这是最安全的支票，我还没有看到它像这样贴在这里：

if (typeof value !== 'undefined' && value) {
    //deal with value'
};

它将涵盖从未定义价值的情况，以及以下任何情况：

无效的undefined（未定义的值与从未定义的参数不同）0“”（空字符串）假的NaN公司

已编辑：更改为严格相等（！==），因为这是现在的标准；）

仅使用“空值合并”检查未定义和空值

if ((myVariable ?? undefined) !== undefined) {
    // handle myVariable has a value, including 0 or ""
}
else {
    // handle undefined or null only
}

来自铬控制台

{const x=undefined; (x ?? undefined) !== undefined}
false

{const x=null; (x ?? undefined) !== undefined}
false

{const x=0; (x ?? undefined) !== undefined}
true

{const x=""; (x ?? undefined) !== undefined}
true

{const x={}; (x ?? undefined) !== undefined}
true

{const x=[]; (x ?? undefined) !== undefined}
true

{const x="a"; (x ?? undefined) !== undefined}
true

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]"        

可以直接使用相等运算符

<script>
    var firstName;
    var lastName = null;
    /* Since null == undefined is true, the following statements will catch both null and undefined */
        if(firstName == null){
            alert('Variable "firstName" is undefined.');
        }    
        if(lastName == null){
           alert('Variable "lastName" is null.');
        }
</script>

demo@如何使用JavaScript确定变量是否未定义或为空

我认为使用？操作员稍微干净一点。

var ? function_if_exists() : function_if_doesnt_exist();

return val || 'Handle empty variable'

是在许多地方处理它的一种非常好且干净的方法，也可以用于分配变量

const res = val || 'default value'