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How to create an array of objects with all objects from multiple arrays?

Given de following arrays:
const classes = [
{
students: [
Student1,
Student2,
]
},
{
students: [
Student3,
Student4,
]
}
]
What's the best way performance-wise to generate an array of objects with a similar structure to this:
const allStudents = [
Student1,
Student2,
Student3,
Student4
]

Please note: Donot use reserved keyworks such ass class as a variable name.
Array.reduce implementation
const classes = [
{ students: [ 'Student1', 'Student2' ] },
{ students: [ 'Student3', 'Student4' ] }
];
const allStudents = classes.reduce((acc, cls) => {
acc = cls.students ? acc.concat(cls.students) : acc;
return acc;
}, []);
console.log(allStudents)
Array.flatMap implementation.
const classes = [
{ students: [ 'Student1', 'Student2' ] },
{ students: [ 'Student3', 'Student4' ] }
]
const allStudents = classes.flatMap((cls) => cls.students ? cls.students : []);
console.log(allStudents);

const classes = [
{
students:[
"Student1",
"Student2"
]
},
{
students:[
"Student3",
"Student4"
]
}
]
const n=classes.flatMap(x=>x.students.flatMap(i=>i))
console.log(n)
This would be recommended keeping performance in mind it is faster then regular for loop and ... many online sources say flatMap is better in terms of performance

const classes = [
{
students: ['Student1', 'Student2'],
},
{
students: ['Student3', 'Student4'],
},
];
const allStudents = classes.reduce((acc, curr) => {
return [...acc, ...curr.students];
}, []);
console.log(allStudents);

const allStudents = classes.reduce((acc,curr)=>{
return [...acc,...curr.students]
},[])
I think this is a simple and easy to understand way to group students in one array.

That's what I did, hope it helped:
const classes = [
{
students: [
'Student1',
'Student2',
]
},
{
students: [
'Student3',
'Student4',
]
}
];
var newArray = [];
for(var i = 0; i < classes.length; i++){
var tempArray = Object.values(classes[i]);
for(var j = 0; j < tempArray[0].length; j++){
newArray.push(tempArray[0][j]);
}
}
console.log(newArray);

Pushing values into an object of array

So I have an array of objects called products:
const products = [
{
name: 'Pants'
},
{
name: 'Shoes'
},
{
name: 't shirts'
}
]
And the list contains a lot more products with other values apart from name. So suppose i forgot to give each product, (object) a coupon's array, it would take me quite a while to update each object manually.
Is there a function that I can create which will push a coupons value into each object as an array?
So the final array would look like:
const updatedProducts = [
{
name: 'Pants',
coupons: []
},
{
name: 'Shoes',
coupons: []
},
{
name: 't shirts',
coupons: []
}
]

const updatedProducts = products.map(product => ({ ...product, coupons: [] }))

Yes you can add properties in javascript like this:
products.forEach(product => product.coupons = []);

How to filter array and create new array with specific object keys

I have an array with objects and in each object there is an "items" array. My goal is to combine these "items" into one array.
Goal / Expected Output
[
{ id: 'SUV' },
{ id: 'Compact' },
{ id: 'Gasoline' },
{ id: 'Hybrid' }
]
Sample Array
[
{
"id":"carType",
"items":[
{
"id":"SUV"
},
{
"id":"Compact"
}
]
},
{
"id":"fuelType",
"items":[
{
"id":"Gasoline"
},
{
"id":"Hybrid"
}
]
}
]

You could use Array#flatMap.
const data = [{"id":"carType","items":[{"id":"SUV"},{"id":"Compact"}]},{"id":"fuelType","items":[{"id":"Gasoline"},{"id":"Hybrid"}]}];
const r = data.flatMap(({ items }) => items);
console.log(r);

A one-liner in vanilla JS (earlier than EMCAScript 2019, flatMap is not available, so in case of that...)
[].concat(...arr.map(elt => elt.items))

Something like this?
newArr = []
for (let i = 0;i<arr.length;i++) {
for (let ii = 0;ii<arr[i].items.length;ii++) {
newArr.push(arr[i].items[ii].id)
}
}
console.log(newArr)

