Javascript convert array of objects to an array with only specific keys with ES6

Question

I have the following array

const array = [{name:'one', value:1},{name:'two'},{name:'three', value:3}]

What i want is to get

const finalarr = [1,3]

That is get an array of values with key value

So i have tried

const finalarr = arr.map(obj=>obj.value)

The above returns [1, undefined, 3];

I have also tried

const finalarr = arr.map(obj => {
       if (obj.value) {
         return obj.value;
       }
    })

but i still get [1, undefined,3]

How can i adjust this to only return the values of objects with a key value to eliminate the undefined key?

Ele · Accepted Answer · 2019-04-26 02:28:53Z

1

Use the function reduce because the function map returns an array with the same length of the source array.

const finalarr = arr.reduce((a, {value}) => {
    if (value) a.push(value);
    return a;
}, []);

Important: your approach is skipping the objects with values equal to zero 0.

Example

const arr = [{name:'one', value:1},{name:'two'},{name:'three', value:3}];
const finalarr = arr.reduce((a, {value}) => {
    if (value) a.push(value);
    return a;
}, []);
console.log(finalarr);

.as-console-wrapper { max-height: 100% !important; top: 0; }

edited Apr 26, 2019 at 2:28

answered Apr 26, 2019 at 2:18

Ele

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Comments

yqlim · Accepted Answer · 2019-04-26 02:22:16Z

1

The Array .map method will return another array with the exact same size of the original array.

If you want a result with different size, you cannot do it with pure .map only.

What you can do is to first .filter out objects without .value property:

const array = [{name:'one', value:1},{name:'two'},{name:'three', value:3}]

const result = array
        .filter(obj => typeof obj.value !== 'undefined')
        .map(obj => obj.value);
        
console.log(result);

Otherwise, you can also use .reduce:

const array = [{name:'one', value:1},{name:'two'},{name:'three', value:3}]

const result = array.reduce((arr, obj) => {

  if (typeof obj.value !== 'undefined')
    arr.push(obj.value);
    
  return arr;
  
}, []);
        
console.log(result);

edited Apr 26, 2019 at 2:22

answered Apr 26, 2019 at 2:17

yqlim

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Comments

Code Maniac · Accepted Answer · 2019-04-26 02:30:32Z

1

You can use reduce

const array = [{name:'one;', value:1},{name:'two'},{name:'three', value:3}]

let final = array.reduce((op,{value})=>{
  if(typeof value !== 'undefined'){
    op.push(value)
  }
  return op
},[])


console.log(final)

map used to map complete array back with changed values it doesn't skip value you can use map and filter for you purpose

const arr = [{name:'one', value:1},{name:'two'},{name:'three', value:3}]

const finalarr = arr.filter(({value})=>typeof value !== 'undefined').map(({value})=> value)


console.log(finalarr)

edited Apr 26, 2019 at 2:30

answered Apr 26, 2019 at 2:15

Code Maniac

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