Question 192074 in Javascript Question posted in
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How can I push to an array thats inside an object's element in Javascript

I have the following object:

var watchObject = {};

Here is the following code I’m trying to accomplish; if it exists in the object I want to append it and if it doesn’t exist I want to create a new key/value pair. The key value pair looks like "23": [blah]. So if the "23"(data.room) exists in the object I will push another to it like "23":[blah,bleh].

//assume data.room is defined elsewhere and data.videoId is defined elsewhere

if (data.room in watchObject) {
    watchObject[data.room].push(data.videoId);
} else {
    watchObject[data.room] = data.videoId;
}

I tried the above, I thought it would work but It keeps saying:

"TypeError: watchObject[data.room].push is not a function"

I’ve also tried assinging data.room into a variable with no luck:

if (data.room in watchObject) {
   let tempRoom = data.room;
   watchObject.tempRoom.push(data.videoId);
   //watchObject[data.room].push(data.videoId);
} else {
    watchObject[data.room] = data.videoId;
}

and get this error:

"TypeError: Cannot read property ‘push’ of undefined"

Tags:

4

Answers


  1. 0

    It’s because watchObject[data.room] is undefined.

    You need this:

    if(watchObject[data.room] == null) {
  watchObject[data.room] = []
}
// now you can safely push to the array
watchObject[data.room].push(data.videoId)
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  2. 0

    The issue is that once you create the entry via this code…

    watchObject[data.room] = data.videoId;

    it is not an array so the next time you try this code…

    watchObject[data.room].push(data.videoId);

    it will fail with the error you see.

    A simple option here is to lazily create an array before pushing

    const watchObject = {};
const arr = [{ room: "23", videoId: "blah" }, { room: "23", videoId: "bleh" }];

arr.forEach((data) => {
  // create an empty array if it does not exist
  watchObject[data.room] ??= [];

  // now push
  watchObject[data.room].push(data.videoId);
});

console.log(watchObject);

    See Nullish coalescing assignment (??=)

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  3. 0

    You almost done. But you have some mistake.
    Try this.

    let watchObject = {};

    // Init data

    if (watchObject[data.room]) {
    watchObject[data.room].push(data.videoId);
} else {
    watchObject[data.room] = **[data.videoId]**;
}

    This will works well.

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  4. 0

    To push an element to an array that is inside an object in JavaScript, you can access the object, then access the array property using dot notation or square brackets, and finally use the push() method to add the element. Here’s an example:

    const myObject = {
myArray: []
};

myObject.myArray.push("newElement");
console.log(myObject.myArray); // Output: ["newElement"]

    This code snippet demonstrates accessing the myArray property of myObject and pushing the element "newElement" to it.

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