Solució Triangle bonic

var c = ' ';
result="";
for (i=1; i<=10; i++){
     // escriure el caracters buits
	for(j=1; j<=10-i; j++){	
		result = result + c;
        }
     
     // escriure la seqüència creixent
  	var n = i;
  	for(j=1; j<=i;j++){
   		if(n == 10) n = 0;
		result = result + n;
		n++;
	}
     // escriure la seqüència decreixent
	n = n-2;
	for (j=1; j<=i-1; j++){
		if(n == -1) n =9;
		result = result + n;
		n--;
	}
	result = result + "<br>";
}
document.write(result);

Una altre solució sense bucles:

var alfa = ["","1","2","3","4","5","6","7","8","9","0"];
console.log(alfa.reduce(function(data, item, index){
    
    var b = alfa.reduce(function(linea, item2, index2){
        //espais en blanc
        if (index2 > index) 
            linea["blancs"] = linea["blancs"] + " ";
        //cadena ascendent
        if((index > (index2 - index)) & index <= index2  )
            linea["asc"] = linea["asc"] + item2;
        //cadena descenc¡dent
        if((index > (index2 - index)+1) & index <= index2  )
            linea["desc"] =  item2 + linea["desc"];
        
        //part del mig
        var n = alfa.length / 2;
        if (index > n)
        {
            //cadena del mig ascendent
            if(index2 <= index - (alfa.length - index)){
                linea["medasc"] = linea["medasc"] + item2;
            }
            //cadena del mig descendent
            if(index2 < index - (alfa.length - index)){
                linea["meddesc"] = item2 + linea["meddesc"];
            }
        }
        return linea;
        },{blancs: "", asc:"", desc:"", medasc:"", meddesc:""});
        
    data.push(b["blancs"] + b["asc"] + b["medasc"] + b["meddesc"] + b["desc"]+"\n");
    return data; 
},[]));