Develop Reference

Reduce function creating an array with half the data

I am trying to sort data into separate arrays based on the value of a property inside each object. I am using a reduce function which is new to me and still quite confusing.
My data looks like this:
const data = [{
score: 2,
category: "date of admission"
subcategory: 2-days
}, { score: 12,
category: "Vitals",
subcategory: "bloodPressure",
},{
score: 4,
category: "date of admission"
subcategory: 3-days
}...]
My reduce function looks like this:
const grouped = data.reduce((cat, item, index, data) => {
for (index = 0; data.length; index++) {
cat[item.category] = [];
cat[item.category].push(item);
return cat;
}
}, {});
The problem Im having is the output when I console.log(grouped) each array will have the correct category label but inside the array there is only ever 1 object. Even tho the data may have more than one item with the same category. Im not sure what Im doing wrong here. Any advice would be helpful.

I think you're looking for something like this:
const grouped = data.reduce((catetories, item) => {
const category = catetories[item.category] || [];
return {
...catetories,
[item.category]: [
...category,
item,
]
}
}, {});
const groupedArr = Object.keys(grouped).map((key) => grouped[key]);

const data = [{
score: 2,
category: "date of admission",
subcategory: "2-days"
}, { score: 12,
category: "Vitals",
subcategory: "bloodPressure",
},{
score: 4,
category: "date of admission",
subcategory: "3-days"
}];
const cat = {};
for (let c of data) {
if (cat[c.category]) {
cat[c.category].push(c);
} else {
cat[c.category] = [c];
}
}
console.log(cat)

Change particular key in nested array to object in javascript

If the fields key in a object is array, change the first value of arrays as a key value pair object in javascript.
var obj =
[
{ id:1, fields:["item", "2", "list"]},
{ id:2, fields:["sample", "1", "check"]}
]
function newObj(obj) {
let objFields = {};
modifiedObj.fields.forEach(field => objFields[field] = field);
modifiedObj.fields= objFields;
return modifiedObj;
}
var result = this.newObject(obj)
Expected Output
{
item: "item",
sample: "sample"
}

Try this:
var obj =
[
{ id:1, fields:["item", "2", "list"]},
{ id:2, fields:["sample", "1", "check"]}
]
function newObject(obj) {
let objFields = {};
obj.forEach(e => {
if(e.fields && e.fields.length>0)
objFields[e.fields[0]] = e.fields[0];
});
return objFields;
}
var result = this.newObject(obj);
console.log(result);

Here is a functional approach that makes use of Object.assign(), spread operator, and Array.map() to create the object you need.
const input = [
{ id: 1, fields: ["item", "2", "list"] },
{ id: 2, fields: ["sample", "1", "check"] }
];
const process = (input) => (Object.assign(...input.map(({ fields }) => (
fields.length ? { [fields[0]]: fields[0] } : {}
))));
console.log(process(input));

Your snippet was close, you just needed to clean up the variable names, and then using map makes it a bit neater too:
const obj = [
{id: 1, fields: ["item", "2", "list"]},
{id: 2, fields: ["sample", "1", "check"]}
]
function newObj(inputArray) {
let outputObject = {};
inputArray.map(item => item.fields[0])
.forEach(field => outputObject[field] = field);
return outputObject;
}
var result = newObj(obj)
console.log(result)

Inside an Object.value loop how can I get the key

I have to create match condition based on an array my array will look like below
var groupData={
A:[
{rollnum: 1, name:'Arya', age:15},
{rollnum: 2, name:'Aryan', age:15}
],
B:[
{rollnum:11, name:'Biba', age:15},
{rollnum:12, name:'Bimisha', age:15}
]
}
I am looping using for loop. How can reduce the loops. Can any one suggest me a proper way for this
Object.values(groupData).flat().forEach((rowitem)=>{
query={};
Object.keys(rowitem).forEach(eachField=>{
query[eachField]["$in"].push(rowitem[eachField])
});
fullarray[Object.keys(groupData)]=matchQuery;
})
I need an output (fullarray) like below
{
'A':{
rollnum:{'$in':[1,2]},
name: {'$in':['Arya', 'Aryan']},
age: {'$in':[15]}
},
'B':{
rollnum:{'$in':[11,12]},
name: {'$in':['Biba', 'Bimisha']},
age: {'$in':[15]}
}
}
Here 'A' 'B' is not coming correctly

Don't use Object.values() since that discards the A and B keys.
Use nested loops, one loop for the properties in the object, and a nested loop for the arrays.
You need to create the nested objects and arrays before you can add to them.
var groupData = { A:
[ { rollnum: 1,
name: 'Arya',
age:15},
{ rollnum: 2,
name: 'Aryan',
age:15}, ],
B:
[ { rollnum: 11,
name: 'Biba',
age:15},
{ rollnum: 12,
name: 'Bimisha',
age:15} ] }
result = {};
Object.entries(groupData).forEach(([key, arr]) => {
if (!result[key]) {
result[key] = {};
}
cur = result[key];
arr.forEach(obj => {
Object.entries(obj).forEach(([key2, val]) => {
if (!cur[key2]) {
cur[key2] = {
"$in": []
};
}
cur[key2]["$in"].push(val);
});
});
});
console.log(result);

ImmutableJS - pushing into a nested array

const List = Immutable.List;
const items = [
{ id: 1, subList: [] },
{ id: 2, subList: [] },
{ id: 3, subList: [] }
];
const newItem = { name: "sublist item" };
let collection = List(items);
collection = collection.updateIn([0, 'subList'], function (items) {
return items.concat(newItem)
});
https://jsbin.com/midimupire/edit?html,js,console
Results in:
Error: invalid keyPath
I think that perhaps I need to set subList as a List(); I get the same error when trying this.

