javascript - javascript如何初始化变量打印未定义？

Question

01  print(x === undefined); // prints "true"
02  var x = 3;
03   
04  //will return a value of undefined
05  var myvar = "my value";
06   
07  (function() {
08    alert(myvar);//undefined       -- here i don't understand ho it print undefined 
09    var myvar = "local value"
10  })();

Answer 1

因为hoisting 。当函数执行时，var myvar = "local value"被推到函数的顶部，值为 undefined (覆盖先前定义的 var myvar = "my value"; )。

编辑:添加了来自 Adequately Good 的稍微修改的示例

这意味着像这样的代码:

function foo() { 
    bar(); 
    var x = 1; 
}

实际上是这样解释的:

function foo() { 
    var x; 
    bar();
    alert(x); //alerts undefined 
    x = 1; 
}