js的拾遗

1.Re-Declaring JavaScript Variables

• If you re-declare a JavaScript variable, it will not lose its value.
  The variable carName will still have the value "Volvo" after the execution of these statements:
  Example
  var carName = "Volvo";
  var carName;
  PS:这个应该是重新定义时没有初始化对应的内存区吧，是编译器和语言的要求吗？！
  2.Example
  var x = "5" + 2 + 3;
  Note:If you add a number to a string, the number will be treated as string, and concatenated.
  3.JavaScript evaluates expressions from left to right. Different sequences can produce different results:
  ---------------------------
  JavaScript:
  var x = 16 + 4 + "Volvo";
  Result:
  20Volvo
  ---------------------
  JavaScript:
  var x = "Volvo" + 16 + 4;
  Result:
  Volvo164
  [popexizhi:从左到右的啊！
  测试了一下
  var x =10+4+"Volvo"+20*3;
  Result:
  14Volvo60
  应该是按运算符顺序计算，但遇到字符串后变成字符串运算的。
  ]
  
  4.typeof
  The typeof Operator
  You can use the JavaScript typeof operator to find the type of a JavaScript variable:
  Example
  typeof "John"                // Returns string
  typeof 3.14                  // Returns number
  typeof false                 // Returns boolean
  typeof [1,2,3,4]             // Returns object
  typeof {name:'John', age:34} // Returns object
  5.----
  Undefined
  In JavaScript, a variable without a value, has the value undefined. The typeof is also undefined.
  Example
  var person;                  // The value is undefined, the typeof is undefined