m-Sum.js

var mSum = (m, n, a) => 
    a.length ? m > 2 ? mSum(m, n, a.slice(0, a.length - 1)).concat(
        mSum(m - 1 , n - a[a.length - 1], a.slice(0, a.length - 1)).map(e => e.concat([a[a.length - 1]]))) :
        twoSum(n, a) :
        [];
var twoSum = (n, a) => {
    a.sort((x, y) => x - y);
    var r = [];
    if(a.length < 2) return r;
    
    for(var i = 0, j = a.length - 1; i < j;) {
        if(a[i] + a[j] == n)
            r.push([a[i], a[j]]), 
            i++;
        if(a[i] + a[j] > n )
            j--;
        if(a[i] + a[j] < n )
            i++;
    }
    return r;
}
mSum(3, 10, [1, 2, 3, 4, 5, 6, 7, 8])