JavaScript ReferenceError – 赋值给未声明的变量

如果该值已分配给未声明的变量，则此 JavaScript 异常分配给未声明的变量会在严格模式下发生。

信息：

ReferenceError: assignment to undeclared variable "x" (Firefox)
ReferenceError: "x" is not defined (Chrome)
ReferenceError: Variable undefined in strict mode (Edge)

错误类型：

ReferenceError

错误原因：在代码的某处，有一个没有 var、let 或 const 关键字的赋值。将值分配给未声明的变量时，会发生此错误。

例1：在这个例子中， const关键字与变量赋值一起使用，所以没有发生错误。

HTML

  
     
   
 
      
        GeeksforGeeks  
     
      
    
        JavaScript ReferenceError-
        Assignment to undeclared variable 
    
     
    
        click here
    
     
    
         
     
        var el_down = document.getElementById("GFG_DOWN");
        function GFG() { 
          'use strict'; 
           const var_1 = "Value assigned without declaration"; 
        }
        function Geeks() { 
            try {
                GFG();
                el_down.innerHTML = 
                "'Assignment to undeclared variable'"+
                "error has not occurred";
            } catch(e) {
                el_down.innerHTML = 
                "'Assignment to undeclared variable'"+
                "error has occurred";
            }
        } 
     
   

输出：

例2：在本例中，变量赋值没有使用var、let或const关键字，所以出现了错误。

HTML

  
     
   
 
      
        GeeksforGeeks  
     
      
    
        JavaScript ReferenceError -
        Assignment to undeclared variable 
    
     
    
        click here
    
     
    
         
     
        var el_down = document.getElementById("GFG_DOWN");
        function GFG() { 
          'use strict'; 
          var_1 = true; 
        }
        function Geeks() { 
            try {
                GFG();
                el_down.innerHTML = 
                "'Assignment to undeclared variable'"+
                "error has not occurred";
            } catch(e) {
                el_down.innerHTML = 
                "'Assignment to undeclared variable'"+
                "error has occurred";
            }
        } 
     
   

输出：