Develop Reference

How to add elements in an object while traversing through an array using map?

I am having an array like this :
arr = [ {id:0,name:Mark} , {id:1,name:Ron}, {id:2,name:Henry}, {id:3,name:Rose}].
I want to create an object like this :
obj1 = { Mark:false, Ron:false, Henry:false, Rose:flase }
I am using map to traverse through the array like this
let obj1 = {};
obj1 = arr.map((item)=> {
obj1[item.name] = false;
})
How can I achieve the following result?

You could map entries and build an object from the pairs.
const
data = [{ id: 0, name: 'Mark' }, { id: 1, name: 'Ron' }, { id: 2, name: 'Henry' }, { id: 3, name: 'Rose' }],
result = Object.fromEntries(data.map(({ name }) => [name, false]));
console.log(result);

Object.fromEntries() is probably the best idea. But you could also use reduce, if you got more operations on the array and want to stick to the "pipe" approach.
const arr = [
{ id: 0, name: 'Mark' },
{ id: 1, name: 'Ron' },
{ id: 2, name: 'Henry' },
{ id: 3, name: 'Rose' }
];
const objA = arr
.reduce((previous, { name }) => ({ ...previous, [name]: false }), {});
const objB = arr
.reduce((previous, { name }) => {
previous[name] = false;
return previous;
}, {});
The spreach operation {...obj} for objA does effectivly copy the object on each extension, which might not be desirable. But the modern JavaScript engines simplify those expressions anyways.
objB is the more standard approach for me. One additional benefit, in regards to Object.fromEntries() is that you can have some form of standard or default object / settings, which you can use as the start of the reduce (the second parameter) and evaluate in the collection function.
All three options are valid and depend on your code style.

How to combine _.map and _.filter in a more efficient way?

I am using Lodash in my Angular project and I was wondering if there is a better way to write the following code:
$scope.new_arr = _.map(arr1, function(item){
return _.assign(item, {new_id: _.find(arr2, {id: item.id})});
});
$scope.new_arr = _.filter($scope.new_arr, function (item) {
return item.new_id !== undefined;
});
I am trying to combine values from one array to same objects in other array, and I want to ignore the objects that not appear in both arrays (it is something like join or left outer join in the sql language).
Here is a fiddle with an example of this code: Click me!

i think is better to use chaining
$scope.new_arr = _.chain(arr1)
.map(function(item) {
return _.merge(
{}, // to avoid mutations
item,
{new_id: _.find(arr2, {id: item.id})}
);
})
.filter('new_id')
.value();

https://jsfiddle.net/3xjdqsjs/6/
try this:
$scope.getItemById = (array, id) => {
return array.find(item => item.id == id);
};
$scope.mergeArrays = () => {
let items_with_ids = arr1.filter(item => !_.isNil($scope.getItemById(arr2,item.id)));
return items_with_ids.map(item => _.assign(item, {new_id: $scope.getItemById(arr2,item.id)}));
};

The answers provided here are all runtime of O(n^2), because they first run an outer loop on the first array, with an inner loop on the second array. You can instead run this in O(n). First, create a hashmap of all the ids in arr2 in a single loop; this will allow us an order 1 lookup. In the second loop on arr1, check this hashmap to determine if those items exist with O(n). Total Complexity is n + n = 2n, which is just O(n).
// provision some test arrays
var arr1 = [
{
id: 2
},
{
id: 4
},
{
id: 6
}
]
var arr2 = [
{
id: 3
},
{
id: 4
},
{
id: 5
},
{
id: 6
}
]
// First, we create a map of the ids of arr2 with the items. Complexity: O(n)
var mapIdsToArr2Items = _.reduce(arr2, function(accumulator, item) {
accumulator[item.id] = item;
return accumulator;
}, {});
// Next, we use reduce (instead of a _.map followed by a _.filter for slightly more performance.
// This is because with reduce, we loop once, whereas with map and filter,
// we loop twice). Complexity: O(n)
var combinedArr = _.reduce(arr1, function(accumulator, item) {
// Complexity: O(1)
if (mapIdsToArr2Items[item.id]) {
// There's a match/intersection! Arr1's item matches an item in arr 2. Include it
accumulator.push(item);
}
return accumulator;
}, []);
console.log(combinedArr)

You could first make a Map with arr1 and then map the items of arr2 with the properties of arr1.
var arr1 = [{ id: 1, title: 'z' }, { id: 2, title: 'y' }, { id: 3, title: 'x' }, { id: 4, title: 'w' }, { id: 5, title: 'v' }],
arr2 = [{ id: 2, name: 'b' }, { id: 3, name: 'c' }, { id: 4, name: 'd' }, { id: 5, name: 'e' }],
map = new Map(arr1.map(a => [a.id, a])),
result = arr2.map(a => Object.assign({}, a, map.get(a.id)));
console.log(result);
.as-console-wrapper { max-height: 100% !important; top: 0; }

How to add non duplicate objects in an array in javascript?

