Javascript object property not in scope

2019-08-27 19:38发布

问题:

I seem to have a problem with objects' properties' scope. I would like to output each of the Message objects' title and message properties to a select element, but it is Not Working! What am I doing incorrectly

<html><head>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.5.0/jquery.min.js"></script>
<script type="text/javascript">
$(function(){
    function Message(title, message) {
        this.title=title;
        this.message=message;
        this.getTitle = function(){
            return this.title;
        };
        this.getMessage = function(){
            return this.message;
        };
    }
    var messages = new Array(
        new Message("First Title", "This is the first message"),
        new Message("Second Title", "This is another message")
    );
    function updateSelect () {
        $("#cannedMessages_button").empty();
        for (c in messages) {
            // First try, with getters and setters
            $("#cannedMessages_button").append($('<option>', { value : c.getMessage() , text : c.getTitle() }));
            // Second try, directly
            $("#cannedMessages_button").append($('<option>', { value : c.message , text : c.title }));
        }
    }
    updateSelect();
});
</script>
</head><body>
<form><select id="cannedMessages_button"></select></form>
</body></html>

I can verify that the foreach is in fact running two iterations, but I cannot get the values out of the objects.

回答1:

don't use for (c in messages).

in is for iterating over properties of an object, not for iterating over values in an array.

Use the tried and true

for(var i = 0; i < messages.length; i++) {
...
}

Also, you are not putting your getTitle and getMessage methods on a prototype, which is kind of wasteful.

回答2:

the syntax for the for in loop in js is :

for(var key in obj)
    {
        var currentElement = obj[key];
    }