TypeError: Cannot read property '#' of undefined In simple JavaScript Game -

i'm making small battleship game friend of mine. thought practice learned. have 2 dimensional array 10x10 filled zeros. change zeros ones in places our ships go. made function place 1x5 ship down. function prompts user 2 coordinates , assigns variables outside of function. gets coordinates of start point , asks direction player wants rest wrest of ship go. if there no obstacles ship should placed. if there function should restart. thing error. # replaced number in prompt asking vertical coordinate position.

here error. typeerror: cannot read property '#' of undefined in battleship game

here function

function firstship(){     window.alert("we placing 1 5 length ship.  take note playing on 10 10 board.");     userx = parseint(prompt("horizontal coordinate position first unit of ship")-1);     usery = parseint(prompt("vertical coordinate position first unit of ship")-1);     direction = prompt("now choose direction want rest of ship face.  may   use words up, down left, or right.").tolowercase();       //making sure ship fit , nothing there!     if ((userx+4)>9 && direction=== "right"){      window.alert("you close right edge of board that. restarting...");      firstship();     }     else if ((userx-4)<0 && direction=== "left"){      window.alert("you close left edge of board that. restarting...");      firstship();     }     else if ((usery+4)>9 && direction=== "down"){      window.alert("you close bottom edge of board that. restarting...");      firstship();     }     else if ((usery-4)<0 && direction=== "up"){      window.alert("you close top edge of board that. restarting...");      firstship();     }     else if (user[userx][usery] === 1) {         window.alert("coordinate used. please try again");         firstship();     }       else if (user[userx][usery] === null || user[userx][usery] === ""){         window.alert("that coordinate isn't on board. restarting...");         firstship();     }      else if(direction !=="up" || direction !=="down" || direction !=="left" || direction !=="right") {         for(i=1; i<5; i++){             if(user[userx+i][usery] === 1 && direction=== "right"){                 window.alert("can't place ship in direction, ship in way.");                 isthere=true;             }             if(user[userx-i][usery] === 1 && direction=== "left"){                 window.alert("can't place ship in direction, ship in way.");                 isthere=true;             }             if(user[userx][usery+i] === 1 && direction=== "down"){                 window.alert("can't place ship in direction, ship in way.");                 isthere=true;             }             if(user[userx][usery-i] === 1 && direction=== "up"){                 window.alert("can't place ship in direction, ship in way.");                 isthere=true;             }             if(isthere===true){                 isthere = false;                 firstship();                  return false;             }             else{                 user[userx][usery] = 1;             }         }      }     else{         window.alert("sorry didn't type in direction wanted ship go correctly. restarting...");         firstship();     }       // building ship 1x5     for(i=1; i<4; i++){          if (direction==="up"){             user[userx][usery-i] =1;         }         else if (direction==="down"){             user[userx][usery+i] =1;         }         else if (direction==="left"){             user[userx-i][usery] =1;         }         else if (direction==="right"){             user[userx][usery+i] =1;         }     } } firstship(); //here new cpu creation. there may way shorten if statements, other that, set 

this happening because using negative array index.

if try code here: http://plnkr.co/edit/g4i1qay6cou18xtk6qzi?p=preview

and enter 1, 1, right answers questions, see error on line 28 of script.js:

if(user[userx-i][usery] === 1 && direction=== "left"){ 

the error is:

uncaught typeerror: cannot read property '0' of undefined  

what happens userx = 0 , = 1, evaluating user[-1][0]. since user[-1] undefined, that's why you're seeing error.