JavaScript for ... in 循环

for (variable in object) {
 ...
 }

// Initialize object.
 a = { "a": "Athens", "b": "Belgrade", "c": "Cairo" }

 // Iterate over the properties.
 var s = ""
 for (var key in a) {
    s += key + ": " + a[key];
    s += "<br />";
    }
 document.write (s);

 // Output:
 // a: Athens
 // b: Belgrade
 // c: Cairo

 // Initialize the array.
 var arr = new Array("zero", "one", "two");

 // Add a few expando properties to the array.
 arr["orange"] = "fruit";
 arr["carrot"] = "vegetable";

 // Iterate over the properties and elements.
 var s = "";
 for (var key in arr) {
    s += key + ": " + arr[key];
    s += "<br />";
 }

 document.write (s);

 // Output:
 //   0: zero
 //   1: one
 //   2: two
 //   orange: fruit
 //   carrot: vegetable

 // Efficient way of getting an object's keys using an expression within the for-in loop's conditions
 var myObj = {a: 1, b: 2, c:3}, myKeys = [], i=0;
 for (myKeys[i++] in myObj);

 document.write(myKeys);

 //Output:
 //   a
 //   b
 //   c