检查默认值

function typeOfVar (obj) {
      return {}.toString.call(obj).split(' ')[1].slice(0, -1).toLowerCase();
}
function isVariableHaveDefaltVal(variable) {
    if ( typeof(variable) === 'string' ) {  // number, boolean, string, object 
        console.log(' Any data Between single/double Quotes is treated as String ');        
        return (variable.trim().length === 0) ? true : false;
    }else if ( typeof(variable) === 'boolean' ) {
      console.log('boolean value with default value \'false\'');
        return (variable === false) ? true : false;
    }else if ( typeof(variable) === 'undefined' ) {
        console.log('EX: var a; variable is created, but has the default value of undefined.'); 
        return true;
    }else if ( typeof(variable) === 'number' ) { 
        console.log('number : '+variable);
        return (variable === 0 ) ? true : false;
    }else if ( typeof(variable) === 'object' ) {
   //   -----Object-----
        if (typeOfVar(variable) === 'array' && variable.length === 0) {
            console.log('\t Object Array with length = ' + [].length); // Object.keys(variable)
            return true;
        }else if (typeOfVar(variable) === 'string' && variable.length === 0 ) {
            console.log('\t Object String with length = ' + variable.length);
            return true;
        }else if (typeOfVar(variable) === 'boolean' ) {
            console.log('\t Object Boolean = ' + variable);
            return (variable === false) ? true : false;
        }else if (typeOfVar(variable) === 'number' ) {
            console.log('\t Object Number = ' + variable);
            return (variable === 0 ) ? true : false;
        }else if (typeOfVar(variable) === 'regexp' && variable.source.trim().length === 0 ) {
       console.log('\t Object Regular Expression : ');
        return true;
        }else if (variable === null) {
       console.log('\t Object null value');
        return true;
        }
    }
    return false;
}
var str = "A Basket For Every Occasion";
str = str.replace(/\s/g, "-");
//The "g" flag in the regex will cause all spaces to get replaced.

检查结果：

isVariableHaveDefaltVal(' '); // string          
isVariableHaveDefaltVal(false); // boolean       
var a;           
isVariableHaveDefaltVal(a);               
isVariableHaveDefaltVal(0); // number             
isVariableHaveDefaltVal(parseInt('')); // NAN isNAN(' '); - true         
isVariableHaveDefaltVal(null);              
isVariableHaveDefaltVal([]);               
isVariableHaveDefaltVal(/ /);              
isVariableHaveDefaltVal(new Object(''));               
isVariableHaveDefaltVal(new Object(false));            
isVariableHaveDefaltVal(new Object(0)); 
typeOfVar( function() {} );

我使用@Vix函数（）来检查哪种类型的对象。

使用instansof«

var prototypes_or_Literals = function (obj) {
    switch (typeof(obj)) {
        // object prototypes
        case 'object':
            if (obj instanceof Array)
                return '[object Array]';
            else if (obj instanceof Date)
                return '[object Date]';
            else if (obj instanceof RegExp)
                return '[object regexp]';
            else if (obj instanceof String)
                return '[object String]';
            else if (obj instanceof Number)
                return '[object Number]';

            else
                return 'object';
        // object literals
        default:
            return typeof(obj);
    }   
};
output test «
prototypes_or_Literals( '' ) // "string"
prototypes_or_Literals( new String('') ) // "[object String]"
Object.prototype.toString.call("foo bar") //"[object String]"        

我认为使用？操作员稍微干净一点。

var ? function_if_exists() : function_if_doesnt_exist();

var myNewValue = myObject && myObject.child && myObject.child.myValue;

这永远不会抛出错误。如果myObject、child或myValue为空，则myNewValue将为空。不会引发任何错误

此函数检查空对象｛｝、空数组[]、空、未定义和空字符串“”

function isEmpty(val) {
  //check for empty object {}, array []
  if (val !== null && typeof val === 'object') {
    if (Object.keys(obj).length === 0) {
      return true;
    }
  }
  //check for undefined, null and "" 
  else if (val == null || val === "") {
    return true;
  }
  return false;
}

var val={}；isEmpty（val）->trueval=[]；isEmpty（val）->trueisEmpty（未定义）->trueisEmpty（null）->trueisEmpty（“”）->trueisEmpty（false）->falseisEmpty（0）->false

对于我的案例，我尝试了if null，“”！变量，但它不起作用。

查看下面的代码以从html字段获取文本

var status=$(this).text(); //for example (for my case)

如果状态变量中没有值（没有文本），我试图将值“novalue”设置为状态变量。

以下代码有效。

if(status == false)
{
   status='novalue';
} 

当没有找到satus变量的文本时，上面的代码将“novalue”分配给状态变量

对于每一个有类似问题的人来说，以下内容非常有用，我在过去的几年里都把它放在了我的图书馆里：

(function(g3, $, window, document, undefined){
   g3.utils = g3.utils || {};
/********************************Function type()********************************
* Returns a lowercase string representation of an object's constructor.
* @module {g3.utils}
* @function {g3.utils.type}
* @public
* @param {Type} 'obj' is any type native, host or custom.
* @return {String} Returns a lowercase string representing the object's 
* constructor which is different from word 'object' if they are not custom.
* @reference http://perfectionkills.com/instanceof-considered-harmful-or-how-to-write-a-robust-isarray/
* http://stackoverflow.com/questions/3215046/differentiating-between-arrays-and-hashes-in-javascript
* http://javascript.info/tutorial/type-detection
*******************************************************************************/
g3.utils.type = function (obj){
   if(obj === null)
      return 'null';
   else if(typeof obj === 'undefined')
      return 'undefined';
   return Object.prototype.toString.call(obj).match(/^\[object\s(.*)\]$/)[1].toLowerCase();
};
}(window.g3 = window.g3 || {}, jQuery, window, document));