adding arrays to a list of arrays is blank

Question

So I'm writing a method to find recursively all of the permutations of a list of numbers.

I'm adding the result of each run, which is an number array, to a larger combos array. However, combos is becoming filled with empty arrays like this: [[],[],[]] instead of [[1,2,3],[3,2,1],[2,3,1] ...] but I know the combinations are being generated

Here is what I have:

var combos = [];
permuteHelper([1,2,3],[]);
console.log(combos); //printing blank array or arrays [[],[]]
function permuteHelper(list, combination){

    if(list.length == 0){
        combos.push(combination); //ERROR: combos only holds blank arrays (e.g. [[],[]] )
        console.log(combination); //although this is printing correct list after each recursive run (e.g. [3,2,1] )
    }
    for(var i = 0; i < list.length; i++ ){

        //pick a digit
        var digit = list[i];
        list.splice(i,1); //remove the character from the array
        combination.push(digit);

        //recursively keep picking
        permuteHelper(list, combination);

        //backtrack and put the digit back for next time
        list.splice(i,0,digit);
        combination.pop();
    }

}

I've tried making combos non-global and updated the function header

function permuteHelper(list, combination,combos)

but combos is still not populated correctly. I'm new to js, not sure what I'm missing.

Answer 1

When you get combination as a parameter to your function, it's treated as a reference. Any change to combination will change the original variable. You hand over that reference to each new call of permuteHelper and therefore always modify the original array.

A primitive solution with few code changes would be:

var combos = [];
permuteHelper([1, 2, 3], [], []);

function permuteHelper(list, _combination) {
    var combination = Object.assign([], _combination);
    if (list.length == 0) {
        combos.push(combination); //this only has a blank array of 
        arrays(e.g. [[], []])
        console.log(combination); //prints correct permuted list after each recursive trial (e.g. [3,2,1] )
    }
    for (var i = 0; i < list.length; i++) {

        //pick a digit
        var digit = list[i];
        list.splice(i, 1); //remove the character from the array
        combination.push(digit);

        //recursively keep picking
        permuteHelper(list, combination, combos);

        //backtrack and put the digit back for next time
        list.splice(i, 0, digit);
        combination.pop();
    }
}

Like that, you create a new object combination from the parameter _combination that can be modified as you please.