javascript 中 in操作符

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

"make"
in mycar  // returns true

"model"
in mycar // returns true

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// Arrays

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)

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var
color1 = new
String("green");

"length"
in color1 // returns true

var
color2 = "coral";

"length"
in color2 // generates an error (color is not a String object)

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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