仓库源文，站点原文

title: JavaScript中需要注意的几个问题 categories:

• JavaScript
• 前端杂烩 tags:
• tech
• cnblogs warning: true date: 2013-09-22 03:40:00 fromUrl: http://www.cnblogs.com/hustskyking/archive/2013/09/22/javascript-attention.html from: 博客园

var t = event.keyCode;
switch (t) {
   case '65':
      alert("Yay!");
      break;
}

"use strict";
var global = (function() {
    console.log(this); //undefined
})();

"use strict";
var global = (function() {
    var t = new Function("return this")();
    console.log(t);
})();

"use strict";
var global = (function() {
    var t = window.eval("this");
    console.log(t);
})();

(function () {
   var local = 1;
   new Function("console.log(typeof local);")(); // logs undefined
}());

var t = "global";
function foo(){
    console.log(t); //undefined
    return; 　　 var t = "local";
}

[Global Scope]
    |----- t   [String] undefined -> "global"
    |----- foo [Reference] [foo Scope]

[foo Scope]
    |----- t   [String] undefined -> "local"

var t = "global";
function foo(){
    var t; // 等同于 -> var t = undefined;
    console.log(t); //undefined     return; 　     var t = "local"; }

function foo(a, b, c) {
    var x = 1,
        bar,
        baz = "something";
}

<h3>附：javascript严格模式下要注意的地方（转自<a href="http://www.web-tinker.com/article/20125.html" target="_blank">次碳酸钴</a>）</h3>

"use strict";
a=1; //缺少var语句做声明，因此报错

"use strict";
var a=b=1; //错误 b未声明

"use strict";
(function(){
  //闭包中是允许使用函数声明语句的
  function func(){};
})();
{
  var f=function(){}; //函数声明表达式允许
  function func(){}; //函数声明语句在普通闭包中，错误
};

"use strict";
(function(){
  alert(this); //输出undefined
})();

"use strict";
var o={a:1,a:1};
//这个对象定义了两个a属性，因此报错

"use strict";
function func(a,a){};
//这个函数的两个形参都是a，因此报错

"use strict";
var a=1,b=1;
eval("var a=2");
window.eval("var b=2");
alert(a); //输出1 因为运行的a变成了eval作用域的局部变量
alert(b); //输出2 window.eval依然是全局作用域

"use strict";
function func(){
  return arguments.callee; //错误 callee无法使用
};
func();

"use strict";
with({});

"use strict";
var a=0999; //十进制，可以使用
var b=0123; //八禁止，无法使用

"use strict";
var a=1;
delete a;
//错误 无法删除var声明的变量

"use strict";
var o={get a(){}};
o.a=1;
//错误 给只读属性赋值