map function and its return value

Question

For deeper understanding purpose i used val.key instead of val[ ] at function extractKey which i thought would give me [ name, name, name, name ] as result but instead i get [undefined, undefined, undefined, undefined] Why is this ? :

function extractKey(arr, key){



return arr.map(function(val){

    return val.key;

})



}    
extractKey([{name: 'Elie'}, {name: 'Tim'}, {name: 'Matt'}, {name: 
'Colt'}], 'name') // ['Elie', 'Tim', 'Matt', 'Colt']





(4) [undefined, undefined, undefined, undefined]

For deeper understanding purpose know that `val.key` and `val[key]` do different things. https://stackoverflow.com/questions/4244896/dynamically-access-object-property-using-variable — Thilo, Feb 24 '19 at 05:01

score 1 · Accepted Answer · answered Feb 24 '19 at 05:04

Use (square brackets) array notation instead of (dot) object notation.

function extractKey(arr, key) {
  return arr.map(function(val) {

    return val[key];

  })
}

const names = extractKey([{
  name: 'Elie'
}, {
  name: 'Tim'
}, {
  name: 'Matt'
}, {
  name: 'Colt'
}], 'name'); // ['Elie', 'Tim', 'Matt', 'Colt']

console.log(names);

map function and its return value

1 Answers1