I want to add non-duplicate objects into a new array.
var array = [
{
id: 1,
label: 'one'
},
{
id: 1,
label: 'one'
},
{
id: 2,
label: 'two'
}
];
var uniqueProducts = array.filter(function(elem, i, array) {
return array.indexOf(elem) === i;
});
console.log('uniqueProducts', uniqueProducts);
// output: [object, object, object]
live code

I like the class based approach using es6. The example uses lodash's _.isEqual method to determine equality of objects.
var array = [{
id: 1,
label: 'one'
}, {
id: 1,
label: 'one'
}, {
id: 2,
label: 'two'
}];
class UniqueArray extends Array {
constructor(array) {
super();
array.forEach(a => {
if (! this.find(v => _.isEqual(v, a))) this.push(a);
});
}
}
var unique = new UniqueArray(array);
console.log(unique);
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.16.4/lodash.min.js"></script>

Usually, you use an object to keep track of your unique keys. Then, you convert the object to an array of all property values.
It's best to include a unique id-like property that you can use as an identifier. If you don't have one, you need to generate it yourself using JSON.stringify or a custom method. Stringifying your object will have a downside: the order of the keys does not have to be consistent.
You could create an objectsAreEqual method with support for deep comparison, but this will slow your function down immensely.
In two steps:
var array=[{id:1,label:"one"},{id:1,label:"one"},{id:2,label:"two"}];
// Create a string representation of your object
function getHash(obj) {
return Object.keys(obj)
.sort() // Keys don't have to be sorted, do it manually here
.map(function(k) {
return k + "_" + obj[k]; // Prefix key name so {a: 1} != {b: 1}
})
.join("_"); // separate key-value-pairs by a _
}
function getHashBetterSolution(obj) {
return obj.id; // Include unique ID in object and use that
};
// When using `getHashBetterSolution`:
// { '1': { id: '1', label: 'one' }, '2': /*etc.*/ }
var uniquesObj = array.reduce(function(res, cur) {
res[getHash(cur)] = cur;
return res;
}, {});
// Convert back to array by looping over all keys
var uniquesArr = Object.keys(uniquesObj).map(function(k) {
return uniquesObj[k];
});
console.log(uniquesArr);
// To show the hashes
console.log(uniquesObj);

You can use Object.keys() and map() to create key for each object and filter to remove duplicates.
var array = [{
id: 1,
label: 'one'
}, {
id: 1,
label: 'one'
}, {
id: 2,
label: 'two'
}];
var result = array.filter(function(e) {
var key = Object.keys(e).map(k => e[k]).join('|');
if (!this[key]) {
this[key] = true;
return true;
}
}, {});
console.log(result)

You could use a hash table and store the found id.
var array = [{ id: 1, label: 'one' }, { id: 1, label: 'one' }, { id: 2, label: 'two' }],
uniqueProducts = array.filter(function(elem) {
return !this[elem.id] && (this[elem.id] = true);
}, Object.create(null));
console.log('uniqueProducts', uniqueProducts);
Check with all properties
var array = [{ id: 1, label: 'one' }, { id: 1, label: 'one' }, { id: 2, label: 'two' }],
keys = Object.keys(array[0]), // get the keys first in a fixed order
uniqueProducts = array.filter(function(a) {
var key = keys.map(function (k) { return a[k]; }).join('|');
return !this[key] && (this[key] = true);
}, Object.create(null));
console.log('uniqueProducts', uniqueProducts);

You can use reduce to extract out the unique array and the unique ids like this:
var array=[{id:1,label:"one"},{id:1,label:"one"},{id:2,label:"two"}];
var result = array.reduce(function(prev, curr) {
if(prev.ids.indexOf(curr.id) === -1) {
prev.array.push(curr);
prev.ids.push(curr.id);
}
return prev;
}, {array: [], ids: []});
console.log(result);
.as-console-wrapper{top:0;max-height:100%!important;}
If you don't know the keys, you can do this - create a unique key that would help you identify duplicates - so I did this:
concat the list of keys and values of the objects
Now sort them for the unique key like 1|id|label|one
This handles situations when the object properties are not in order:
var array=[{id:1,label:"one"},{id:1,label:"one"},{id:2,label:"two"}];
var result = array.reduce(function(prev, curr) {
var tracker = Object.keys(curr).concat(Object.keys(curr).map(key => curr[key])).sort().join('|');
if(!prev.tracker[tracker]) {
prev.array.push(curr);
prev.tracker[tracker] = true;
}
return prev;
}, {array: [], tracker: {}});
console.log(result);
.as-console-wrapper{top:0;max-height:100%!important;}

