Adding an Array Value as a Javascript Object Property

Question

function select(arr, obj) {
  var myKeys = Object.keys(obj);
  var myValues = Object.values(obj);
  var newObj = {};

  for(var i=0; i<myKeys.length; i++) {

    if(arr[i] === myKeys[i]) {
      newObj[myKeys[i]] = myValues[i];   
    }

  }
  
  return newObj;
}

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 }

/*

If keys are present in the given array, but are not in 
the given object, it should ignore them.

It does not modify the passed in object.

*/

I'm having trouble adding an array as an object property. I created a new Object to store the values in, however it only stores the first instance of arr[i]. I'm confused at this point any help?

I'm confused by the question. You say you want an array as a property, but your output does not show any arrays whatsoever. So which property to you want as an array? — Heretic Monkey
– Heretic Monkey, Commented Mar 13, 2021 at 21:28

Jkarttunen · Accepted Answer · 2021-03-13 21:19:18Z

2

Do this instead

for(var i=0; i<arr.length; i++) {
    if(myKeys[arr[i]] !== undefined) {
        newObj[arr[i]] = myValues[i];   
    }
}

Your code works only if the index of matching keys is exactly the same.

answered Mar 13, 2021 at 21:19

Jkarttunen

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1 Comment

Heretic Monkey Over a year ago

Note that you can edit your answers instead of deleting them. You can also undelete answers you've deleted yourself, after editing them.

trincot · Accepted Answer · 2021-03-13 21:38:04Z

1

The problem in your code is that it assumes that the i^th value in the array must correspond to the i^th key of the object, but that order is not guaranteed.

Here is a functional programming style solution, that uses Obect.fromEntries to construct the returned object:

const select = (arr, obj) =>
    Object.fromEntries(arr.filter(key => key in obj).map(key => [key, obj[key]]));

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

answered Mar 13, 2021 at 21:38

trincot

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Comments

Heretic Monkey · Accepted Answer · 2021-03-13 21:35:06Z

I'd use the relatively recently added Object.fromEntries to create an object directly from a map of a filtered set of keys from your object.

function select (arr, obj) {
   // get the keys from obj and filter to those present in arr
   var keys = Object.keys(obj).filter(key => arr.includes(key));
   // create an array of arrays where each inner array has a key and value
   var entries = keys.map(key => [key, obj[key]]);
   // call fromEntries with that new array.
   return Object.fromEntries(entries);
}

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 }

/*

If keys are present in the given array, but are not in 
the given object, it should ignore them.

It does not modify the passed in object.

*/

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