JavaScript中是否有常量？

如果您不介意使用功能:

var constant = function(val) {
   return function() {
        return val;
    }
}

这种方法为您提供了函数而不是常规变量,但它保证^*一旦设置了值,任何人都无法改变它.

a = constant(10);

a(); // 10

b = constant(20);

b(); // 20

我个人认为这很令人愉快,特别是在从淘汰观察中习惯了这种模式之后.

^{_{*除非有人constant在你打电话之前重新定义了这个功能}}

在可能的情况下将常量分组为结构:

例如,在我目前的游戏项目中,我使用了以下内容:

var CONST_WILD_TYPES = {
    REGULAR: 'REGULAR',
    EXPANDING: 'EXPANDING',
    STICKY: 'STICKY',
    SHIFTING: 'SHIFTING'
};

分配:

var wildType = CONST_WILD_TYPES.REGULAR;

比较:

if (wildType === CONST_WILD_TYPES.REGULAR) {
    // do something here
}

最近我使用,比较:

switch (wildType) {
    case CONST_WILD_TYPES.REGULAR:
        // do something here
        break;
    case CONST_WILD_TYPES.EXPANDING:
        // do something here
        break;
}

IE11采用新的ES6标准,具有'const'声明.
以上工作在早期的浏览器,如IE8,IE9和IE10.

您可以轻松地为脚本配备一个可以设置但不能更改的常量机制.尝试更改它们将产生错误.

/* author Keith Evetts 2009 License: LGPL  
anonymous function sets up:  
global function SETCONST (String name, mixed value)  
global function CONST (String name)  
constants once set may not be altered - console error is generated  
they are retrieved as CONST(name)  
the object holding the constants is private and cannot be accessed from the outer script directly, only through the setter and getter provided  
*/

(function(){  
  var constants = {};  
  self.SETCONST = function(name,value) {  
      if (typeof name !== 'string') { throw new Error('constant name is not a string'); }  
      if (!value) { throw new Error(' no value supplied for constant ' + name); }  
      else if ((name in constants) ) { throw new Error('constant ' + name + ' is already defined'); }   
      else {   
          constants[name] = value;   
          return true;  
    }    
  };  
  self.CONST = function(name) {  
      if (typeof name !== 'string') { throw new Error('constant name is not a string'); }  
      if ( name in constants ) { return constants[name]; }    
      else { throw new Error('constant ' + name + ' has not been defined'); }  
  };  
}())  


// -------------  demo ----------------------------  
SETCONST( 'VAT', 0.175 );  
alert( CONST('VAT') );


//try to alter the value of VAT  
try{  
  SETCONST( 'VAT', 0.22 );  
} catch ( exc )  {  
   alert (exc.message);  
}  
//check old value of VAT remains  
alert( CONST('VAT') );  


// try to get at constants object directly  
constants['DODO'] = "dead bird";  // error  

忘记IE并使用const关键字.

然而,没有确切的跨浏览器预​​定义方法来实现它,您可以通过控制变量的范围来实现它,如其他答案所示.

但我建议使用名称空间来区分其他变量.这样可以将碰撞的几率从其他变量降到最小.

正确的命名空间就像

var iw_constant={
     name:'sudhanshu',
     age:'23'
     //all varibale come like this
}

因此,使用它将是iw_constant.name或iw_constant.age

您还可以使用Object.freeze方法阻止添加任何新密钥或更改iw_constant中的任何密钥.但是旧版浏览器不支持它.

例如:

Object.freeze(iw_constant);

对于旧版浏览器,您可以使用polyfill进行冻结方法.

如果你可以调用函数,那么最好用跨浏览器的方式来定义常量.在自执行函数中确定对象的范围,并为常量返回get函数:

var iw_constant= (function(){
       var allConstant={
             name:'sudhanshu',
             age:'23'
             //all varibale come like this

       };

       return function(key){
          allConstant[key];
       }
    };

//获取值使用 iw_constant('name')或iw_constant('age')

**在这两个示例中,您必须非常小心名称间距,以便您的对象或函数不应该通过其他库替换.(如果对象或函数本身将被替换,您的整个常量将会去)

有一段时间,我在传递给with()语句的对象文字中指定了"常量"(它实际上不是常量).我觉得它太聪明了.这是一个例子:

with ({
    MY_CONST : 'some really important value'
}) {
    alert(MY_CONST);
}

在过去,我还创建了一个CONST命名空间,我将放置所有常量.再次,与开销.啧.

现在,我只是尽var MY_CONST = 'whatever';到KISS.