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/

How can I get a unique array based on object property using underscore

I have an array of objects and I want to get a new array from it that is unique based only on a single property, is there a simple way to achieve this?
Eg.
[ { id: 1, name: 'bob' }, { id: 1, name: 'bill' }, { id: 1, name: 'bill' } ]
Would result in 2 objects with name = bill removed once.

Use the uniq function
var destArray = _.uniq(sourceArray, function(x){
return x.name;
});
or single-line version
var destArray = _.uniq(sourceArray, x => x.name);
From the docs:
Produces a duplicate-free version of the array, using === to test object equality. If you know in advance that the array is sorted, passing true for isSorted will run a much faster algorithm. If you want to compute unique items based on a transformation, pass an iterator function.
In the above example, the function uses the objects name in order to determine uniqueness.

If you prefer to do things yourself without Lodash, and without getting verbose, try this uniq filter with optional uniq by property:
const uniqFilterAccordingToProp = function (prop) {
if (prop)
return (ele, i, arr) => arr.map(ele => ele[prop]).indexOf(ele[prop]) === i
else
return (ele, i, arr) => arr.indexOf(ele) === i
}
Then, use it like this:
const obj = [ { id: 1, name: 'bob' }, { id: 1, name: 'bill' }, { id: 1, name: 'bill' } ]
obj.filter(uniqFilterAccordingToProp('abc'))
Or for plain arrays, just omit the parameter, while remembering to invoke:
[1,1,2].filter(uniqFilterAccordingToProp())

If you want to check all the properties then
lodash 4 comes with _.uniqWith(sourceArray, _.isEqual)

A better and quick approach
var table = [
{
a:1,
b:2
},
{
a:2,
b:3
},
{
a:1,
b:4
}
];
let result = [...new Set(table.map(item => item.a))];
document.write(JSON.stringify(result));
Found here

You can use the _.uniqBy function
var array = [ { id: 1, name: 'bob' }, { id: 2, name: 'bill' }, { id: 1, name: 'bill' },{ id: 2, name: 'bill' } ];
var filteredArray = _.uniqBy(array,function(x){ return x.id && x.name;});
console.log(filteredArray)
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.5/lodash.js"></script>
In the above example, filtering is based on the uniqueness of combination of properties id & name.
if you have multiple properties for an object.
then to find unique array of objects based on specific properties, you could follow this method of combining properties inside _.uniqBy() method.

I was looking for a solution which didn't require a library, and put this together, so I thought I'd add it here. It may not be ideal, or working in all situations, but it's doing what I require, so could potentially help someone else:
const uniqueBy = (items, reducer, dupeCheck = [], currentResults = []) => {
if (!items || items.length === 0) return currentResults;
const thisValue = reducer(items[0]);
const resultsToPass = dupeCheck.indexOf(thisValue) === -1 ?
[...currentResults, items[0]] : currentResults;
return uniqueBy(
items.slice(1),
reducer,
[...dupeCheck, thisValue],
resultsToPass,
);
}
const testData = [
{text: 'hello', image: 'yes'},
{text: 'he'},
{text: 'hello'},
{text: 'hell'},
{text: 'hello'},
{text: 'hellop'},
];
const results = uniqueBy(
testData,
item => {
return item.text
},
)
console.dir(results)

In case you need pure JavaScript solution:
var uniqueProperties = {};
var notUniqueArray = [ { id: 1, name: 'bob' }, { id: 1, name: 'bill' }, { id: 1, name: 'bill' } ];
for(var object in notUniqueArray){
uniqueProperties[notUniqueArray[object]['name']] = notUniqueArray[object]['id'];
}
var uniqiueArray = [];
for(var uniqueName in uniqueProperties){
uniqiueArray.push(
{id:uniqueProperties[uniqueName],name:uniqueName});
}
//uniqiueArray

unique array by id property with ES6:
arr.filter((a, i) => arr.findIndex(b => b.id === a.id) === i); // unique by id
replace b.id === a.id with the relevant comparison for your case