var collection = {    2548: {      album: "Slippery When Wet",      artist: "Bon Jovi",      tracks: [         "Let It Rock",         "You Give Love a Bad Name"       ]    },    2468: {      album: "1999",      artist: "Prince",      tracks: [         "1999",         "Little Red Corvette"       ]    },    1245: {      artist: "Robert Palmer",      tracks: [ ]    },    5439: {      album: "ABBA Gold"    }};// 深拷贝 collection，用于测试var collectionCopy = JSON.parse(JSON.stringify(collection));function update(id, prop, value) { if(value !==""){   if(prop !== "tracks"){     collection[id][prop] = value;   }else{      collection[id][prop].push(value);   }       }else{   delete collection[id][prop]; }  return collection;}update(5439, "artist", "ABBA");