javascript - Same Function Gives Different Values -

i have function calling find derivative @ point, if hard code number script, right answer, if use variable same number, wrong answer.

html

point<br> <input type="text" name="point" id="point" class="form"><br> <p id="submit" onclick="submit()" name="solve">click</p><br> 

script

<script type="text/javascript"> function diff(f) { return function(x) { return (f(x+.0000000001)-f(x))/.0000000001 }; }  function f(x) { return x*x+2*x+1; }  fprime = diff(f);  function submit() { var point = document.getelementbyid("point").value;  console.log(point); //-0.5  var q = fprime(-0.5); //will output 1 var w = fprime(point); //will output 749999998.98 } 

any idea on why or how fix?

the problem point string, not number, convert value number using parsefloat() or use unary operator below

var point = +document.getelementbyid("point").value; 

demo: fiddle