JS 严格模式

• 如何开启

  'use strict' // 全局开启
   
  function fn() {
    'use strict' // 某个函数开启
   
  }

• 特点
  • 全局变量必须先声明

    "use strict";
    n = 10; // ReferenceError: n is not defined

  • 静止使用 with

    "use strict";
    var obj = { x: 10 };
    with (obj) {
      // Uncaught SyntaxError: Strict mode code may not include a with statement
      console.log(x);
    }

  • 创建 evel 作用域

    "use strict";
    var x = 10;
    eval("var x = 20; console.log(x)");
    console.log(x);

  • 静止 this 指向 window

    "use strict";
    function fn() {
      console.log("this", this); // undefined
    }
    fn();

  • 函数参数不能重名

    "use strict";
    // Uncaught SyntaxError: Duplicate parameter name not allowed in this context
    function fn(x, x, y) {
      return;
    }