[0, n)の数字からm個の値を抽出

combination.js

function combination(n, m) {
    var arr = [];
    var num;
    var i = 0;
    while(i < m) {
        num = Math.floor(Math.random() * n);
        if(~arr.indexOf(num)) continue;
        arr.push(num);
        i++;
    }

    return arr;
}