Access Values in Array of Objects?

Question

I've seen a lot of different ways to access values in an array of objects. One being arr.forEach(Function) in which the function is simply a for...in

But im curious, how come two for...in functions does not work, for example:

[{ first: "Romeo", last: "Montague" }, { first: "Mercutio", last: null }, { first: "Tybalt", last: "Capulet" }]

(Taken from freeCodeCamp).

This works:

function myFunction (item, index) {

  for( var key in item ) {
    console.log(item[key])
  }
}

arr.forEach(myFunction ( prints all the values fine)

but how come this does not work:

for(key in arr)
{
   for(value in key)
   {
    console.log(key[value];
   }
}

I would think this would work, since we can do arr[key] (and print out the objects in the outer loop, but im not able to access the values this way)

edit:

Sidenote: is it possible for me to print out each sub key/value pair: IE for example on Array index 0 to print out "first: "Romeo" and then "last: "Montague"

for/in should not be used on arrays as they will iterate all properties of the object, including inherited ones. for/in should be exclusively used on objects. — Scott Marcus
– Scott Marcus, Commented Apr 21, 2017 at 18:27
Have a look at this stackoverflow.com/questions/500504/… for more discussion. — Karl Reid
– Karl Reid, Commented Apr 21, 2017 at 18:29

JLRishe · Accepted Answer · 2017-04-21 18:30:45Z

3

but how come this does not work:

Because key in that case would be a string, not an element of the array.

You shouldn't use for..in to iterate over arrays: MDN.

Newer versions of JavaScript (ECMAScript) provide the ability to use for..of loops, which are perfect for iterating over arrays:

for (var item of arr) {
    console.log(item);
}

answered Apr 21, 2017 at 18:30

JLRishe

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3 Comments

msmith1114 Over a year ago

Ah, well...is there a way for me to grab a subsection object in the array, for example if I wanted to print out the first key/value pair in the arr index 1: Like printing out: first: "Romeo" and then last: "Montague"

Karl Reid Over a year ago

You could just use a regular for(var i=0; i < arr.length; i++) and then if(i > limit) break. Or use the for..of loop and keep a separate iteration count variable.

JLRishe Over a year ago

@msmith1114 Yeah, just get the item at index 1 and print its values: var idx1 = arr[1]; if (idx1) { for (var key in idx1) { console.log(key, ':', idx1[key]); } }

williamolojede · Accepted Answer · 2017-04-21 18:31:05Z

1

Its not working because key in the second for ... in is dynamic, same reason you can't do item.key in the first example

answered Apr 21, 2017 at 18:31

williamolojede

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Comments

Nina Scholz · Accepted Answer · 2017-04-21 18:31:37Z

0

You need to loop over the indices of the array and then over the keys of an item of the array.

var array = [{ first: "Romeo", last: "Montague" }, { first: "Mercutio", last: null }, { first: "Tybalt", last: "Capulet" }],
    key, index;

for (index = 0; index < array.length; index++) {
   for(key in array[index]) {
      console.log(array[index][key]);
   }
}

answered Apr 21, 2017 at 18:31

Nina Scholz

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