Group and merge similar javascript objects

I have the following data and I am stuck between a logic
[
{
"user.employeeId": "10081",
"objectives": [
"Improve consultation"
],
"param": "dueByDays"
},
{
"user.employeeId": "10081",
"objectives": [
"Building a strong team"
],
"param": "overdue"
},
]
How can I get the data in the below format, I have tried Map(), ForEach(), etc. but I am getting stuck,
This is the requirement/Output
[
{
'user.employeeId': '10081',
params: [
{
objectives: ['Improve consultation'],
param: 'dueByDays'
},
{
objectives: ['Building a strong team'],
param: 'overdue'
},
{
objectives: null,
param: 'dueToday'
}
]
}];
If suppose param is not available, we still need to set objective as null, please help me out regarding this query. It would be better if anyone can help me do this using lodash
There will be only 3 params as mentioned above

The approach here would be something like this:
create some "storage" object (can be a simple object);
iterate through items;
if this is a new employee, then create new record in the "storage";
if this is a known employee, just update its info.
After this, the storage object has to be converted to an array, as per requirement:
const userEntries = /* initial array */;
const tempStorage = Object.create(null); // or just '{}'
for (const entry of userEntries) {
const id = entry["user.employeeId"];
const param = {
objectives: entry.objectives,
param: entry.param,
};
if (id in tempStorage === false) { // this is a new employee
tempStorage[id] = [ param ];
} else { // this is a known employee, it exists in storage
tempStorage[id].push(param);
}
}
// All the necessary data is in tempStorage.
// Now on to converting.
const output = [];
for (const employeeId in tempStorage) {
output.push({
["user.employeeId"]: employeeId,
params: tempStorage[employeeId],
});
}

Something like this should work:
const employeesData = {};
for (let item of array) {
const id = item["user.employeeId"];
if (!employeesData[id]) {
employeesData[id] = {};
}
employeesData[id][item.param] = item.objectives;
}
const params = ['dueByDays', 'overdue', 'dueToday'];
const result = [];
for (let key of Object.keys(employeesData)) {
result.push({
'user.employeeId': key,
params: params.map(param => ({
param,
objectives: employeesData[key][param] || null,
})),
});
}
We first build an object containing all the data then we iterate through the keys to build a proper array.

I don't test it but it should work.
const usersHash = users.reduce((acc, curr) => {
if(acc[curr["user.employeeId"]])
acc[curr["user.employeeId"]].params.push(curr.param ? {...curr.param} : {})
else
acc[curr["user.employeeId"]] = { "user.employeeId": curr["user.employeeId"], params: [{...curr.param}] }
return acc;
},{})
const mergedUsers = Object.values(usersHash);

You can use .reduce() method. The following code should work.
const arr = [
{
"user.employeeId": "10081",
"objectives": [
"Improve consultation"
],
"param": "dueByDays"
},
{
"user.employeeId": "10081",
"objectives": [
"Building a strong team"
],
"param": "overdue"
},
{
"user.employeeId": "10081",
"objectives": [
"A team"
],
},
]
const result = arr.reduce((accumulator, currentValue) => {
const element = accumulator.find(item => item['user.employeeId'] === currentValue['user.employeeId'])
if (element) {
element.params.push({objectives: currentValue.objectives, param: currentValue.param || null})
} else {
accumulator.push({'user.employeeId': currentValue['user.employeeId'], params: [{objectives: currentValue.objectives, param: currentValue.param || null}]})
}
return accumulator
},[])
console.log(result)

JavaScript make object to array only by messageid

Let's say I have the following:
[
{ messageid: '582b4350af7cb8f21e4b7b43',
_id: 582b4c79105387dd21e08004 } ,
{ messageid: '582b4350af7cb8f21e4b7xb43',
_id: 582b4c79105387dd21e08s004 }
]
How can I make it produce a array that would only be the messageid `s values?
Like:
{582b4350af7cb8f21e4b7b43,582b4350af7cb8f21e4b7xb43} ?
Note: plain javascript or angular

You can use .map
var output = input.map(function(item) {
return item.messageid;
});
This will create an array that will look like [582b4350af7cb8f21e4b7b43,582b4350af7cb8f21e4b7xb43]

You could map the property messageid.
var objectArray = [{ messageid: '582b4350af7cb8f21e4b7b43', _id: '582b4c79105387dd21e08004' }, { messageid: '582b4350af7cb8f21e4b7xb43', _id: '582b4c79105387dd21e08s004' }],
newArray = objectArray.map(function (o) {
return o.messageid;
});
console.log(newArray);

Use forEach to loop through the array
var orArray = [{
messageid: '582b4350af7cb8f21e4b7b43',
_id: '582b4c79105387dd21e08004'
}, {
messageid: '582b4350af7cb8f21e4b7xb43',
_id: '582b4c79105387dd21e08s004'
}
];
var newArr = [];
orArray.forEach(function(item) {
newArr.push(item.messageid)
});
console.log(newArr)
DEMO