Eight Queen Problem

eight_queen.js

let answerIndex=0;

let board=[];
for(let i=0;i<64;i++)board[i]=false;
findNextState(board,1,-1);

function findNextState(board,order,last){
	//console.log("find Next State for board with order ",order," since index ",last);
	//printBoard(board);
	for(let i=last+1;i<64;i++){
		if(i%8==0 && i-last>8){
			//console.log("No hope, stop here, i as "+i);
			return;
		}
		if(checkCellSafe(board,i)){
			//console.log("Found a cell safe as ",i);
		
			let b1=JSON.parse(JSON.stringify(board));
			b1[i]=true;
			findNextState(b1,order+1,i);
		}
	}
	if(order>8){
		answerIndex++;
		console.log("Finally order as "+order+" (Answer No."+answerIndex+")");
		printBoard(board);
		return;
	}
}
function checkCellSafe(board,index){
/*
same row: index/8==i/8
same column: index%8==i%8
same \: abs(x-i)%9==0
same /: abs(x-i)%7==0

 0  1  2  3  4  5  6  7
 8  9 10 11 12 13 14 15
16 17 18 19 20 21 22 23
24 25 26 27 28 29 30 31
32 33 34 35 36 37 38 39

however, for 22, 22+7x would be normal, but 22-7x would be strange.
see,
Not	 8 --- %8=0 	
	15 --- %8=7 above, mod should not smaller then 6
	22 --- %8=6 ------
	29 --- %8=5 below, mod should not greater than 6

and for 25, 
	 7 --- %8=7
	16 --- %8=0 above, mod should not greater than 1
	25 --- %8=1 ----
	34 --- %8=2 below, mod should not smaller than 1

*/
	let note='';
	let result=true;
	if(board[index]){
		note='X Q';
		result=false;
	}
	else for(let i=0;i<64;i++){
		if(!board[i])continue;
		if(Math.floor(index/8)%8==Math.floor(i/8)%8){
			note='X Row '+i;
			result=false;
			break;
		}
		if(Math.floor(index%8)==Math.floor(i%8)){
			note='X Col '+i;
			result=false;
			break;
		}
		if(Math.abs(index-i)%9==0){
			if(
				(i<index && (i%8)<(index%8))
			||	(i>index && (i%8)>(index%8))
			){
				note='X \\ '+i;
				result=false;
				break;
			}
		}
		if(Math.abs(index-i)%7==0){
			if(
				(i<index && (i%8)>(index%8))
			||	(i>index && (i%8)<(index%8))
			){
				note='X // '+i;
				result=false;
				break;
			}
		}
	}
	//console.log('check cell safe',index,' of board --> '+(result?'SAFE':'FATAL')+ " reason: "+note);
	return result;
}
function printBoard(board){
/*
	console.log("--------");
	for(let i=0;i<8;i++){
		let s='';
		for(let j=0;j<8;j++)s+=(board[i*8+j]?'Q':'X');
		console.log(s);
	}
	console.log("========");
*/
	let s='';
	for (let i=0;i<64;i++)if(board[i])s+=" "+i;
	console.log("# "+s);
}