Why did I write this addition didn't work?

https://codepen.io/niusz/pen/.

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Javascript
Jul.06,2021

buy and sell. Fnum and snum, take a good value before the program runs, but not onclick, which is equivalent to a constant, so it is (initial value) 0.

value takes the current value when clicked 

var eleFirstNumber = document.getElementById("first-number");
var eleSecondNumber = document.getElementById("second-number");
document.getElementById("add-btn").onclick = function(){
  var fnum = Number(eleFirstNumber.value);
  var snum = Number(eleSecondNumber.value);
  document.getElementById("result").innerHTML =  fnum + snum;
}

should get the input value in onclick, that is, 

document.getElementById("add-btn").onclick = function(){
  var fnum = Number(document.getElementById("first-number").value);
  var snum = Number(document.getElementById("second-number").value);
  document.getElementById("result").innerHTML =  fnum + snum;
}

fnum and snum in your code are always the initial values of input 0

get the value first, but not when you click on the event 


document.getElementById("add-btn").onclick = function(){
  var fnum = Number(document.getElementById("first-number").value);
  var snum = Number(document.getElementById("second-number").value);
  document.getElementById("result").innerHTML =  fnum + snum;
}

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