JavaScript ReferenceError – 初始化前无法访问词法声明`variable'

JavaScript ReferenceError – 初始化前无法访问词法声明`variable'

如果在初始化之前访问了词法变量，则此 JavaScript 异常无法在初始化之前访问词法声明“变量” 。当未定义的 let 或 const 声明被访问时，这可能发生在任何块语句中。

信息：

ReferenceError: Use before declaration (Edge)
ReferenceError: can't access lexical declaration `variable' before 
                initialization (Firefox)
ReferenceError: 'variable' is not defined (Chrome)

错误类型：

ReferenceError

错误原因：在代码的某处，有一个词法变量在初始化之前被访问过。

示例1：在这个示例中，const关键字与if语句中的变量一起使用，所以发生了错误。

HTML

    
    
    
        
            GeeksforGeeks
        
         
         

          JavaScript ReferenceError -
          Can't access lexical declaration`variable'
          before initialization
        
 
 
        
            click here
        
        
 
 
        
            var el_down = document.getElementById("GFG_DOWN");
            function GFG() {
                const var_1 = "This is";
                if (true) {
                    const var_1 = var_1 + "GeeksforGeeks";
                }
            }
            function Geeks() {
                try {
                    GFG();
                    el_down.innerHTML =
                      "'Can't access lexical declaration"+
                      "`variable'before initialization' "+
                      "error has not occurred";
                } catch (e) {
                    el_down.innerHTML =
                      "'Can't access lexical declaration"+
                      "`variable' before initialization'"+
                      " error has occurred";
                }
            }
        
    

输出：

例2：在这个例子中，关键字与变量一起使用，所以发生了错误。

HTML

    
    
    
        
            GeeksforGeeks
        
         
         

          JavaScript ReferenceError -
          Can't access lexical declaration`variable'
          before initialization
        
 
 
        
            click here
        
        
 
 
        
            var el_down = document.getElementById("GFG_DOWN");
            function GFG() {
                let var_1 = 3;
                if (true) {
                     var_1 = var_1 + 5;
                }
            }
            function Geeks() {
                try {
                    GFG();
                    el_down.innerHTML =
                      "'Can't access lexical declaration"+
                      "`variable' before initialization'"+
                      " error has not occurred";
                } catch (e) {
                    el_down.innerHTML =
                      "'Can't access lexical declaration"+
                      "`variable'before initialization'"+
                      " error has occurred";
                }
            }
        
    

输出：