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Asked: 2026-05-18T07:36:04+00:00

I was writing a javascript program and running it in Chrome 7, when I

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I was writing a javascript program and running it in Chrome 7, when I encountered some strange behavior. Now, in my code, with all the other stuff going on, it took me some time to figure out it wasn’t me.

I have distilled the essence of the code below. 

<html>

<script>

var data = [1,2,3,4,5];

var data_copy = [];

for (var i=0; i<data.length; i++){
    data_copy.push(data[i]);
}

console.log("Printing before:");
console.log(data_copy);

//alert(data_copy);

console.log("------------------------");

for (var i=0; i<data_copy.length; i++){
    data_copy[i] = data_copy[i] * 1000;
}

console.log("Printing after:");
console.log(data_copy);

</script>

</html>

When I run this on Chrome 7, I get the output that follows in the Javascript console:

Printing before:
[1000, 2000, 3000, 4000, 5000]
------------------------
Printing after:
[1000, 2000, 3000, 4000, 5000]

How come the first call to console.log prints the changed version of data_copy?

Now, if I uncomment the “alert” and run the same code, I get what you would normally expect:

Printing before:
[1, 2, 3, 4, 5]
------------------------
Printing after:
[1000, 2000, 3000, 4000, 5000]

I also tried the code in node.js and I get the second (normal) output.

Any ideas?

Is this some JIT optimization gone awry?

Or am I missing something obvious?

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1 Answer

  1. Editorial Team

    Change console.log(data_copy) to console.log(String(data_copy)).

    console.log effectively sends the object by reference to Chrome’s Console. alert interrupts your script so the first logged data_copy gets rendered before the later modification; without, the entire script runs to completion before the console renders either data_copy reference.

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