变量提升

// will return a value of undefined
var myvar = "my value";

(function() {
  console.log(myvar); // undefined
  var myvar = "local value";
})();


将等同于

var myvar = "my value";
 
(function() {
  var myvar;
  console.log(myvar); // undefined
  myvar = "local value";
})();