window.onload = function(){
  // get the canvas DOM element by its ID
     var canvas = document.getElementById("myCanvas");
  // declare a 2-d context using the getContext() method of the 
  // canvas object
     var context = canvas.getContext("2d");
    
  // set the line width to 10 pixels
     context.lineWidth = 10;
  // set the line color to blue
     context.strokeStyle = "blue";

  // position the drawing cursor
     context.moveTo(50, canvas.height - 50);
  // draw the line
     context.lineTo(canvas.width - 50, 50);
  // make the line visible with the stroke color
     context.stroke();
};

<canvas id="myCanvas" width="600" height="250" style="border:1px solid black;">
</canvas>