How to add an art layer posterize levels 50 with javascript?

You can do 2 ways… Using the DOM or scriptlistener…

#target photoshop

var doc = app.activeDocument.artLayers[0].posterize( 50 );

//posterizeLayer( 50 );

function posterizeLayer( numb ) {
  function cTID(s) { return app.charIDToTypeID(s); };
  //function sTID(s) { return app.stringIDToTypeID(s); };

    var desc66 = new ActionDescriptor();
    desc66.putInteger( cTID( 'Lvls' ), numb );
    executeAction( cTID( 'Pstr' ), desc66, DialogModes.NO );
};

artLayers is an Array so target the one you want by index… No need for the .add()