How Do I Write A Script Resizing A Layer To Canvas & Keeping The Aspect Ratio

As JJ Mack stated it's just math: figuring out your file size in pixels and you layer size in pixels, then sizing the layer.

#target photoshop  

var oldPref = app.preferences.rulerUnits  
app.preferences.rulerUnits = Units.PIXELS;  
var doc = activeDocument; 
var iLayer = doc.activeLayer;  
doc.activeLayer = iLayer;  
var scale = Math.max(doc.width/(iLayer.bounds[2]-iLayer.bounds[0]),doc.height/(iLayer.bounds[3]-iLayer.bounds[1]));  
iLayer.resize (scale*100,scale*100);  
iLayer.translate(doc.width/2-(iLayer.bounds[0]+iLayer.bounds[2])/2,doc.height/2-(iLayer.bounds[1]+iLayer.bounds[3])/2);  

Moving to Photoshop Scripting forum

There is a scripting forum and if you search it you will find code there that will answer your question. You can also read Adobe Fit Image Plug-in script to see how whole documents can be resized and maintain aspect ratio. Its simple math.

https://forums.adobe.com/search.jspa?place=%2Fplaces%2F1383833&sort=updatedDesc&q=resize+layer+aspect+ratio

Photoshop Scripting

You can also fit an imag into and area without distorting it by  resizing it to fill the area then masking off any excess . A virtual center crop to the new aspect ratio.  For example the script will resize the current layer to fit within four guides you first set.

http://www.mouseprints.net/old/dpr/FitImageToGuides.jsx