Defines the "charCount" method, which returns the number of characters in a string. Assumes that the browser / environment supports the ECMAScript 6 String.prototype.codePointAt() method (IE 11+).

charCount.js

(function (window) {
    'use strict';
    
    var charCodeAt, codePointAt;
    
    if (!window.charCount) {
        
        charCodeAt = String.prototype.charCodeAt;
        codePointAt = String.prototype.codePointAt;
        
        /**
         * Determines the number of characters present in a string. Accounts for the
         * presence of any UTF-16 surrogate pairs, and counts them as a single
         * character (as opposed to String.prototype.length, which counts them as 2).
         * 
         * @param {string} str The string whose characters will be counted.
         * @returns {number} The number of characters in the string.
         */
        function charCount(str) {
            var s = String(str),
                l = s.length,
                i = r = 0;
            for (i, l; i < l; i++) {
                if (charCodeAt.call(s, i) === codePointAt.call(s, i)) {
                    r++;
                }
            }
            return r;
        }
        window.charCount = charCount;
        
    }
})(self);

charCount.min.js

(function(T,h,e,f,t,s){'use strict';var v=h.prototype,C=v[f],P=v[t];if(!T[e])T[e]=function(m){var r=0,i=0,S=h(m),l=S.length;for(i,l;i<l;i++){C[s](S,i)===P[s](S,i)&&r++;}return r;};})(window,String,'charCount','charCodeAt','codePointAt','call');