嵌套 JSON：如何向对象添加(推送)新项目

我只是从数组、对象和JSON开始——希望这里有一些简单的东西我忽略了.我在try 向json对象中添加新项时遇到错误.

I'm encountering the following error: Result of expression 'library.push' [undefined] is not a function (towards the bottom of my code snippet).

// This is my JSON object generated from a database
var library = {
    "Gold Rush" : {
        "foregrounds" : ["Slide 1","Slide 2","Slide 3"],
        "backgrounds" : ["1.jpg","","2.jpg"]
    },
    "California" : {
        "foregrounds" : ["Slide 1","Slide 2","Slide 3"],
        "backgrounds" : ["3.jpg","4.jpg","5.jpg"]
    }
}

// These will be dynamically generated vars from editor
var title = "Gold Rush";
var foregrounds = ["Howdy","Slide 2"];
var backgrounds = ["1.jpg",""];

function save () {

    // If title already exists, modify item
    if (library[title]) {
        // Replace values with new
        library[title].foregrounds = foregrounds;
        library[title].backgrounds = backgrounds;

        // Save to Database. Then on callback...
        document.write('Changes Saved to <b>'+title+'</b>');

    // If title does not exist, add new item
    else {
        // Format it for the JSON object
        var item = ('"'+title+'" : {"foregrounds" : '+foregrounds+',"backgrounds" : '+backgrounds+'}');


        // THE PROBLEM SEEMS TO BE HERE??
        // Error: "Result of expression 'library.push' [undefined] is not a function"
        library.push(item);


        // Save to Database. Then on callback...
        document.write('Added: <b>'+title+'</b>');
    }
}

save();