how to get the value of a function from a button click in javascript? -

i new javascript , trying go through tutorials understand concepts.

this function

function tocelsius(fahrenheit){      (5/9) * (fahrenheit-32);         document.getelementbyid("demotemp").onclick.tostring();  } 

and trying invoke here:

<form> <br> enter number: <input type="text" name="firstname">  <!-- <input type="text" name="lastname"> --> <button type="button" onclick="tocelsius()"> new temp in celsius </button> </form>  <p id="demotemp"></p> 

i want user able enter value , hit button tell them new temperature fahrenheit celsius

just put result in inner html.

function tocelsius(){     var fahrenheit = document.getelementsbyname("firstname")[0].value;     var cel = (5 / 9) * (fahrenheit - 32);         document.getelementbyid("demotemp").innerhtml = cel; }