How to get bounding box of a layer mask?

It seems that  a layer mask  and a layers bounds are the same somwtimes.  When the layer actual content  smaller then then layers mask selection boynds. The layers bounds  is the layer pixels bounds.

// enable double-clicking from Mac Finder or Windows Explorer
#target photoshop // this command only works in Photoshop CS2 and higher
// bring application forward for double-click events
app.bringToFront();
// Save the current preferences
var startRulerUnits = app.preferences.rulerUnits;
// Set Photoshop to use pixels 
app.preferences.rulerUnits = Units.PIXELS;
try{
var LB = activeDocument.activeLayer.bounds;
var LWidth = (LB[2].value) - (LB[0].value);
var LHeight = (LB[3].value) - (LB[1].value);
MarkX((LB[0].value + LWidth/2));
MarkX(LB[0].value);
MarkX(LB[2].value);
MarkY((LB[1].value + LHeight/2));
MarkY(LB[1].value)
MarkY(LB[3].value)
alert("'" + activeDocument.activeLayer.name + "' Layer Bounds\nTop Left " +  LB[0].value + "X," +  LB[1].value + "Y   Bottom Right "  +  LB[2].value + "X," +  LB[3].value
+ "Y\nWidth " +  LWidth + "px   Height " + LHeight +"px" 
+ "\nLayer center relative to canvas " + (LB[0].value + LWidth/2) + "X," +  (LB[1].value + LHeight/2) +"Y" 
);
}
catch(e){alert("Requires a layer targered");}
// Return the app preferences
app.preferences.rulerUnits = startRulerUnits;
function MarkX(x) {
// =======================================================
var idMk = charIDToTypeID( "Mk  " );
    var desc61 = new ActionDescriptor();
    var idNw = charIDToTypeID( "Nw  " );
        var desc62 = new ActionDescriptor();
        var idPstn = charIDToTypeID( "Pstn" );
        var idPxl = charIDToTypeID( "#Pxl" );
        desc62.putUnitDouble( idPstn, idPxl, x);
        var idOrnt = charIDToTypeID( "Ornt" );
        var idOrnt = charIDToTypeID( "Ornt" );
        var idVrtc = charIDToTypeID( "Vrtc" );
        desc62.putEnumerated( idOrnt, idOrnt, idVrtc );
    var idGd = charIDToTypeID( "Gd  " );
    desc61.putObject( idNw, idGd, desc62 );
executeAction( idMk, desc61, DialogModes.NO );
}
function MarkY(y) {
// =======================================================
var idMk = charIDToTypeID( "Mk  " );
    var desc63 = new ActionDescriptor();
    var idNw = charIDToTypeID( "Nw  " );
        var desc64 = new ActionDescriptor();
        var idPstn = charIDToTypeID( "Pstn" );
        var idPxl = charIDToTypeID( "#Pxl" );
        desc64.putUnitDouble( idPstn, idPxl, y );
        var idOrnt = charIDToTypeID( "Ornt" );
        var idOrnt = charIDToTypeID( "Ornt" );
        var idHrzn = charIDToTypeID( "Hrzn" );
        desc64.putEnumerated( idOrnt, idOrnt, idHrzn );
    var idGd = charIDToTypeID( "Gd  " );
    desc63.putObject( idNw, idGd, desc64 );
executeAction( idMk, desc63, DialogModes.NO );
}

Here is a script to fit  a layer to the current selection the is a selection bounds you can use. Also add a layer mask  to shape the layer to the selection.  Slection Bounds was broken in CS2 I had to use other means back than.

var startRulerUnits = app.preferences.rulerUnits;    
app.preferences.rulerUnits = Units.PIXELS;   
  
if (app.activeDocument.activeLayer.kind == LayerKind.NORMAL  || app.activeDocument.activeLayer.kind == LayerKind.SMARTOBJECT && hasLayerMask()) { deleteLayerMask ();}
  
