How to code a scroll in an After Effects Script?

I actually meant to add a vertical scrollbar because the script is too high to fit the dimensions of my laptop screen and with that scrollbar I can keep the window at a defined height and everything that doesn't fit within that size is viewable by scrolling down.

Scroll? Left to right movement?

Try adding this expression to position:

t = time;

s = 300; // number of pixels you want to move per second

v = t * s

[value[0] + v, value[1]]

Here's how that works. In expression language, "time" returns the running time of the layer from the start of the composition. You just multiply time by however many pixels you want the layer to move in one second. If you are working in an HD comp then it is 1920 pixels wide. 1920 / 5 = 384 so if you set the speed to 300 it will take a little longer than 5 seconds for the layer (text) to scroll from left to right. Use whatever value you want for s.

Then there is just a simple multiplication function. Time in seconds multiplied by the speed gives a value we can add to the X value of the layer. This gives us the 'v' value.

The most confusing part of this expression for a newbie is the array at the bottom. I'll break that down. X and Y values for position are always in an array. value[0] is the current x value, value [1] is the current y value. If the layer was 3D there would also be a value for z and that would be described as value[2]. So the last line of the expression reads like this:

Take the current value of X and add to it the product of multiplying time by the speed in pixels per second you want for movement but just keep the current value for Y.

I hope that makes sense.

If you want a roll instead of a scroll just change the last line, the array so 'v' is subtracted from value[1]. If you are doing a scroll or a roll you always want to move the layer an even number of pixels so never use something like 250.5 for a speed value. You will have aliasing issues with fine lines and could easily introduce a stroboscopic visual effect called Judder, which makes scrolling or rolling text layers look pretty awful.