accessing an array within an object javascript

Question

I have the following object:

myObject: {

 myArray1: [1,2,3],
 myArray2: [4,5,6],
 myArray3: [7,8,9]
}

This is an object that keeps growing in Arrays(dynamic array?). So I need to figure out a method to access it. I came across using a for( var key in myObject) with something like this:

    for (var key in myObject) {
     var obj = myObject[key];
       for (var prop in obj) {
           //thinking that this will print the first value of the array
       console.log(prop[0]);
     }
    }

but it doesn't work it prints undefined. I know using a for in is not the way to access an object correctly. I'm wondering if anyone could suggest a method to access the values of this object through a loop.

Thanks!

possible duplicate of Access / process (nested) objects, arrays or JSON — Felix Kling
– Felix Kling, Commented Apr 16, 2014 at 3:36
Why do you think that for...in works differently for objects than it works for arrays? — Felix Kling
– Felix Kling, Commented Apr 16, 2014 at 3:36
@FelixKling This is very interesting. I thought for some reason it wasn't recommended. I don't remember where I read it. — mauricioSanchez
– mauricioSanchez, Commented Apr 16, 2014 at 3:41
Well, yeah, you shouldn't use for..in for arrays, but that doesn't mean that it works differently for arrays than it does for objects. — Felix Kling
– Felix Kling, Commented Apr 16, 2014 at 3:55
@FeliKling It makes sense now for me. Your link is very helpful in how to use for in when the data is unknown — mauricioSanchez
– mauricioSanchez, Commented Apr 16, 2014 at 4:04

thefourtheye · Accepted Answer · 2014-04-16 03:23:19Z

1

Iterating an object with for..in is okay, but not an array. Because when you sue for..in with an array, it will not get the array values, but the array indices. So, you should be doing something like this

for (var key in myObject) {
    var currentArray = myObject[key];
    for(var i = 0; i < currentArray.length; i += 1) {
        console.log(currentArray[i]);
    }
}

answered Apr 16, 2014 at 3:23

thefourtheye

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5 Comments

mauricioSanchez Over a year ago

This works. And I guess this is the right way to do it??

thefourtheye Over a year ago

@MauricioSanchez Indeed, this is how arrays have to iterated (or Array.forEach). NEVER use for..in with Arrays.

mauricioSanchez Over a year ago

Although the method that vivek_nk suggested, it worked as well. I guess it is a matter of better coding practices??

thefourtheye Over a year ago

@MauricioSanchez Well, if you really know what you are doing, that is fine. But don't use for..in with arrays.

thefourtheye Over a year ago

@MauricioSanchez Also read this section from the MDN Docs. It says why you should not use that method.

vivek_nk · Accepted Answer · 2014-04-16 03:24:59Z

1

You made a mistake in 2nd loop. obj is the array and prop is the index

for (var key in myObject) {
  var obj = myObject[key];
   for (var prop in obj) {
       //this will print the first value of the array
   console.log(obj[prop]); //obj is the array and prop is the index
 }
}

answered Apr 16, 2014 at 3:24

vivek_nk

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Comments

juvian · Accepted Answer · 2014-04-16 03:25:21Z

1

prop is the index of the array, its not the array. obj is the array. So it should be:

console.log(obj[prop]);

answered Apr 16, 2014 at 3:25

juvian

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