Get array of keys based on values from another array

Question

Say I have an array of objects that looks like this

let myArray = [
  {item1: true},
  {item2: false},
  {item3: true},
  {item4: false}
]

How would I iterate though this to return a new array of true values that looks like this:

let newArray = ['item1', 'item3']

I found this function but it only returns single items:

function findKey(map, term) {
  var found = [];
  for(var property in map) {
    if(map.hasOwnProperty(property)) {
      for(var key in map[property]) {
        if(map[property].hasOwnProperty(key) && key === term) {
          found.push(property);
        }
      }
    }
  }
  return found;
}

junvar · Accepted Answer · 2018-11-08 23:12:45Z

2

Assuming myArray always contains objects with only 1 property.

let newArray = myArray
    .map(item => Object.entries(item)[0])
    .filter(([key, value]) => value)
    .map(([key, value]) => key)

answered Nov 8, 2018 at 23:12

junvar

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Comments

Dacre Denny · Accepted Answer · 2018-11-08 23:00:37Z

1

You could access the first key of each array item via Object.keys(), and use this to filter items with a true value for that first key, and then complete the process with a call to map() to transform the item to a value based on the same "first key" technique:

let myArray = [
  {item1: true},
  {item2: false},
  {item3: true},
  {item4: false}
]

let result = myArray
.filter(item => item[ Object.keys(item)[0] ] === true)
.map(item => Object.keys(item)[0])

console.log(result)

answered Nov 8, 2018 at 23:00

Dacre Denny

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6 Comments

cup_of Over a year ago

wait, is this only going to work id the keys are in the format item + i? those are just placeholder keys

Dacre Denny Over a year ago

@cup_of you're welcome! Please consider accepting this answer if it has been helpful :-)

Dacre Denny Over a year ago

@cup_of this will work by selecting the first key of each item - does that suit your requirements?

cup_of Over a year ago

yup this will work. thanks man. ill accept it when it lets me

Dacre Denny Over a year ago

@cup_of no problems :-)

|

Ele · Accepted Answer · 2018-11-08 23:07:04Z

1

Use the function reduce to build the desired output. The handler of the function reduce will get the keys and check for each value === true.

This approach checks for the whole set of keys within an object. Further, this way you only use one loop.

let myArray = [{item1: true},{item2: false},{item3: true},{item4: false}],
    result = myArray.reduce((a, c) => a.concat(Object.keys(c).filter(k => c[k] === true)), []);
    
console.log(result);

answered Nov 8, 2018 at 23:07

Ele

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Comments

V. Sambor · Accepted Answer · 2018-11-08 23:21:18Z

1

Something much optimized than the accepted answer would look like this:

const arr = [
  { item1: true },
  { item2: false },
  { item3: true },
  { item4: false }
]

const result = [];
const len = arr.length;

for (let i = 0; i < len; ++i) {
  const obj = arr[i];
  const key = Object.keys(obj)[0];

  if(obj[key]) {
    result.push(key);
  }
}

console.log(result);

There is only one loop over the array, instead of map and filter which ends up looping twice.

answered Nov 8, 2018 at 23:21

V. Sambor

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Comments

Kamil Kiełczewski · Accepted Answer · 2018-11-09 18:09:41Z

1

Shortest

let newArray = myArray.map( x=>Object.keys(x)[0] ).filter( (k,i)=>myArray[i][k] );

In above solution first we use: map which works as for-loop to get array of keys (using Object.keys) ["item1", "item2", "item3", "item4"]. Then we filter that array by choose only those keys for which original array object has true. e.g myArray[0]["item1"] -> true (we use fact that filter funtion takes array element (k) and its index (i) which is the same for elements in myArray). In map and filter we use arrow functions.

edited Nov 9, 2018 at 18:09

answered Nov 8, 2018 at 23:09

Kamil Kiełczewski

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3 Comments

Roy Scheffers Over a year ago

While this might answer the authors' question, it lacks some explaining words and/or links to documentation. Raw code snippets are not very helpful without some phrases around them. You may also find how to write a good answer very helpful. Please edit your answer.

Kamil Kiełczewski Over a year ago

@RoyScheffers This is very short code which use standard js functions (ES6) however I can explain it - I update my answer

Roy Scheffers Over a year ago

Hi Kamil. I know it's a short example and to those who are familiar with .map() .filter() and arrow functions will be able to read it and understand it, however, not everyone is at that level. By the clarification you provided, you've added value that will help newcomers to the language.

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