jquery applying css tag

$($("#myDiv"开发者_C百科)).append("<b>Hello</b>");

how can i apply to mydiv display:none

Don't manipulate the display CSS property with css(). Use hide():

$("#myDiv").hide().append("Hello");

Why are you doing:

$($("#myDiv"))...

?

$("#myDiv").css("display","none"); should do the trick but look into the jquery hide method too (hide) which can hide the div "smoothly" if you want that.