Getting selected node Text with Javascript

Question

Hi,
I have treeview like this
Parent1
    child1
     child2
     child3
parent2
    child21
     child22
I want to select One of the child,when i select it childnode text should copy to label(don't want parent text in any case).Don't need post back also
   So how  to write Javascript code for that and and in which property i should call that.Bcz If i use onclick,while expanding parent node also function is executing.
can any one help on this?