关于js正则表达式的疑问

/**
 * 替换字符串中 {placeholder} 或者 {0}, {1} 等模式部分为参数中传入的字符串
 * 使用方法:
 *     formatString('I can speak {language} since I was {age}', {language: 'Javascript', age: 10})
 *     formatString('I can speak {0} since I was {1}', 'Javascript', 10)
 * 输出都为:
 *     I can speak Javascript since I was 10
 *
 * @param str 带有 placeholder 的字符串
 * @param replacements 用来替换 placeholder 的 JSON 对象或者数组
 * @return 格式化后的字符串
 */
var formatString = function (str, replacements) {
    replacements = (typeof replacements === 'object') ? replacements : Array.prototype.slice.call(arguments, 1);
 
    return str.replace(/\{\{|\}\}|\{(\w+)\}/g, function(m, n) {
        if (m == '{{') { return '{'; }
        if (m == '}}') { return '}'; }
        return replacements[n];
    });
};