Why does assigning a value to an array in a for loop result in the array having the same value for each iteration when passed to console.log()?

Question

Given the following code:

var tmp = [0];
for(var i=0;i<100;i++) {
  tmp[0] = i;
  console.log(tmp);
}

I'd expect output of [0], [1], [2], [3], etc

But I instead get [99], [99], [99], [99], etc

Stepping through the code in a debugger (firebug) however nets me the correct result of [0], [1], [2].

Answer 1

console.log(tmp[0])

When you put console.log(tmp) you are logging the entire array object. Firebug only creates a link to the object and then when you "look" at the object in firebug you are looking at its current state (after the for loop has completed).

Answer 2

That is interesting. Even without a for loop:

var tmp = [], i = 0;
tmp[0] = i;
console.log(tmp);
i++;
tmp[0] = i;
console.log(tmp);

produces

[1]
[1]

...as well. I was not aware that console.log behaved that way. Thanks to @Joshua for a good explanation.