activeDocument.saveAs issues

I am trying to save a tif file via script. The generated value for filename is /d/RAW%20Compare/teststack/Untitled-1_02_45_48.tif. The filename is generated, and the path was generated by using var folder = new Folder('D:/RAW Compare/teststack'); and adding filename to the end of it.

The Error I get is Error 1242: Illegal argument - argument 1

- File/Folder expected

function saveTiff(filename){
	tiffSaveOptions = new TiffSaveOptions();   
	tiffSaveOptions.embedColorProfile = true;   
	tiffSaveOptions.alphaChannels = true;   
	tiffSaveOptions.layers = true;  
	tiffSaveOptions.imageCompression = TIFFEncoding.NONE;  
	activeDocument.saveAs(filename, tiffSaveOptions, true, Extension.LOWERCASE);   
}

I get Error 24: file is not a function when I try to use the following

	activeDocument.saveAs(file(filename), tiffSaveOptions, true, Extension.LOWERCASE);   

edit:

Changing the function to the following helped make it work

function saveTiff(filename){
	tiffFile = new File(filename);
	tiffSaveOptions = new TiffSaveOptions();   
	tiffSaveOptions.embedColorProfile = true;   
	tiffSaveOptions.alphaChannels = true;   
	tiffSaveOptions.layers = true;  
	tiffSaveOptions.imageCompression = TIFFEncoding.NONE;  
	//alert(filename);
	activeDocument.saveAs(tiffFile, tiffSaveOptions, true, Extension.LOWERCASE);   
}