Looping over two arrays in javascript simultaneously

Question

let masterList=[{id:1,name:'Abc'},{id:2,name:'Def'},{id:3,name:'Ghi'}];
let selectedList=[2,3];

The desired result is to have

//desiredList=[{id:2,name:'Def'},{id:3,name:'Ghi'}]

Currently what I am doing is

     let parsedArray = [];
      masterList.forEach(mItem => {
            selectedList.forEach(sItem => {
                if (mItem.id === sItem) {
                    parsedArray.push(mItem);
                }
            });
        });
     desiredList=parsedArray

I do not find this method efficient when iterating over large arrays, is there any logic, any inbuilt javascript operators using which I can achieve the same?

Do you want the code to shorter/simpler or you want to have a single loop instead of nested loops? — Maheer Ali
– Maheer Ali, Commented Jul 8, 2019 at 10:25
@MaheerAli I'd say having a simpler code and having a simple loop is a win win situation. But, I'd primarily would want the code to be faster and effecient. — PCK
– PCK, Commented Jul 8, 2019 at 10:28
50 is not enough elements to worry about optimizing for performance. Unless you have numbers that specifically show otherwise, I'd focus on making the lookup readable and measure. — Davin Tryon
– Davin Tryon, Commented Jul 8, 2019 at 10:42

Nina Scholz · Accepted Answer · 2019-07-08 10:29:31Z

6

You could take a map with id as key and the object as value and map the wanted values from the map by mapping selectedList.

This approach uses the order from selectedList.

var masterList = [{ id: 1, name: 'Abc' }, { id: 2, name: 'Def' }, { id: 3, name: 'Ghi' }],
    selectedList = [2, 3],
    result = selectedList.map(Map.prototype.get, new Map(masterList.map(o => [o.id, o])));

console.log(result);

answered Jul 8, 2019 at 10:29

Nina Scholz

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Comments

Davin Tryon · Accepted Answer · 2019-07-08 10:19:03Z

4

It should be a simple filter on the masterList:

masterList.filter(item => selectedList.includes(item.id));

answered Jul 8, 2019 at 10:19

Davin Tryon

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2 Comments

Maheer Ali Over a year ago

OP said: "I do not find this method efficient when iterating over large arrays". I think by this OP means he wants a better solution regarding time complexity. He don't want short code I guess.

Davin Tryon Over a year ago

Until the OP defines what large is and what kind of timing they want, it is difficult to comment on performance. However, the OP did ask for natvie methods to do what they were doing.

Cuong Le Ngoc · Accepted Answer · 2019-07-08 10:19:26Z

4

You can use Array filter() to do this.

Demo:

let masterList=[{id:1,name:'Abc'},{id:2,name:'Def'},{id:3,name:'Ghi'}];
let selectedList=[2,3];

let desiredList = masterList.filter((val) => selectedList.includes(val.id));
console.log(desiredList)

answered Jul 8, 2019 at 10:19

Cuong Le Ngoc

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Comments

Maheer Ali · Accepted Answer · 2019-07-08 10:19:03Z

3

You can first convert selectedList to Set and then use filter() method array of objects.

You can use Set.prototype.has to check whether the id of the objects exists in the set or not. And this method has O(1) time-complexity. So the time-complexity of the whole algorithm will be linear.

let masterList=[{id:1,name:'Abc'},{id:2,name:'Def'},{id:3,name:'Ghi'}];
let selectedList = [2,3];
let set = new Set(selectedList);

let res = masterList.filter(x => set.has(x.id));
console.log(res)

answered Jul 8, 2019 at 10:19

Maheer Ali

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Comments

LoneWolf · Accepted Answer · 2023-06-09 19:24:57Z

2

Turn the first array into an object indexed by id first, so you can look up the appropriate matching object in O(1) time, and then you can .map the selectedList:

const masterList=[{id:1,name:'Abc'},{id:2,name:'Def'},{id:3,name:'Ghi'}];
const selectedList=[2,3,4];

const masterById = masterList.reduce((a, obj) => {
  a[obj.id] = obj;
  return a;
}, {});

const desiredList = selectedList.map(id => masterById[id]).filter(Boolean);
console.log(desiredList);

edited Jun 9, 2023 at 19:24

LoneWolf

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answered Jul 8, 2019 at 10:18

CertainPerformance

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Comments

Ghoul Ahmed · Accepted Answer · 2019-07-08 10:20:03Z

2

Try this:

let masterList=[{id:1,name:'Abc'},{id:2,name:'Def'},{id:3,name:'Ghi'}];
let selectedList=[2,3];

const result = masterList.filter(({id})=> selectedList.includes(id));

console.log(result);

answered Jul 8, 2019 at 10:20

Ghoul Ahmed

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