * Input: "Ignore spaces at end    "
 * Output: 3
 *
 * Input: "NoSpaces"
 * Output: 8
 */
function lengthOfLastWord(s: string): number {
    // place a pointer at the end of the string
    let ptr = s.length;
    let length = 0;
    while (ptr > 0) {
        ptr -= 1;
        // currently iterating through the last word
        if (s[ptr] !== ' ') {
            length += 1;
        }
        // boundary of the last word has been reached
        else if (length > 0) {
            break;
        }
    }
    return length;
};