How to change the border color of a textarea frequently ?

You can use setInterval() to change the border color every N milliseconds. Store all colors you want in an array, and always pick the next element when changing the color, and go back to the first element when there are no elements left.

var milliseconds = 2000; // 2 seconds
var colors = ["red", "green", "blue"]; // add more if you want
var currentIndex = 0;
setInterval(function () {
    var newColor = colors[currentIndex];
    document.getElementById("yourArea").style.borderColor = newColor;
    currentIndex++;
    if (currentIndex >= colors.length) {
        currentIndex = 0;
    }
}, milliseconds);