var LB = activeDocument.selection.bounds;  
var width = LB[2]-LB[0];  
var height = LB[3]-LB[1];  
var IB = activeDocument.activeLayer.bounds;  
var dwidth = IB[2]-IB[0];  
var dheight = IB[3]-IB[1];  
var ratio = height/width;   
var dratio = dheight/dwidth;   
if (dratio>ratio) {                           // layer's Aspect Ratio less the Canvas area Aspect Ratio 
var percentageChange = ((width/dwidth)*100);  // Resize to canvas area width
app.activeDocument.activeLayer.resize(percentageChange,percentageChange,AnchorPosition.MIDDLECENTER);
}
else { 
var percentageChange = ((height/dheight)*100); // resize to canvas area height
app.activeDocument.activeLayer.resize(percentageChange,percentageChange,AnchorPosition.MIDDLECENTER);
}
align('AdCH'); // align to horizontal centers
align('AdCV'); // align to vertical centers
addLayermask(); // add layer mask to mask off excess
app.preferences.rulerUnits = startRulerUnits;           // restore ruler units 
/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Helper Functions
/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
function align(method) {
var desc = new ActionDescriptor();
var ref = new ActionReference();
ref.putEnumerated( charIDToTypeID( "Lyr " ), charIDToTypeID( "Ordn" ), charIDToTypeID( "Trgt" ) );
desc.putReference( charIDToTypeID( "null" ), ref );
desc.putEnumerated( charIDToTypeID( "Usng" ), charIDToTypeID( "ADSt" ), charIDToTypeID( method ) );
try{executeAction( charIDToTypeID( "Algn" ), desc, DialogModes.NO );}
catch(e){}
}
function hasLayerMask() {   
var hasLayerMask = false;   
try {   
var ref = new ActionReference();   
var keyUserMaskEnabled = app.charIDToTypeID( 'UsrM' );   
ref.putProperty( app.charIDToTypeID( 'Prpr' ), keyUserMaskEnabled );   
ref.putEnumerated( app.charIDToTypeID( 'Lyr ' ), app.charIDToTypeID( 'Ordn' ), app.charIDToTypeID( 'Trgt' ) );   
var desc = executeActionGet( ref );   
if ( desc.hasKey( keyUserMaskEnabled ) ) { hasLayerMask = true; }   
}
catch(e) { hasLayerMask = false; }   
return hasLayerMask;   
} 
function deleteLayerMask (apply) {
// Delet Layer mask default to not apply first
if (apply == undefined) {var apply = false};
try {
var idDlt = charIDToTypeID( "Dlt " );
    var desc9 = new ActionDescriptor();
    var idnull = charIDToTypeID( "null" );
        var ref5 = new ActionReference();
        var idChnl = charIDToTypeID( "Chnl" );
        var idChnl = charIDToTypeID( "Chnl" );
        var idMsk = charIDToTypeID( "Msk " );
        ref5.putEnumerated( idChnl, idChnl, idMsk );
    desc9.putReference( idnull, ref5 );
    var idAply = charIDToTypeID( "Aply" );
    desc9.putBoolean( idAply, apply );
executeAction( idDlt, desc9, DialogModes.NO );
}
catch (e) {}
};
function addLayermask(){
// Add layer Mask
var idMk = charIDToTypeID( "Mk  " );
    var desc52 = new ActionDescriptor();
    var idNw = charIDToTypeID( "Nw  " );
    var idChnl = charIDToTypeID( "Chnl" );
    desc52.putClass( idNw, idChnl );
    var idAt = charIDToTypeID( "At  " );
        var ref19 = new ActionReference();
        var idChnl = charIDToTypeID( "Chnl" );
        var idChnl = charIDToTypeID( "Chnl" );
        var idMsk = charIDToTypeID( "Msk " );
        ref19.putEnumerated( idChnl, idChnl, idMsk );
    desc52.putReference( idAt, ref19 );
    var idUsng = charIDToTypeID( "Usng" );
    var idUsrM = charIDToTypeID( "UsrM" );
    var idRvlS = charIDToTypeID( "RvlS" );
    desc52.putEnumerated( idUsng, idUsrM, idRvlS );
executeAction( idMk, desc52, DialogModes.NO );
// Un link layer mask just added fron the layers content
var idsetd = charIDToTypeID( "setd" );
    var desc2 = new ActionDescriptor();
    var idnull = charIDToTypeID( "null" );
        var ref1 = new ActionReference();
        var idLyr = charIDToTypeID( "Lyr " );
        var idOrdn = charIDToTypeID( "Ordn" );
        var idTrgt = charIDToTypeID( "Trgt" );
        ref1.putEnumerated( idLyr, idOrdn, idTrgt );
    desc2.putReference( idnull, ref1 );
    var idT = charIDToTypeID( "T   " );
        var desc3 = new ActionDescriptor();
        var idUsrs = charIDToTypeID( "Usrs" );
        desc3.putBoolean( idUsrs, false );
    var idLyr = charIDToTypeID( "Lyr " );
    desc2.putObject( idT, idLyr, desc3 );
executeAction( idsetd, desc2, DialogModes.NO );
}

Here is how I got the Selection bounds in CS2