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Here is the source code of the algorithm:

// FAST LINE OF SIGHT ALGORITHM BY ANDRE MICHELLE FOR FLASH
//
// http://www.andre-michelle.com
//
// map format is: map = [[1,0,1],[1,1,1],[1,0,1]];
// example - 2d Array
//
// size is the width / height of a tile in px
// tested with 8 px

var round = Math.round;
var floor = Math.floor;
var abs = Math.abs;

var line_of_sight = function ( x0 , y0 , x1 , y1 , map , size ) {

	var lty , tmp;

	var dx = x1 - x0;
	var dy = y1 - y0;

	var dm = dy / dx;

	var acute = abs( dm ) > 1;
	if ( acute ) {
		tmp = x0;
		x0 = y0;
		y0 = tmp;
		tmp = dx;
		dx = dy;
		dy = tmp;
		dm = dy / dx;
	};
	if ( dx > 0 ) {
		for ( var x = size - x0 % size ; x < dx ; x += size ) {
			var tx = x0 + x;
			var ty = y0 + x * dm;
			if ( tx % size == 0 && ty % size == 0 ) {	// exact diagonal
				tx = round ( tx / size );
				if ( dy > 0 ) { ty = round ( ty / size ) } 
					else { ty = floor ( ty / size ) - 1 };
				if ( !acute ) {
					if ( map[ty][tx] == 1 ) return true;
				} else {
					if ( map[tx][ty] == 1 ) return true;
				}
			} else {
				tx = round ( tx / size );
				ty = floor ( ty / size );
				if ( !acute ) {
					if ( map[ty][tx] == 1 ) return true;
				} else {
					if ( map[tx][ty] == 1 ) return true;
				}
				if ( ty != lty ) {
					tx = round ( tx ) - 1;
					if ( !acute ) {
						if ( map[ty][tx] == 1 ) return true;
					} else {
						if ( map[tx][ty] == 1 ) return true;
					}
				}
			}
			lty = ty;
		}
	} else {
		for ( var x = x0 % size ; x < -dx ; x += size ) {
			var tx = x0 - x - size;
			var ty = y0 - x * dm;
			if ( tx % size == 0 && ty % size == 0 ) {	// exact diagonal
				tx = round ( tx / size );
				if ( dy > 0 ) { ty = round ( ty / size ) } 
					else { ty  = floor ( ty / size ) - 1 };
				if ( !acute ) {
					if ( map[ty][tx] == 1 ) return true;
				} else {
					if ( map[tx][ty] == 1 ) return true;
				}
			} else {
				tx = round ( tx / size );
				ty = floor ( ty / size );
				if ( !acute ) {
					if ( map[ty][tx] == 1 ) return true; )
				} else {
					if ( map[tx][ty] == 1 ) return true; )
				}
				if ( ty != lty ) {
					tx = floor ( tx ) + 1;
					if ( !acute ) {
						if ( map[ty][tx] == 1 ) return true;
					} else {
						if ( map[tx][ty] == 1 ) return true;
					}
				}
			}
			lty = ty;
		}
	}
	return false;
}