changing values in array

For a battleship, the way I would do it would be (assuming your grid is already filled with 0s):

For each ship

• 1. randomly select a starting position
• 2. randomly select a direction (up, down, left, right)
• 3. add your ship (by changing however many 1s you need to, based on the size of the ship).

The checks you need to add would be:

• At step 1, make sure there isn't a boat there already, in which case pick again.
• At step 2, make sure you're not going to hit the side of your game board, or another ship, in which case try another direction. If all 4 directions have been tried and there isn't enough space for a ship, back to step 1.

I usually don't give full answers when OP doesn't really show they tried but I liked the challenge.

The idea is to:

• Set your empty board.
• Choose a random point in the board where the ship will start
• Choose direction (H or V)
• With the random point and direction, make sure there is room for the ship according to the limits of the board
• Create a list of positions the ship will take
• Test all positions to make sure they are free
• Set the positions on the board as filled.

At any given time, if a check is not fulfilled I've put continue; this will stop the current iteration and back to the beginning of the while. That way, the code runs until it finds a spot, and return to leave the loop.

Also, I've made a 1d array instead of 2d because it felt easier for mathing it out and manipulations. Feel free to convert to 2D afterward, or not.

let battlefield = new Array(10*10).fill(0);
placeShip(3);
placeShip(4);
placeShip(4);
placeShip(5);
console.log(battlefield);

function placeShip(length){
  while(true){
      const start = Math.round(Math.random()*99);
      if(battlefield[start]==='1') continue;
      const orientation = Math.random() <=.5?'H':'V';
      // Fill the positions where the ship will be placed.
      const positions = new Array();
      if(orientation === 'H'){
          // First make sure we have room to place it
          if(10-((start+1) % 10) < length)continue;
          for(let p=start;p<start+length;p++){
              // Set for each length the position the ship will take.
              positions.push(p);
          }
      }else if(orientation === 'V'){
          // Same as for H but we divide by 10 because we want rows instead of cells.
          if(10-((start/10) % 10) < length)continue;
          for(let p=start;p<start+length*10;p+=10){
              // Set for each length the position the ship will take.
              positions.push(p);
          }
      }
      // Now let's check to make sure there is not a ship already in one of the positions
      for(let i=0,L=positions.length;i<L;i++){
          if(battlefield[positions[i]]!=="0")continue;
      }
      // Now let's put the ship in place
      for(let i=0,L=positions.length;i<L;i++){
          battlefield[positions[i]] = 1;
      }
      return;
  }
}