gistfile1.txt

minified: 
var n=9;for(var i=1;i<n*2;i++){for(var s=n-(Math.abs(i-n)),k=s-1;k>0;k--){s=[k,s,k].join("");}console.log(new Array(Math.abs(i-n)+2).join(" ")+s);}


146 characters

Clean:
var num = 9;
for(var i = 1; i < num*2; i++ ) {
    var seq = num-(Math.abs(i-num));

    for(var k = seq-1; k > 0; k--) {
        seq = [k,seq,k].join('');
    }
    var space = new Array(Math.abs(i-num)+1).join(" ");
    console.log(space+seq+"\n");
}
/*
        1
       121
      12321
     1234321
    123454321
   12345654321
  1234567654321
 123456787654321
12345678987654321
 123456787654321
  1234567654321
   12345654321
    123454321
     1234321
      12321
       121
        1
*/

-----------------------------------------------------------------------
minified: 
var n=9,s=n;for(var k=n-1;k>0;k--){s=[k,s,k].join('');}var out=[s];for(var i=n;i>1;i--){s=s.split(''+i+(i-1)).join('');out.unshift([new Array(Math.floor((out[Math.floor(out.length/2)].length-s.length))/2+1).join(' ')+s]);out.push(out[0]);}console.log(out.join('\n'));


var n=9,s=n;                                                                                   
for(var k=n-1;k>0;k--){                                                                        
    s=[k,s,k].join('');                                                                        
}                                                                                              
var out=[s];                                                                                   
for(var i=n;i>1;i--){                                                                          
    s=s.split(''+i+(i-1)).join('');                                                            
    out.unshift([new Array(Math.floor((out[Math.floor(out.length/2)].length-s.length))/2+1).join(' ')+s]);
    out.push(out[0]);                                                                          
}                                                                                              
console.log(out.join('\n'));                                                                   

fewer loop iterations and one output statement, slightly longer code