58

/**
 * @param {string} s
 * @return {number}
 */
var lengthOfLastWord = function(s) {
    var arr = s.trim().split(' ')
    return arr[arr.length - 1].length
};

faster than 100%

var lengthOfLastWord = function(s) {
    return s
            .trim()
            .split(' ')
            .splice(-1)
            .toString()
            .split('')
            .length;
};