相关疑难解决方法(0)

我不完全理解为什么以下显示"悬挂"到最后.

var x = 'set';
var y = function () 
{
    // WHAT YOU DON'T SEE -> var x; 
    // is effectively "hoisted" to this line!

    if (!x) 
    { 
        // You might expect the variable to be populated at this point...it is not
        // though, so this block executes
        var x = 'hoisted'; 
    }

    alert(x); 
}

//... and this call causes an alert to display "hoisted"
y();

任何指针将不胜感激.

为什么x下面示例中的变量返回undefined而不是25？

var x = 25;

(function() {
  console.log(x);
  var x = 10;
})();

我在这里对可变提升概念有点困惑。为什么第一个console.log(flag)输出undefined？它不应该捕获已经初始化的 false 值在作用域链中向上移动吗？

var flag = false;
(function(){
    console.log(flag);
    var flag = true; // JavaScript only hoists declarations, not initialisations

    console.log(flag);

    if(flag){
        let name = "John";
        const age = "24";

        console.log(name);
        console.log(age);
    }

    //console.log(name); //ReferenceError: name is not defined ( as name is block scoped here )
    //console.log(age);  //ReferenceError: age is not defined ( as age is block scoped )

})();

如果在同一个函数中重新声明和定义了相同的全局变量，为什么这个全局变量在函数中未定义？

var a = 1;
function testscope(){
 console.log(a, 'inside func');
 //var a=2;
};
testscope();
console.log(a, 'outside func');

output:
1 "inside func"
1 "outside func" 

考虑相同的代码，其中 var a = 2; 内部功能块未注释

var a = 1;
function testscope(){
 console.log(a, 'inside func');
 var a=2; 
};
testscope();
console.log(a, 'outside func');

Output
undefined "inside func"
1 "outside func"