Comparing Array Values With Object Keys

Question

Write a function called "select".

Given an array and an object, "select" returns a new object whose properties are those in the given object AND whose keys are present in the given array.

var arr = ['a', 'c', 'e'];
var obj = {
  a: 1,
  b: 2,
  c: 3,
  d: 4
};
var output = select(arr, obj);
console.log(output); // --> { a: 1, c: 3 }

My Solution:

function select(arr, obj) {
  for (var k in obj) {
    return arr.reduce((o, c) => {
      if (obj.hasOwnProperty(c)) {
        o[c] = obj[c]
      }
      return o
    }, {})
  }
}

var array = ['a', 'c', 'e'];

var object = {
  a: 1,
  b: 2,
  c: 3,
  d: 4
};

console.log(select(array, object));

My solution is working, but I have a feeling that I'm not using the best practices or most eloquent code. For example I'm using a for/in to search the object, but I never use the 'k' from for (var k in obj). Any tips would be appreciated.

Yes, just omit the for (var k in obj) {. It has absolutely no purpose in your code. Can you explain what you meant it to do? — Bergi
– Bergi, Commented Aug 29, 2017 at 12:25
@jhazelton1 you don't need to iterate over obj as you're using arr.reduce to do that. — evolutionxbox
– evolutionxbox, Commented Aug 29, 2017 at 12:26
And you are returning on the first iteration, so there won't be a lookup over all properties of the object either way — baao
– baao, Commented Aug 29, 2017 at 12:30

Nenad Vracar · Accepted Answer · 2017-08-29 12:35:10Z

1

You can use Object.assign() with reduce() and inside check if property exists in object.

var arr = ['a', 'c', 'e'];
var obj = { a: 1,b: 2,c: 3, d: 4};

let select = (arr, obj) => arr.reduce((r, e) => Object.assign(r, obj[e] ? {[e]: obj[e]} : null), {})

var output = select(arr, obj);
console.log(output); // --> { a: 1, c: 3 }

edited Aug 29, 2017 at 12:35

answered Aug 29, 2017 at 12:32

Nenad Vracar

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1 Comment

Bergi Over a year ago

Don't pass a string to Object.assign. Pass null.

Bergi · Accepted Answer · 2020-09-02 22:54:19Z

1

To get the intersection between the keys of the objects and the keys in the array, there are two approaches:

enumerate the object property keys and check which of them are also in the array
iterate the keys in the array and check which of them are also an object property

You don't have to do both loops. The first approach will be more efficient for small objects, the second one works well for large objects and small subsets as well.

function select(arr, obj) {
  let o = {};
  for (let k in obj)
    if (arr.includes(k))
      o[k] = obj[k];
  return o;
}

function select(arr, obj) {
  let o = {};
  for (let k of arr)
    if (k in obj)
      o[k] = obj[k];
  return o;
}

You can also use reduce instead of the for … of loop like you successfully did (I won't repeat that solution), but which of the two is easier to read and understand only you can decide.

edited Sep 2, 2020 at 22:54

answered Aug 29, 2017 at 12:30

Bergi

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5 Comments

LaravelSucks Over a year ago

I think you are missing a curly brace.

Bergi Over a year ago

@LaravelSucks Rather, having one too much :-) Thanks, fixed!

London804 Over a year ago

This only matches the first key and then it returns.

Bergi Over a year ago

@London804 No it doesn't? The return statements are after the respective loop

London804 Over a year ago

@Bergi I'm sorry you're right. It looks like my console.log was in the loop. I will upvote as soon as it lets me.

Ori Drori · Accepted Answer · 2017-08-29 12:30:23Z

0

Iterate the array with Array#reduce and pluck all properties that exist on the object to a new object:

const arr = ['a', 'c', 'e'];
const obj = {
  a: 1,
  b: 2,
  c: 3,
  d: 4
};

const select = (arr, obj) => arr.reduce((r, prop) => {
    obj.hasOwnProperty(prop) && (r[prop] = obj[prop]);
    
    return r;
  }, {});

const output = select(arr, obj);
console.log(output); // --> { a: 1, c: 3 }

answered Aug 29, 2017 at 12:30

Ori Drori

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