How to push elements into array based on dynamic variable

Question

I have an object and I want to fill an array with the object property and repeat each property a number of times, based on its value. An example:

obj = {
   watches: 3
   rings: 1
}
// => ['watches', 'watches', 'watches', 'rings']

Below is what I have so far. I'm having a hard time figuring how to repeat each property based on the associated value?

function arrayBuilder(obj) {
  let objToArr = [];

  for (let [property, value] of Object.entries(obj)) {
    objToArr.push(property);
  }
  return objToArr;
}

console.log(arrayBuilder({watches: 3, rings: 1}));
// => [ 'watches', 'rings' ]

score 2 · Answer 1 · answered Dec 07 '18 at 22:50

2

You were just missing an inner loop:

function arrayBuilder(obj) {
  let objToArr = [];

  for (let [property, value] of Object.entries(obj)) {
    for(var i=0; i<value; i++){
      objToArr.push(property);
    }
  }
  return objToArr;
}

console.log(arrayBuilder({watches: 3, rings: 1}));

answered Dec 07 '18 at 22:50

Jonathan Wilson

4,138
1
24
36

Ah, gosh. So obvious in hindsight. Thank you so much. – Ralitza Dec 07 '18 at 22:52

Ori Drori · Answer 2 · 2018-12-07T23:00:29.143

2

You can use Array.flatMap() (note the support) with Array.fill():

const obj = {
   watches: 3,
   rings: 1
}

const result = Object.entries(obj).flatMap(([k, v]) => Array(v).fill(k));

console.log(result);

Or Array.reduce() with array spread, if flatMap is not supported:

const obj = {
   watches: 3,
   rings: 1
}

const result = Object.entries(obj)
  .reduce((r, [k, v]) => [...r, ...Array(v).fill(k)], []); // or r.concat(Array(v).fill(k)) instead of the spread

console.log(result);

edited Dec 07 '18 at 23:00

answered Dec 07 '18 at 22:52

Ori Drori

183,571
29
224
209

1

Thanks. I hadn't seen `Array.fill` before. – Jonathan Wilson Dec 07 '18 at 22:52
Is there any reason to not use `r.concat(Array(v).fill(k))` in the second example? – Herohtar Dec 07 '18 at 22:58
1

It's the same in this case, so not really. – Ori Drori Dec 07 '18 at 22:59

score 1 · Answer 3 · answered Dec 07 '18 at 22:50

1

Just add another loop:

 for (let [property, value] of Object.entries(obj)) {
   for(let i = 0; i < value; i++) {
     objToArr.push(property);
   }
 }

answered Dec 07 '18 at 22:50

Jonas Wilms

132,000
20
149
151

score 1 · Answer 4 · answered Dec 07 '18 at 23:06

To achieve expected result, use below option using repeat and split methods

var obj = {
   watches: 3,
   rings: 1
}
let result = []
for(key in obj){
  result.push(...key.concat(" ").repeat(obj[key]).trim().split(" "))
}

console.log(result)

codepen - https://codepen.io/nagasai/pen/ZVzoGB?editors=0010

score 1 · Answer 5 · answered Dec 07 '18 at 23:16

Go through the keys, for each key, create an array of the required length, fill the resulting array with the names of the key, and combine the resulting arrays:

function arrayBuilder(obj) {
  return [].concat.apply(
    [],
    Object
      .entries(obj)
      .map(([key, value]) => new Array(value).fill(key))
  )
}
console.log(arrayBuilder({watches: 3, rings: 1}))

score 1 · Answer 6 · answered Dec 08 '18 at 03:58

You can do this via Array.reduce and Object.keys:

const obj = { watches: 3, rings: 1 }

const result = Object.keys(obj).reduce((acc, key) => 
   [...acc, ...new Array(obj[key]).fill(key)], [])

console.log(result)

The idea is to get the keys from the object and use them to get the length of the array needed for the Array.fill. Using Array.fill you can fill the array with the same values once you have it setup with the correct length.

How to push elements into array based on dynamic variable

6 Answers6

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