nodejs declare array of objects with dynamic size - node.js

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?

Answer 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
}
}

Answer 2

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.

Answer 3

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.