var canvas = document.getElementById('c');
var ctx = canvas.getContext('2d');
canvas.width = 400;
canvas.height = 600;

// game object
var G = {};
var current = 0;
var x = 0;
var y = 0;

//GRID
G.grid = [];
G.gridColumns = 10;
G.gridRows = 15;
for (var i = 0; i < G.gridColumns; i++) {
  G.grid[i] = [];
  for (var j = 0; j < G.gridRows; j++) {
    G.grid[i][j] = 0;
  }
}

// Array with all different blocks
G.blocks = [];
//block constructor
function block() {};
G.blocks[0] = new block();
G.blocks[0].matrix = [
  [1, 0, 0, 0],
  [1, 1, 0, 0],
  [0, 1, 0, 0],
  [0, 0, 0, 0]
];
G.blocks[0].width = 2;
G.blocks[0].height = 3;

function transpose(m) {
  // dont understand this completely, but works because j<i
  for (var i = 0; i < m.matrix.length; i++) {
    for (var j = 0; j < i; j++) {
      var temp = m.matrix[i][j];
      m.matrix[i][j] = m.matrix[j][i];
      m.matrix[j][i] = temp;
    }
  }
}

function reverseRows(m) {
  for (var i = 0; i < m.matrix.length; i++) {
    m.matrix[i].reverse();
  }
}

function rotate(m) {
  transpose(m);
  reverseRows(m);
}

function add(m1, m2) {
  for (var i = 0; i < m1.matrix.length; i++) {
    for (var j = 0; j < m1.matrix[i].length; j++) {
      m2[i + x][j + y] = m1.matrix[i][j];
    }
  }
}

function draw(matrix) {
  for (var i = 0; i < matrix.length; i++) {
    for (var j = 0; j < matrix[i].length; j++) {
      if (matrix[i][j] === 1) {
        ctx.fillRect(j * 20, i * 20, 19, 19);
      }
    }
  }
  ctx.strokeRect(0, 0, G.gridColumns * 20, G.gridRows * 20);
}


window.addEventListener("click", function(e) {
  rotate(G.blocks[current]);
});

function tick() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  add(G.blocks[current], G.grid);
  draw(G.grid);
}
setInterval(tick, 1000 / 30);

<canvas id="c"></canvas>

var canvas = document.getElementById('c');
var ctx = canvas.getContext('2d');
canvas.width = 100;
canvas.height = 100;

var shape = [
  [ 0x4640, 0x0E40, 0x4C40, 0x4E00 ], // 'T'
  [ 0x8C40, 0x6C00, 0x8C40, 0x6C00 ], // 'S'
  [ 0x4C80, 0xC600, 0x4C80, 0xC600 ], // 'Z'
  [ 0x4444, 0x0F00, 0x4444, 0x0F00 ], // 'I'
  [ 0x44C0, 0x8E00, 0xC880, 0xE200 ], // 'J'
  [ 0x88C0, 0xE800, 0xC440, 0x2E00 ], // 'L'
  [ 0xCC00, 0xCC00, 0xCC00, 0xCC00 ]  // 'O'
];

var curShape = 0, curRotation = 0;
draw(curShape, curRotation);

function draw(s, r) {
  ctx.fillStyle = 'white';
  ctx.fillRect(0, 0, 100, 100);
  ctx.fillStyle = 'black';

  for (var y = 0; y < 4; y++) {
    for (var x = 0; x < 4; x++) {
      if (shape[s][r] & (0x8000 >> (y * 4 + x))) {
        ctx.fillRect(x * 20, y * 20, 19, 19);
      }
    }
  }
}

function next() {
  curShape = (curShape + 1) % 7;
  draw(curShape, curRotation);
}

function rotate() {
  curRotation = (curRotation + 1) % 4;
  draw(curShape, curRotation);
}

<canvas id="c"></canvas>
<button onclick="rotate()">Rotate</button>
<button onclick="next()">Next shape</button>