test.js

class Grid {
  constructor(length) {
    this.length = length;
    this.grid = [];
    for (let v = 0; v < length; v += 1) {
      for (let h = 0; h < length; h += 1) {
        this.grid.push(' ');
      }
    }
  }
  render() {
    let buffer = '';
    for (let v = 0; v < this.length; v += 1) {
      for (let h = 0; h < this.length; h += 1) {
        buffer += this.grid[v * this.length + h];
      }
      buffer += '\n';
    }
  
    return buffer;
  }
  drawAt(x, y, symbol) {
    this.grid[y * this.length + x] = symbol;
  }
}

const directions = [
  'right',
  'down',
  'left',
  'up'
];

function fillGrid(grid) {
  let currentLength = grid.length;
  let direction = 0;
  let x = -1;
  let y = 0;

  while (currentLength > 0) {
    for (let i = 0; i < currentLength; i += 1) {
      let symbol = '*';

      // Next coordinate.
      switch (directions[direction]) {
        case 'right':
          x += 1;
          symbol = '-';
          break;
        case 'down':
          y += 1;
          symbol = '|';
          break;
        case 'left':
          x -= 1;
          symbol = '-';
          break;
        case 'up':
          y -= 1;
          symbol = '|';
          break;
      }
      if (i + 1 === currentLength) {
        symbol = '+';
      }
      
      grid.drawAt(x, y, symbol);
    }

    currentLength -= 1;
    direction = (direction + 1) % directions.length;
  }
}

function main() {
  // Check if all arguments are there.
  if (process.argv.length >= 3) {
    const length = Number.parseInt(process.argv[2]);
    const grid = new Grid(length);
    fillGrid(grid)
    console.log(grid.render());
  } else {
    console.log(`Error: missing length argument.`);
  }
}

main();