If I understand the question correctly, you want to return collection with the first element as:
{
id : 1,
subList: [
{name: "sublist item"}
]
}
To do this we'll need to make a few changes.
Use Immutable.fromJS to deeply convert the plain JS array of objects to an Immutable List of Maps
Use List.update() to return a new List with the updated value
Use Map.updateIn() to return a new LMapist with the updated value
Here's the whole thing:
const List = Immutable.List;
const items = [{
id: 1,
subList: []
},
{
id: 2,
subList: []
},
{
id: 3,
subList: []
}
];
const newItem = {
name: "sublist item"
};
let collection = Immutable.fromJS(items);
collection = collection.update(0, item => {
return item.updateIn(['subList'], subList => {
return subList.concat(newItem);
});
});
console.log(collection)
<script src="https://cdnjs.cloudflare.com/ajax/libs/immutable/3.8.2/immutable.js"></script>
And the result:
[
{
"id": 1,
"subList": [
{
"name": "sublist item"
}
]
},
{
"id": 2,
"subList": []
},
{
"id": 3,
"subList": []
}
]
Update: List.updateIn() can use an index as the keypath, so you can simplify this to the following:
collection = collection.updateIn([0, 'subList'], subList => {
return subList.concat(newItem);
});
Like this:
const List = Immutable.List;
const items = [{
id: 1,
subList: []
},
{
id: 2,
subList: []
},
{
id: 3,
subList: []
}
];
const newItem = {
name: "sublist item"
};
let collection = Immutable.fromJS(items);
collection = collection.updateIn([0, 'subList'], subList => {
return subList.concat(newItem);
});
console.log(collection)
<script src="https://cdnjs.cloudflare.com/ajax/libs/immutable/3.8.2/immutable.js"></script>

Use the object you got, update the subArray and return the whole object.
const List = Immutable.List;
const items = [
{ id: 1, subList: [] },
{ id: 2, subList: [] },
{id: 3, subList: [] }
];
const newItem = { name: "sublist item" };
let collection = List(items);
collection = collection.update([0], function (obj) {
obj.subList = obj.subList.concat(newItem)
return obj;
});

This doesn’t work because the elements of your Immutable.List are plain-old JavaScript objects (POJO), not Immutable.Maps, so updateIn doesn’t know how to work with them. You can either:
Make the objects Immutable.Maps by using Immutable.fromJS instead of Immutable.List as the constructor to convert the entire object graph to Immutable objects. (See JS Bin)
Use update([0]) instead of updateIn to just get the POJO and mutate it (as in #Navjot’s answer).

Create new array from iterating JSON objects and getting only 1 of its inner array

See jsfiddle here: https://jsfiddle.net/remenyLx/2/
I have data that contains objects that each have an array of images. I want only the first image of each object.
var data1 = [
{
id: 1,
images: [
{ name: '1a' },
{ name: '1b' }
]
},
{
id: 2,
images: [
{ name: '2a' },
{ name: '2b' }
]
},
{
id: 3
},
{
id: 4,
images: []
}
];
var filtered = [];
var b = data1.forEach((element, index, array) => {
if(element.images && element.images.length)
filtered.push(element.images[0].name);
});
console.log(filtered);
The output needs to be flat:
['1a', '2a']
How can I make this prettier?
I'm not too familiar with JS map, reduce and filter and I think those would make my code more sensible; the forEach feels unnecessary.

First you can filter out elements without proper images property and then map it to new array:
const filtered = data1
.filter(e => e.images && e.images.length)
.map(e => e.images[0].name)
To do this in one loop you can use reduce function:
const filtered = data1.reduce((r, e) => {
if (e.images && e.images.length) {
r.push(e.images[0].name)
}
return r
}, [])

You can use reduce() to return this result.
var data1 = [{
id: 1,
images: [{
name: '1a'
}, {
name: '1b'
}]
}, {
id: 2,
images: [{
name: '2a'
}, {
name: '2b'
}]
}, {
id: 3
}, {
id: 4,
images: []
}];
var result = data1.reduce(function(r, e) {
if (e.hasOwnProperty('images') && e.images.length) r.push(e.images[0].name);
return r;
}, [])
console.log(result);

All answers are creating NEW arrays before projecting the final result : (filter and map creates a new array each) so basically it's creating twice.
Another approach is only to yield expected values :
Using iterator functions
function* foo(g)
{
for (let i = 0; i < g.length; i++)
{
if (g[i]['images'] && g[i]["images"].length)
yield g[i]['images'][0]["name"];
}
}
var iterator = foo(data1) ;
var result = iterator.next();
while (!result.done)
{
console.log(result.value)
result = iterator.next();
}
This will not create any additional array and only return the expected values !
However if you must return an array , rather than to do something with the actual values , then use other solutions suggested here.
https://jsfiddle.net/remenyLx/7/