无法将值推入对象数组？

Question

So I have some code im working through on FreeCodeCamp: https://jsfiddle.net/matt40413/m8wh7sxz/

Everything is working great until I try to push a value onto a new array (that doesn't exist yet) I get the error Uncaught TypeError: collectionCopy[id].tracks.push is not a function .

  if(prop == "tracks")
  {
    collectionCopy[id]["tracks"].push(value);
  }

This is where it's failing. But the funny thing is, if I MANUALLY put everything in such as: collectionCopy[5439]["tracks"].push("Whatever") works just fine!

So what's going on here? Im essentially doing the same thing? but just using the parameters instead (Which btw my other functions do work, so im a bit confused).

Answer 1

Three comments about the code:

1. You are using the if-else statements as separated cases, that is not true! In order to separate each case, you should use else if statement

2. The problem in the code is that your first case was

   if(!collectionCopy[id][[prop]])

That is saying: if the key [prop] doesn't exists in collectionCopy[id] , then ...

Since [prop] is an array and not a number neither a string, the expression was always true. Therefore, it was executing collectionCopy[id][prop] = value .

Note: the keys are always string or numbers.

3. The rest of the code was correct and actually cool.

This is the corrected snippet.

// Setup
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"
    }
};
// Keep a copy of the collection for tests
var collectionCopy = JSON.parse(JSON.stringify(collection));

// Only change code below this line
function updateRecords(id, prop, value) {

  if(!collectionCopy[id][prop])
  {
  collectionCopy[id][prop] = value;
  }
  else if(value == "")
  {
  console.log("DELETED");
    delete collectionCopy[id][prop.push]
  }
  else if(prop == "tracks")
  {
    collectionCopy[id]["tracks"].push(value);
  }
    console.log(collectionCopy);





  return collection;
}

// Alter values below to test your code
updateRecords(5439, "tracks", "Take a Chance on Me");

Working example https://jsfiddle.net/m8wh7sxz/2/

Answer 2

Change this part:

collectionCopy[id]["tracks"].push(value);

To:

collectionCopy[id]["tracks"] = value;

Because collectionCopy[id]["tracks"] is not initialized yet, there for you can't call push() function

Answer 3

You simply initialized your object incorrectly, doing this will fix it:

updateRecords(5439, "tracks", ["Take a Chance on Me"]);

Now your push will append to this array! Before you were trying to run .push() on a string!

Edit:

If you need to dynamically construct the array when updating, you can do:

if (prop == "tracks") {
  console.log(collectionCopy[id]["tracks"])
  if (!(collectionCopy[id]["tracks"] instanceof Array)) {
    collectionCopy[id]["tracks"] = [value]
  } else collectionCopy[id]["tracks"].push(value)
}

Answer 4

You don't have "tracks" in "5439" object.

You should add it if you want to access it as array.

"5439": {
  "album": "ABBA Gold",
  "tracks":[]
}

EDIT:

This should work for you then.

else if(prop == "tracks")
{
   if(!("tracks" in collectionCopy[id]) ||  !(collectionCopy[id]
     ["tracks"].constructor === Array)){
       collectionCopy[id]["tracks"]=[];
   }
   collectionCopy[id]["tracks"].push(value);      

}

Answer 5

if(prop == "tracks")
  {
    if(!Array.isArray(collectionCopy[id]["tracks"]){
      collectionCopy[id]["tracks"] = [];
    }
    collectionCopy[id]["tracks"].push(value);
  }

create an array if it is not an array first

Answer 6

You can't push onto an array property if it hasn't been initialized yet. You can use the following trick:

if(prop == "tracks")
{
  (collectionCopy[id]["tracks"] = collectionCopy[id]["tracks"] || []).push(value);
}

If the array is initilized, then collectionCopy[id]["tracks"] || [] collectionCopy[id]["tracks"] || [] evaluates to collectionCopy[id]["tracks"] || [] collectionCopy[id]["tracks"] || [] , otherwise it will evaluate to [] thus a new property of array type being initialized.