ES6语法中常量声明（const）的实现原理

var const_customer = function(param, value) {
    // 目前是在浏览器端测试全局对象window，如果是在node 环境全局对象global
    var _global = window; 

    var KEY_WORD = ['const', 'let', 'var','class', 'return']; //关键字列表（不全）
    var REG_NUMBER =  '^[1-9][0-9]*([.][0-9]+)?$|0' ; //数字正则表达式
    var _reg = new RegExp(REG_NUMBER)

    //检测键值是否存在并且被支持
    if(!param||_reg.test(parseFloat(param))||KEY_WORD.indexOf(param) > -1) {
        throw new Error(`Unexpected token: ${param}`);
    }

    //检测是否被定义
    if(_global.hasOwnProperty(param)) {
        throw new Error(`${param} has already been declared !`);
    }

    _global[param] = value;
    Object.defineProperty(_global, param, {
        enumerable: false,
        configurable: false,
        get: function () {
            return value
        },
        set: function (data) {
            
            // 检测赋值异常
            if (_global.hasOwnProperty(param)) { 
                throw new Error(`${param} is read-only!`);
            } else {
                return value
            }
        }
    })
}

//功能测试

const_customer('TEST', "test_value"); 
console.log(TEST) // test_value
const_customer('TEST', "test_value"); // TEST has already been declared !
TEST = "TEST"; // Uncaught Error: TEST is read-only!