为什么我不能构建这个Javascript对象？

Question

I have an object I'm trying to populate from another object (that is, iterate over a return object to produce an object with only selected values from the original). My code looks like this:

var collect = {};

function getHistoricalData(username){
$.getJSON("http://url/" + username + ".json?params",
            function(data){
                for (var i=0; i < data.length; i++) {
                    console.log(i);
                    collect = { i : {text : data[i].text}};
                    $("#wrap").append("<span>" + data[i].text + "</span><br />");

                };
                console.log(collect);
            }); 

}

So I'm using Firebug for debugging, and here's what I know:

• The JSON object is intact
• console.log(i); is showing the numbers 1-20 as expected

• When I log the collect object at the end, it's structure is this: var collect = { i : {text : "the last iteration's text"}};

  So the incrementer is "applying" to the data[i].text and returning the text value, but it's not doing what I expected, which is create a new member of the collect object; it's just overwriting collect.i 20 times and leaving me with the last value.

Is there a different syntax I need to be using for assigning object members? I tried collect.i.text = and collect[i].text = and the error was that whatever I tried was undefined.

I'd love to know what's going on here, so the more in-depth an explanation the better.

Thanks!

Answer 1

The reason you're seeing only one property in the object at the end is that you're not augmenting it in each loop iteration - you're overwriting it.

collect = { i : {text : data[i].text}};

This says: create an object with one property, i , whose value is another object with one property, text , and assign it to the variable collect . What you want to do instead is set a property of the existing collect object for each value of i , where i is the name of the property.

In order to create an object property where the name is set dynamically, you must use array-style syntax, like this: collect[i] . I don't think you can mix array syntax and object literal syntax like you've tried. Try this instead (disclaimer, not tested):

collect[i] = { text: data[i]["text"] };

Answer 2

 collect[i] = {text : data[i].text};

If you use collect[i].text , since collect[i] does not exist before the assignment, it will return undefined , which has no text property.

Also:

1. collect.i means collect["i"] . This is totally different from collect[i] .
2. You can use an Array and append with .push .

Answer 3

As you already know, the problem is in this line:

 collect = { i : {text : data[i].text}};

Every iteration of the loop, you are erasing the collect object and assigning it a new object as value. That's why in the end it only shows the last iteration.

Try this:

 collect[i] = {text : data[i].text};