变量被提升

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

因为变量声明和函数声明会被解释为：

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

函数被提升

声明式函数会自动将声明放在前面并且执行赋值过程，而变量式则是先将声明提升，然后到赋值处再执行赋值。

function test(){
  foo();// TypeError "foo is not a function"
  bar();// "this will run!"
  var foo = function(){
    alert("this won't run!");
  }
  function bar(){// function declaration,given the name 'bar'
    alert('this will run !');
  }
}
test();

实际上等价于：

function test(){
  var foo;
  var bar;
  bar = function(){
    alert("this won't run !");
  };
  foo();// TypeError "foo is not a function"
  bar();// "this will run!"
  foo = function(){
    alert("this won't run !");
  }
}
test();