Fade out a google maps polyline.

polylinefade.js

function fadeOut(line, keepAround, fadeDuration){
    keepAround = keepAround || 1000;
    fadeDuration = fadeDuration || 500;

    setTimeout(function(){
        var startingOpacity = line.strokeOpacity,
            startTime = (new Date()).getTime();
                    
        function step(){
            var currentTime = (new Date()).getTime(),
                elapsed = currentTime - startTime,
                targetOpacity = startingOpacity - startingOpacity * (elapsed/fadeDuration);
                            
            line.setOptions({
                strokeOpacity: targetOpacity
            });
                       
            if(elapsed >= fadeDuration){
                line.setMap(null);
            } else {
                setTimeout(step, 30);
            }
        }
        setTimeout(step, 30);
    }, keepAround);
}