with

下面的with语句指定[Math](https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Math "JavaScript/Reference/Global_Objects/Math")对象作为默认对象。with语句里面的变量，分別指向Math对象的PI 、[cos](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/cos "JavaScript/Reference/Global_Objects/Math/cos")和``[sin](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sin "JavaScript/Reference/Global_Objects/Math/sin")函数，不用在前面添加命名空间。后续所有引用都指向Math对象。

如下：with不改变this指向！

var a, x, y;
var r = 10;

with (Math) {
  a = PI * r * r;
  x = r * cos(PI);
  y = r * sin(PI / 2);
}
with({num:11,say:function(){alert(22)},this:33}){
      console.log(num);   // 11
      console.log(say());  // 22
      console.log(this) ;  // Window
}

MDN原文链接地址

严格模式和 new Function

// 严格模式和 new Function
// 严格模式作用范围
// new Function 因为Function === window.Function
(function(){
    'use strict' // 只能在作用域第一句
    console.log('this11:',this);
    (function (){
        new Function('console.log("this33:",this)').bind({name:"haha"})()
    })();
})();
console.log('this22:',this)

严格模式和with

在 ECMAScript 5 严格模式中该标签已被禁止。推荐的替代方案是声明一个临时变量来承载你所需要的属性。

(function(){
    // vue中的解决方案
    'use strict'
    console.log('this1:',this);
    new Function('with(this){console.log(123)}')();
})();
console.log('this2:',this)