console.log(array) shows different array contents than iterating the array and displaying the individual elements

Question

I have the following code:

console.log("start");
for(var i = 0; i < array.length; i++){
    console.log(i + " = " + array[i]);
}
console.log(array);
console.log("end");

This gives me the following output:

[16:34:41.171] start
[16:34:41.171] 0 = 0
[16:34:41.172] 1 = 168
[16:34:41.172] 2 = 171
[16:34:41.172] [0, 168, 171, 139]
[16:34:41.172] end

That is, it doesn't show the 139 element when iterating the array, but console.log does print it when outputting the whole array. WHY? (<-- the question)

I do modify the array later on, is the console.log somehow delayed until after I changed the array? Note tho that change the order of the statements, and putting console.log(array) directly at the start does not change the outcome (still different outputs).

I am using firefox 20.0

How/where is your array defined? I'm not seeing what you are: jsfiddle.net/j08691/vJd6x — j08691
– j08691, Commented May 10, 2013 at 14:43
possible duplicate of JavaScript: console.log() gives different results than alert() — Rob W
– Rob W, Commented May 10, 2013 at 14:46
Is the array an object property? stackoverflow.com/questions/8249136/… — Alex K.
– Alex K., Commented May 10, 2013 at 14:47
Rob W: that answered my question thanks. I had assumed that console.log was instant and blocking, turns out it isn't :) — user1302914
– user1302914, Commented May 10, 2013 at 15:29
It seems I cannot accept the duplicate as a correct answer (which was first). So I have accepted Daniel's answer. Thanks all — user1302914
– user1302914, Commented May 10, 2013 at 22:16

Daniel Aranda · Accepted Answer · 2016-03-05 16:15:45Z

16

Update: If you want to see this behavior, copy and paste the code in the console and execute. Then close developer tools and open again, apparently the pointer thing only happens when the code is executed in the background(which happens when you reopen the console).

Console.log output of objects, is a pointer, no a real value. This means that if the object changes later, console.log object will be updated. Try:

console.log("start");
var array = [1];
for(var i = 0; i < array.length; i++){
    console.log(i + " = " + array[i]);
}
console.log(array);
console.log("end");
array.push(9999);// you will see the 9999 in the console no matter it was added after the output.

To prevent pointer issues try this: console.log(array.join()); because later in some point of your application you are adding the 139 value.

edited Mar 5, 2016 at 16:15

answered May 10, 2013 at 14:53

Daniel Aranda

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