Why does console.log wrongly print an empty array?

Question

I use Firefox.

This code logs [].

var log = console.log;

function new_comb(aComb) {
    var res = [];
    log(aComb); // <- This is the line
    for (var p in aComb) {
        var peg = aComb[p];
        var current = peg[peg.length - 1];
        for (var i = 0; i < aComb.length; i++) {
            if (i == p) continue;
            if (current > aComb[i][aComb[i].length - 1]) continue;
            var tmp = aComb.splice(0);
            tmp[i].push(current);
            tmp[p].pop();
            res.push(tmp);
        }
    }
    return res;
}

var comb = [
    [3, 1],
    [9, 2],
    [15, 0]];
var res = new_comb(comb);

This code logs the correct value.

var log = console.log;

function new_comb(aComb) {
    var res = [];
    log(aComb); // <- This is the line
    // note that I comment this out.
    /*for (var p in aComb) {
        var peg = aComb[p];
        var current = peg[peg.length - 1];
        for (var i = 0; i < aComb.length; i++) {
            if (i == p) continue;
            if (current > aComb[i][aComb[i].length - 1]) continue;
            var tmp = aComb.splice(0);
            tmp[i].push(current);
            tmp[p].pop();
            res.push(tmp);
        }
    }*/
    return res;
}

var comb = [
    [3, 1],
    [9, 2],
    [15, 0]];
var res = new_comb(comb);

Why is this happening?

Not your question, but you should prefer something like var log = console.log.bind(console); to keep context — Paul S.
– Paul S., Commented Aug 14, 2013 at 11:46
Or better yet, with console.log or any other host-provided function: function log(msg) { return console.log(msg); } Host-provided functions can be (and are allowed to be) very squirrelly indeed. They're not guaranteed to have bind, and they're not guaranteed to work correctly when called via other references than their normative one. — T.J. Crowder
– T.J. Crowder, Commented Aug 14, 2013 at 11:47
@learner: Anything you don't see in the JavaScript specification. So console's various functions, alert, prompt, confirm, all the DOM functions, ... — T.J. Crowder
– T.J. Crowder, Commented Aug 14, 2013 at 11:54

Quentin · Accepted Answer · 2013-08-14 11:46:12Z

14

console.log shows live data, not a snapshot of the object at the time you run it.

Since you splice all the data out of the array, it is empty almost as soon as you log it.

Stringify or deep copy the array if you want to log a snapshot of it.

answered Aug 14, 2013 at 11:46

Quentin

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2 Comments

insertusernamehere Over a year ago

+1 Better wording and fully explained in 30 seconds less than my answer. :)

batman Over a year ago

I now realize I wanted to use slice and not splice :)

tothemario · Accepted Answer · 2024-03-20 00:59:33Z

0

If you log objects in the latest versions of Chrome and Firefox, what you get logged on the console is a reference to the object, which is not necessarily the 'value' of the object at the moment in time you call console.log(), but it is the value of the object at the moment you open the console.

answered Mar 20, 2024 at 0:59

tothemario

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