nodejs declare array of objects with dynamic size

Question

Here's how I declare my array of objects:

let arrayOfObjects = [{ _id:0, names:'' }];

And I want to assign the values like below:

for (var i = 0; i < kelasList.length; i++){
  for (var j = 0; j < absentees[i].length; j++){                    
    arrayOfObjects[i]._id = absentees[i][j]._id
    arrayOfObjects[i].names = absentees[i][j].firstName + " " + absentees[i][j].lastName
  }
}

Running the code above will return

UnhandledPromiseRejectionWarning: TypeError: Cannot set property '_id' of undefined at line

which basically points to the line

arrayOfObjects[i]._id

But it will assign the value without any problem, if I write

arrayOfObjects[0]._id

instead of

arrayOfObjects[i]._id

My assumption was let arrayOfObjects = [{ _id:0, names:'' }]; would create an array of objects, so I would be able to access/set its values with

arrayOfObjects[i]._id

but it seems let arrayOfObjects = [{ _id:0, names:'' }]; here will only create a single array of object. So currently for me to pass more values, I will need to declare it something like

let arrayOfObjects = [{ _id:0, names:'' },{ _id:0, names:'' },{ _id:0, names:'' },{ _id:0, names:'' },{ _id:0, names:'' }];

But of course, this isn't feasible because I don't know what should my array size be. So actually how should I declare the array of objects that can contain dynamic size of objects?

Sebastian Speitel · Accepted Answer · 2019-02-03 13:20:25Z

1

You could simply check if the element already exists in the array and if it doesn't, create it.

let arrayOfObjects = [];

for (var i = 0; i < kelasList.length; i++) {
  for (var j = 0; j < absentees[i].length; j++) {
    if (!arrayOfObjects[i]) {
      arrayOfObjects[i] = {
        _id: 0,
        names: ''
      };
    }
    arrayOfObjects[i]._id = absentees[i][j]._id
    arrayOfObjects[i].names = absentees[i][j].firstName + " " + absentees[i][j].lastName
  }
}

answered Feb 3, 2019 at 13:20

Sebastian Speitel

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Comments

Azami · Accepted Answer · 2019-02-03 13:28:39Z

By setting arrayOfObjects[i]._id, you are setting the attribute _id on an already existing object arrayOfObjects[i]. But for i >= 1, this object doesn't exist.

Instead, you need to set the whole object in the loop:

for (var i = 0; i < kelasList.length; i++){
  for (var j = 0; j < absentees[i].length; j++){                    
    arrayOfObjects[i] = {
        _id: absentees[i][j]._id, 
        names: absentees[i][j].firstName + " " + absentees[i][j].lastName}
    }
}

This will work just fine, but will replace the whole object at the index i. If instead you want to set the _id and names attribute of an object that could already be existing with different keys (say, address), you would check if the object exists:

for (var i = 0; i < kelasList.length; i++){
  for (var j = 0; j < absentees[i].length; j++){ 
    let id = absentees[i][j]._id;
    let names = arrayOfObjects[i].names = absentees[i][j].firstName + " " + absentees[i][j].lastName; 

    if(arrayOfObjets[i]){
      arrayOfObjects[i]._id = id;
      arrayOfObjects[i].names = names;
    }else{         
      arrayOfObjects[i] = {
        _id: absentees[i][j]._id, 
        names: absentees[i][j].firstName + " " + absentees[i][j].lastName
      }
    }
  }
}

There is one last problem with your code. You are iterating over i, then over j, and are setting arrayOfObjects[i]. This means that your double loop is basically the same thing as:

for (var i = 0; i < kelasList.length; i++){
  let lastAbsentee = absentees[i][absentees[i].length-1];
  arrayOfObjects[i]._id = lastAbsentee._id
  arrayOfObjects[i].names = lastAbsentee.firstName + " " + lastAbsentee.lastName
}

Are you sure that is what you want to do? Somehow I would be doubting it.

I would also address the odd iterating over kelasList which just seems to be absentees with extra steps.

molamk · Accepted Answer · 2019-02-03 14:32:04Z

Your arrayOfObjects has a size of 1 when you initialize it, but then you look into arrayOfObjects[i] after i === 0 which cause the undefined behavior since there are no items there (yet).
In your for loop, you're also overwriting on arrayOfObjects[i] multiple times (j times to for precision).

Try using this

let arrayOfObjects = [];

for (var i = 0; i < absentees.length; i++) {
    for (var j = 0; j < absentees[i].length; j++) {
        arrayOfObjects.push({
            _id: absentees[i][[j]._id,
            names: absentees[i][j].firstName + " " + absentees[i][j].lastName
        });
    }
}

NOTE: The iteration logic may be subject to change since I didn't really get your intention from your code. The "adding items to the arrray" part is good though.

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