var n=10000;
var a = [2,3,4,100,6,...];
var b = new Array(n);
for (var i = 0; i < a.length; i++) {//改了这里
    b[a[i] - 1] = a[i];
}
for (var k = 0; k < n; k++) {
    if(!b[k]){
        alert(k+1);
    }
}