json函数返回使用mvc中的jquery中的onchange事件在文本框中不显示字符串值

i want to show purchase description . when i enter purchase code onchange event fire and json function call json function return description and i want to show description in Description textbox but function return sting value and value not show in textbox . anyone can tell what is the problem in my code.i want to show purchase description . when i en