The last thing I’m going to do for this tutorial series is show how to implement the scoring and level mechanism. The idea of the game is that as you clear lines, you get points to increase your score. After you get enough points, you move on to the next level, where the blocks fall faster. To add this we’ll use three more global variables, defined at the start of the script.
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var score; //Player's score var level; //Current level var timestep; //Time between calls to gameStep() |
In the initialization function, we’ll set the starting values for these variables.
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score = 0; level = 1; timestep = 1000; //Start the game timer clearInterval(timer); timer = setInterval(function(){gameStep()}, timestep); |
Note that I’ve used the timestep variable when initializing the timer. It’s usually a good idea to use variables whenever possible for things like this, so that you can change the value for all instances at once. If I had made the dimensions of the game state grid variables instead of hard coding them, it would be easy to modify the game to use a giant grid, for example.
The only time the score can change is when a line is cleared, so that’s where we’ll add this next code segment.
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//Update score score++; //Check if ready for the next level if(score >= level*10) { level++; //Update the timer with a shorter timestep timestep *= 0.8; clearInterval(timer); timer = setInterval(function(){gameStep()}, timestep); } //Update score and level display document.getElementById("score").innerHTML = "Level " + level + " Score: " + score; |
Each time a line is cleared, we increment the score variable. Every 10 lines, the level will also increase. When this happens, we compute a new timestep variable and reset the game timer. The values here can be adjusted to set the difficulty of the game by changing the rate at which each new level increases the game speed. The last line updates the score and level display in our HTML document. We need to add an id attribute to the div element containing the score so that it can be referenced in our code.
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<div id="score" style="width:200px;background-color:#CCCCCC"> Level 1 Score: 0 </div> |
The getElementById function searches the document object (in this case our entire file) for an element with an id value of "score". It then sets the HTML content of that element to the string that we provide. JavaScript uses the “+” symbol to concatenate two strings of text, and because it is a weakly typed language, we can include our variables here directly. This effectively updates the score display with the current values of the variables. The final version of the code should look like this:
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<!DOCTYPE html> <html> <head> <title>Tetris</title> <script> //Golbal variables var ctx; //Canvas object var t; //Tetrimino type var x, y; //Tetrimino position var o; //Tetrimino orientation var grid; //Game state grid var timer; //Game timer var score; //Player's score var level; //Current level var timestep; //Time between calls to gameStep() /************************************************ Initialize the drawing canvas ************************************************/ function initialize() { //Get the canvas context object from the body c = document.getElementById("myCanvas"); ctx = c.getContext("2d"); //Initialize tetrimino variables t = 1 + Math.floor((Math.random()*7)); x = 4; y = 18; o = 0; //Create an empty game state grid grid = new Array(20); for(i = 0; i < 20; i++) { grid[i] = new Array(10); for(j = 0; j < 10; j++) grid[i][j] = 0; } //Draw the current tetrimino drawTetrimino(x,y,t,o,1); //Redraw the grid drawGrid(); score = 0; level = 1; timestep = 1000; //Start the game timer clearInterval(timer); timer = setInterval(function(){gameStep()}, timestep); } /************************************************ Draws the current game state grid ************************************************/ function drawGrid() { //Clear the canvas ctx.clearRect(0,0,200,400); //Loop over each grid cell for(i = 0; i < 20; i++) { for(j = 0; j < 10; j++) drawBlock(j, i, grid[i][j]); } } /************************************************ Draws a block at the specified game coordinate x = [0,9] x-coordinate y = [0,19] y-coordinate t = [0,7] block type ************************************************/ function drawBlock(x, y, t) { //Check if a block needs to be drawn if(t > 0) { //Get the block color var c; if(t == 1) //I type c = 180; //Cyan else if(t == 2) //J type c = 240; //Blue else if(t == 3) //L type c = 40; //Orange else if(t == 4) //O type c = 60; //Yellow else if(t == 5) //S type c = 120; //Green else if(t == 6) //T type c = 280; //Purple else //Z type c = 0; //Red //Convert game coordinaes to pixel coordinates pixelX = x*20; pixelY = (19-y)*20; /**** Draw the center part of the block ****/ //Set the fill color using the supplied color ctx.fillStyle = "hsl(" + c + ",100%,50%)"; //Create a filled rectangle ctx.fillRect(pixelX+2,pixelY+2,16,16); /**** Draw the top part of the block ****/ //Set the fill color slightly lighter ctx.fillStyle = "hsl(" + c + ",100%,70%)"; //Create the top polygon and fill it ctx.beginPath(); ctx.moveTo(pixelX,pixelY); ctx.lineTo(pixelX+20,pixelY); ctx.lineTo(pixelX+18,pixelY+2); ctx.lineTo(pixelX+2,pixelY+2); ctx.fill(); /**** Draw the sides of the block ****/ //Set the fill color slightly darker ctx.fillStyle = "hsl(" + c + ",100%,40%)"; //Create the left polygon and fill it ctx.beginPath(); ctx.moveTo(pixelX,pixelY); ctx.lineTo(pixelX,pixelY+20); ctx.lineTo(pixelX+2,pixelY+18); ctx.lineTo(pixelX+2,pixelY+2); ctx.fill(); //Create the right polygon and fill it ctx.beginPath(); ctx.moveTo(pixelX+20,pixelY); ctx.lineTo(pixelX+20,pixelY+20); ctx.lineTo(pixelX+18,pixelY+18); ctx.lineTo(pixelX+18,pixelY+2); ctx.fill(); /**** Draw the bottom part of the block ****/ //Set the fill color much darker ctx.fillStyle = "hsl(" + c + ",100%,30%)"; //Create the bottom polygon and fill it ctx.beginPath(); ctx.moveTo(pixelX,pixelY+20); ctx.lineTo(pixelX+20,pixelY+20); ctx.lineTo(pixelX+18,pixelY+18); ctx.lineTo(pixelX+2,pixelY+18); ctx.fill(); } } /************************************************* Draws a tetrimino at the specified game coordinate with the specified orientation x = [0,9] x-coordinate y = [0,19] y-coordinate t = [1,7] tetrimino type o = [0,3] orientation d = [-1,1] test, erase, or draw *************************************************/ function drawTetrimino(x,y,t,o,d) { //Determine the value to send to setGrid c = -1; if(d >= 0) c = t*d; //Initialize validity test valid = true; /**** Pick the appropriate tetrimino type ****/ if(t == 1) { //I Type //Get orientation if(o == 0) { valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); valid = valid && setGrid(x+2,y,c); } else if(o == 1) { valid = valid && setGrid(x+1,y+1,c); valid = valid && setGrid(x+1,y,c); valid = valid && setGrid(x+1,y-1,c); valid = valid && setGrid(x+1,y-2,c); } else if(o == 2) { valid = valid && setGrid(x-1,y-1,c); valid = valid && setGrid(x,y-1,c); valid = valid && setGrid(x+1,y-1,c); valid = valid && setGrid(x+2,y-1,c); } else if(o == 3) { valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y-1,c); valid = valid && setGrid(x,y-2,c); } } if(t == 2) { //J Type //Get orientation if(o == 0) { valid = valid && setGrid(x-1,y+1,c); valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); } else if(o == 1) { valid = valid && setGrid(x+1,y+1,c); valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y-1,c); } else if(o == 2) { valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); valid = valid && setGrid(x+1,y-1,c); } else if(o == 3) { valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y-1,c); valid = valid && setGrid(x-1,y-1,c); } } if(t == 3) { //L Type //Get orientation if(o == 0) { valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); valid = valid && setGrid(x+1,y+1,c); } else if(o == 1) { valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y-1,c); valid = valid && setGrid(x+1,y-1,c); } else if(o == 2) { valid = valid && setGrid(x-1,y-1,c); valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); } else if(o == 3) { valid = valid && setGrid(x-1,y+1,c); valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y-1,c); } } if(t == 4) { //O Type //Orientation doesn’t matter valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x+1,y+1,c); } if(t == 5) { //S Type //Get orientation if(o == 0) { valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x+1,y+1,c); } else if(o == 1) { valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); valid = valid && setGrid(x+1,y-1,c); } else if(o == 2) { valid = valid && setGrid(x-1,y-1,c); valid = valid && setGrid(x,y-1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); } else if(o == 3) { valid = valid && setGrid(x-1,y+1,c); valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y-1,c); } } if(t == 6) { //T Type //Get orientation if(o == 0) { valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); valid = valid && setGrid(x,y+1,c); } else if(o == 1) { valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y-1,c); valid = valid && setGrid(x+1,y,c); } else if(o == 2) { valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); valid = valid && setGrid(x,y-1,c); } else if(o == 3) { valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y-1,c); valid = valid && setGrid(x-1,y,c); } } if(t == 7) { //Z Type //Get orientation if(o == 0) { valid = valid && setGrid(x-1,y+1,c); valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x+1,y,c); } else if(o == 1) { valid = valid && setGrid(x+1,y+1,c); valid = valid && setGrid(x+1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y-1,c); } else if(o == 2) { valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x,y-1,c); valid = valid && setGrid(x+1,y-1,c); } else if(o == 3) { valid = valid && setGrid(x,y+1,c); valid = valid && setGrid(x,y,c); valid = valid && setGrid(x-1,y,c); valid = valid && setGrid(x-1,y-1,c); } } return valid; } /************************************************* Sets a grid cell in the game state grid x = [0,9] x-coordinate y = [0,19] y-coordinate t = [-1,7] test or block type *************************************************/ function setGrid(x, y, t) { //Check if point is in range if(x >= 0 && x < 10 && y >= 0 && y < 20) { //Return test result if testing if(t < 0) return grid[y][x] == 0; //Otherwise assign block type to the grid grid[y][x] = t; return true; } return false; } /************************************************* Responds to a key press event *************************************************/ function keyDown(e) { if(e.keyCode == 37) { //Left arrow drawTetrimino(x,y,t,o,0); //Erase the current tetrimino x2 = x - 1; if(drawTetrimino(x2,y,t,o,-1)) //Check if valid x = x2; } else if(e.keyCode == 38) { //Up arrow drawTetrimino(x,y,t,o,0); //Erase the current tetrimino o2 = (o + 1) % 4; if(drawTetrimino(x,y,t,o2,-1)) //Check if valid o = o2; } else if(e.keyCode == 39) { //Right arrow drawTetrimino(x,y,t,o,0); //Erase the current tetrimino x2 = x + 1; if(drawTetrimino(x2,y,t,o,-1)) //Check if valid x = x2; } else if(e.keyCode == 40) { //Down arrow drawTetrimino(x,y,t,o,0); //Erase the current tetrimino y2 = y - 1; if(drawTetrimino(x,y2,t,o,-1)) //Check if valid y = y2; } else if(e.keyCode == 32) { //Space-bar drawTetrimino(x,y,t,o,0); //Erase the current tetrimino //Move down until invalid while(drawTetrimino(x,y-1,t,o,-1)) y -= 1; gameStep(); } //Draw the current tetrimino drawTetrimino(x,y,t,o,1); //Redraw the grid drawGrid(); } /************************************************* Updates the game state at regular intervals *************************************************/ function gameStep() { //Erase the current tetrimino drawTetrimino(x,y,t,o,0); //Check if the tetrimino can be dropped 1 block y2 = y - 1; if(drawTetrimino(x,y2,t,o,-1)) y = y2; else { //Redraw last tetrimino drawTetrimino(x,y,t,o,1); //Check if any lines are complete checkLines(); //Create a new tetrimino t2 = 1 + Math.floor((Math.random()*7)); x2 = 4; y2 = 18; o2 = 0; //Check if valid if(drawTetrimino(x2,y2,t2,o2,-1)) { t = t2; x = x2; y = y2; o = o2; } else { alert("Game Over"); initialize(); return; } } //Draw the current tetrimino drawTetrimino(x,y,t,o,1); //Redraw the grid drawGrid(); } /************************************************* Removes completed lines from the grid *************************************************/ function checkLines() { //Loop over each line in the grid for(i = 0; i < 20; i++) { //Check if the line is full full = true; for(j = 0; j < 10; j++) full = full && (grid[i][j] > 0); if(full) { //Update score score++; //Check if ready for the next level if(score >= level*10) { level++; //Update the timer with a shorter timestep timestep *= 0.8; clearInterval(timer); timer = setInterval(function(){gameStep()}, timestep); } //Update score and level display document.getElementById("score").innerHTML = "Level " + level + " Score: " + score; //Loop over the remaining lines for(ii = i; ii < 19; ii++) { //Copy each line from the line above for(j = 0; j < 10; j++) grid[ii][j] = grid[ii+1][j]; } //Make sure the top line is clear for(j = 0; j < 10; j++) grid[19][j] = 0; //Repeat the check for this line i--; } } } </script> <body onload="initialize();" onkeydown="keyDown(event);" style="background-color:#EEEEEE"> <canvas id="myCanvas" height="400px" width="200px" style="background-color:#444444"></canvas> <div id="score" style="width:200px;background-color:#CCCCCC"> Level 1 Score: 0 </div> </body> </html> |
You can see what the final game looks like here. I hope you’ve enjoyed following along with me as I made this game and hopefully you’ve learned something about HTML, JavaScript, or game programming. Be aware that the methods I presented are by no means the only way of doing things. My goal was to give some insight into how I go about solving problems and to do it without assuming any background knowledge. Again, let me recommend w3schools as a great reference resource and a place to learn more about web development. If you’re interested, you can always continue to improve this game by adding more features, such as a window to show which block is coming next, or a way to keep track of high scores. Regardless, I hope you had fun and will continue to make fun and interesting games.