Why does console.log prints array even when the array is updated after the console statements?

Question

I ran the following JS code. Console prints an empty array.

function xyz(){
var a=[]; var b=[];
console.log(a);
for(i=0;i<10;i++){
b.push(i);
}
a=b;
}

but when I try to run this

function xyz(){
var a=[];
console.log(a);
for(i=0;i<10;i++){
a.push(i);
}
}

the console prints the array from 0 to 9. Also, I see here that even though the console statement is above the for loop where we are pushing the values in the array, but still the console is printing but not in the first case (which i think is because the reference which console is printing is empty).Code was tested on chrome browser console.

I cannot see what you are saying... both print an emtpy array — A. Llorente
– A. Llorente, Commented Jun 26, 2018 at 7:01
@AnkitAgarwal Copy paste the code and try running in your chrome browser console.Even I am getting empty array in Stack overflow . — user3847870
– user3847870, Commented Jun 26, 2018 at 7:04

dhaker · Accepted Answer · 2018-06-26 07:22:42Z

2

function xyz(){
   var a=[];
   console.log(a);
   for(var i=0;i<10;i++){
     a.push(i);
   }
}
xyz();

When we see it in chrome console it has a empty array but when we expand it , it is showing array values with length property as well.

You need to pay a close attention there, there is an i (info) which says, it is evaluated just now.

This means, chrome console gets a live reference to the array and showing blank array with length property equal to 0 on console.log() but also adding live reference to that array, so on expanding we see array

You need to use console.log() after array is updated in your code to work properly.

  function xyz(){
      var a=[];
       
      for(i=0;i<10;i++){
         a.push(i); //array is updating here
      }
     console.log(a);
    }
    xyz();

edited Jun 26, 2018 at 7:22

answered Jun 26, 2018 at 7:03

dhaker

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4 Comments

user3847870 Over a year ago

yeah I know that. But try to run the given code in the crude form, as I have written, in your local browser console.

dhaker Over a year ago

@user3847870 added explanation for chrome console as well, see if it answers your question !

user3847870 Over a year ago

It will show evaluated just now in other case also.Please try running both scenarios.

dhaker Over a year ago

@user3847870 Yes, true for other case as well. My point was that it is attaching live reference, it is returning empty array but on expanding it, it gives current array status (elements, length etc.). I think this attaching live reference is true for within its scope

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