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?

Answer 1

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.