我的意见(仅适用于对象).

var constants = (function(){
  var a = 9;

  //GLOBAL CONSTANT (through "return")
  window.__defineGetter__("GCONST", function(){
    return a;
  });

  //LOCAL CONSTANT
  return {
    get CONST(){
      return a;
    }
  }
})();

constants.CONST = 8; //9
alert(constants.CONST); //9

尝试!但要明白 - 这是对象,但不是简单的变量.

也尝试一下:

const a = 9;

我也有这个问题.经过一段时间寻找答案并查看每个人的所有回复,我想我已经想出了一个可行的解决方案.

似乎我遇到的大部分答案都是使用函数来保存常量.正如许多论坛的许多用户所发布的那样,这些功能可以很容易地被客户端的用户覆盖.我对Keith Evetts的回答很感兴趣,即外部不能访问常量对象,而只能从内部的函数访问.

所以我提出了这个解决方案:

将所有内容放在匿名函数中,这样客户端就无法更改变量,对象等.还可以通过让其他函数从内部调用"真实"函数来隐藏"真实"函数.我还想过使用函数来检查客户端的用户是否更改了某个函数.如果功能已更改,请使用内部"受保护"的变量将其更改回来,并且无法更改.

/*Tested in: IE 9.0.8; Firefox 14.0.1; Chrome 20.0.1180.60 m; Not Tested in Safari*/

(function(){
  /*The two functions _define and _access are from Keith Evetts 2009 License: LGPL (SETCONST and CONST).
    They're the same just as he did them, the only things I changed are the variable names and the text
    of the error messages.
  */

  //object literal to hold the constants
  var j = {};

  /*Global function _define(String h, mixed m). I named it define to mimic the way PHP 'defines' constants.
    The argument 'h' is the name of the const and has to be a string, 'm' is the value of the const and has
    to exist. If there is already a property with the same name in the object holder, then we throw an error.
    If not, we add the property and set the value to it. This is a 'hidden' function and the user doesn't
    see any of your coding call this function. You call the _makeDef() in your code and that function calls
    this function.    -    You can change the error messages to whatever you want them to say.
  */
  self._define = function(h,m) {
      if (typeof h !== 'string') { throw new Error('I don\'t know what to do.'); }
      if (!m) { throw new Error('I don\'t know what to do.'); }
      else if ((h in j) ) { throw new Error('We have a problem!'); }
      else {
          j[h] = m;
          return true;
    }
  };

  /*Global function _makeDef(String t, mixed y). I named it makeDef because we 'make the define' with this
    function. The argument 't' is the name of the const and doesn't need to be all caps because I set it
    to upper case within the function, 'y' is the value of the value of the const and has to exist. I
    make different variables to make it harder for a user to figure out whats going on. We then call the
    _define function with the two new variables. You call this function in your code to set the constant.
    You can change the error message to whatever you want it to say.
  */
  self._makeDef = function(t, y) {
      if(!y) { throw new Error('I don\'t know what to do.'); return false; }
      q = t.toUpperCase();
      w = y;
      _define(q, w);
  };

  /*Global function _getDef(String s). I named it getDef because we 'get the define' with this function. The
    argument 's' is the name of the const and doesn't need to be all capse because I set it to upper case
    within the function. I make a different variable to make it harder for a user to figure out whats going
    on. The function returns the _access function call. I pass the new variable and the original string
    along to the _access function. I do this because if a user is trying to get the value of something, if
    there is an error the argument doesn't get displayed with upper case in the error message. You call this
    function in your code to get the constant.
  */
  self._getDef = function(s) {
      z = s.toUpperCase();
      return _access(z, s);
  };

  /*Global function _access(String g, String f). I named it access because we 'access' the constant through
    this function. The argument 'g' is the name of the const and its all upper case, 'f' is also the name
    of the const, but its the original string that was passed to the _getDef() function. If there is an
    error, the original string, 'f', is displayed. This makes it harder for a user to figure out how the
    constants are being stored. If there is a property with the same name in the object holder, we return
    the constant value. If not, we check if the 'f' variable exists, if not, set it to the value of 'g' and
    throw an error. This is a 'hidden' function and the user doesn't see any of your coding call this
    function. You call the _getDef() function in your code and that function calls this function.
    You can change the error messages to whatever you want them to say.
  */
  self._access = function(g, f) {
      if (typeof g !== 'string') { throw new Error('I don\'t know what to do.'); }
      if ( g in j ) { return j[g]; }
      else { if(!f) { f = g; } throw new Error('I don\'t know what to do. I have no idea what \''+f+'\' is.'); }
  };

