in操作符

// Arrays，数组：下标 in array,length也可以

var trees = new Array("redwood", "bay", "cedar", "oak", "maple");

0 in trees        // returns true

3 in trees        // returns true

6 in trees        // returns false

"bay" in trees    // returns false (you must specify the

// index number, not the value at that index)

"length" in trees // returns true (length is an Array property)

// Predefined objects：内置对象

"PI" in Math          // returns true

// Custom objects：对象，key in obj

var mycar = {make: "Honda", model: "Accord", year: 1998};

"make" in mycar  // returns true

"model" in mycar // returns true

//使用字符串对象构造的

var color1 = new String("green");

"length" in color1 // returns true

//string不会使用构造函数转换

var color2 = "coral";

// generates an error (color2 is not a String object)

"length" in color2

//delete之后，就false了

var mycar = {make: "Honda", model: "Accord", year: 1998};

delete mycar.make;

"make" in mycar;  // returns false

var trees = new Array("redwood", "bay", "cedar", "oak", "maple");

delete trees[3];

3 in trees; // returns false

//设为undefined的话，就false

var mycar = {make: "Honda", model: "Accord", year: 1998};

mycar.make = undefined;

"make" in mycar;  // returns true

var trees = new Array("redwood", "bay", "cedar", "oak", "maple");

trees[3] = undefined;

3 in trees; // returns true

//可以找到原型链的key

"toString" in {}; // returns true