function subset(nums) {
    var result = [];
    dfs(0, [], nums, result);
    console.log(result);
}

function dfs(index, path, nums, res) {
    if(path.length === 2) res.push([...path])
    for (var i = index; i < nums.length; i += 1){
        path.push(nums[i]);
        dfs(i + 1, path, nums, res)
        path.pop();
    }
}

subset([1,2,3,4]);