  /*The four variables below are private and cannot be accessed from the outside script except for the
    functions inside this anonymous function. These variables are strings of the four above functions and
    will be used by the all-dreaded eval() function to set them back to their original if any of them should
    be changed by a user trying to hack your code.
  */
  var _define_func_string = "function(h,m) {"+"      if (typeof h !== 'string') { throw new Error('I don\\'t know what to do.'); }"+"      if (!m) { throw new Error('I don\\'t know what to do.'); }"+"      else if ((h in j) ) { throw new Error('We have a problem!'); }"+"      else {"+"          j[h] = m;"+"          return true;"+"    }"+"  }";
  var _makeDef_func_string = "function(t, y) {"+"      if(!y) { throw new Error('I don\\'t know what to do.'); return false; }"+"      q = t.toUpperCase();"+"      w = y;"+"      _define(q, w);"+"  }";
  var _getDef_func_string = "function(s) {"+"      z = s.toUpperCase();"+"      return _access(z, s);"+"  }";
  var _access_func_string = "function(g, f) {"+"      if (typeof g !== 'string') { throw new Error('I don\\'t know what to do.'); }"+"      if ( g in j ) { return j[g]; }"+"      else { if(!f) { f = g; } throw new Error('I don\\'t know what to do. I have no idea what \\''+f+'\\' is.'); }"+"  }";

  /*Global function _doFunctionCheck(String u). I named it doFunctionCheck because we're 'checking the functions'
    The argument 'u' is the name of any of the four above function names you want to check. This function will
    check if a specific line of code is inside a given function. If it is, then we do nothing, if not, then
    we use the eval() function to set the function back to its original coding using the function string
    variables above. This function will also throw an error depending upon the doError variable being set to true
    This is a 'hidden' function and the user doesn't see any of your coding call this function. You call the
    doCodeCheck() function and that function calls this function.    -    You can change the error messages to
    whatever you want them to say.
  */
  self._doFunctionCheck = function(u) {
      var errMsg = 'We have a BIG problem! You\'ve changed my code.';
      var doError = true;
      d = u;
      switch(d.toLowerCase())
      {
           case "_getdef":
               if(_getDef.toString().indexOf("z = s.toUpperCase();") != -1) { /*do nothing*/ }
               else { eval("_getDef = "+_getDef_func_string); if(doError === true) { throw new Error(errMsg); } }
               break;
           case "_makedef":
               if(_makeDef.toString().indexOf("q = t.toUpperCase();") != -1) { /*do nothing*/ }
               else { eval("_makeDef = "+_makeDef_func_string); if(doError === true) { throw new Error(errMsg); } }
               break;
           case "_define":
               if(_define.toString().indexOf("else if((h in j) ) {") != -1) { /*do nothing*/ }
               else { eval("_define = "+_define_func_string); if(doError === true) { throw new Error(errMsg); } }
               break;
           case "_access":
               if(_access.toString().indexOf("else { if(!f) { f = g; }") != -1) { /*do nothing*/ }
               else { eval("_access = "+_access_func_string); if(doError === true) { throw new Error(errMsg); } }
               break;
           default:
                if(doError === true) { throw new Error('I don\'t know what to do.'); }
      }
  };

  /*Global function _doCodeCheck(String v). I named it doCodeCheck because we're 'doing a code check'. The argument
    'v' is the name of one of the first four functions in this script that you want to check. I make a different
    variable to make it harder for a user to figure out whats going on. You call this function in your code to check
    if any of the functions has been changed by the user.
  */
  self._doCodeCheck = function(v) {
      l = v;
      _doFunctionCheck(l);
  };
}())

似乎安全性确实是一个问题,并且无法从客户端"隐藏"您的编程.对我来说一个好主意是压缩你的代码,这样任何人,包括你,程序员,都很难阅读和理解它.您可以访问以下网站:http://javascriptcompressor.com/.(这不是我的网站,不要担心我不做广告.)这是一个允许您免费压缩和混淆Javascript代码的网站.

复制上面脚本中的所有代码并将其粘贴到javascriptcompressor.com页面上的顶部textarea中.

选中Base62编码复选框,选中Shrink Variables复选框.

按"压缩"按钮.

将其全部粘贴并保存在.js文件中,然后将其添加到页面顶部的页面中.

显然,这表明需要标准化的跨浏览器const关键字.

但现在:

var myconst = value;

要么

Object['myconst'] = value;

两者似乎都足够了,其他任何东西就像是用火箭筒射击飞行.