JavaScript Get array of keys based on values from another array | JS

Hello 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;
}

Thanks!

Answer:1

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)
Answer:2

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);
Answer:3

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.

Answer:4

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