以下是嚴(yán)格模式中需要注意的用法，這里需要強(qiáng)調(diào)的是：ES6 的 class 和 模塊內(nèi)都是默認(rèn)的嚴(yán)格模式。其實(shí)，js 開發(fā)也會(huì)逐步走向嚴(yán)格模式，這應(yīng)該是個(gè)趨勢。

(function(){
  //非嚴(yán)格模式
  var a = {name: "Bob"};
  with(a){
    name = "Lily";
  }
  console.log(a.name);  //Lily
})();

(function(){
  "use strict";   //嚴(yán)格模式
  var a = {name: "Bob"}, b = {};
  with(a, b){    //SyntaxError: Strict mode code may not include a with statement
    name = "Lily";
  }
  console.log(a.name);
})();

(function(){
  //非嚴(yán)格模式
  a = 10;
  console.log(a);  //10
})();

(function(){
  "use strict";   //嚴(yán)格模式
  b = 10;   //ReferenceError: b is not defined
  console.log(b);
})();

(function(){
  //非嚴(yán)格模式
  console.log(this);  //window
})();

(function(){
  "use strict";   //嚴(yán)格模式
  console.log(this);    //undefined
})();

(function(){
  //非嚴(yán)格模式
  var o = {
    name: "Lily"
  };
  Object.freeze(o);
  o.name = "Bob";
  o.age = 20;
  console.log(o);  //Object {name: "Bob"}
})();

(function(){
  "use strict";   //嚴(yán)格模式
    var o = {
    name: "Lily"
  };
  Object.freeze(o);
  o.name = "Bob";  //TypeError: Cannot assign to read only property "name" of object "#