Why does function showPI() return empty string?

Question

I don't know why function showPI() returns empty string. I triggered console.log(trimmedPI) in this function before return statement and it showed string. But when I put showPI() function in compare() function it didn't work. String is empty and my app doesn't work. Maybe I am tired, because there is 00:30 AM in my country. I can't see mistake, there is no errors in console.

function getInput() {
  const textInput = document.getElementById("textInput").value;
  const PiWithout_3 = textInput.substring(2, textInput.length);
  let PiDigitsWithSpace = PiWithout_3.match(/.{1,5}/g);
  PiDigitsWithSpace = PiDigitsWithSpace.join(' ');
  PiDigitsWithSpace = "3, " + PiDigitsWithSpace;
  document.getElementById("pFromInput").textContent = PiDigitsWithSpace;
  const textInputLength = textInput.length - 2;
  document.getElementById("numbersCount").innerHTML = "You entered <b>" + textInputLength + "</b> Pi digits after comma.";
  return textInput;
}
let textInput = document.getElementById("textInput");

function showPI() {
  const PI = "3,1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679821480865132823066470938446095505822317253594081284811174502841027019385211055596446229489549303819644288109756659334461284756482337867831652712019091456485669234603486104543266482133936072602491412737245870066063155881748815209209628292540917153643678925903600113305305488204665213841469519415116094330572703657595919530921861173819326117931051185480744623799627495673518857527248912279381830119491298336733624406566430860213949463952247371907021798609437027705392171762931767523846748184676694051320005681271452635608277857713427577896091736371787214684409012249534301465495853710507922796892589235420199561121290219608640344181598136297747713099605187072113499999983729780499510597317328160963185950244594553469083026425223082533446850352619311881710100031378387528865875332083814206171776691473035982534904287554687311595628638823537875937519577818577805321712268066130019278766111959092164201989";
  const trimmed = document.getElementById("trimmed").value;
  const trimmedInt = parseInt(trimmed) + 2;
  const trimmedPI = PI.substring(0, trimmedInt);
  document.getElementById("pShowPi").innerHTML = trimmedPI;
  return trimmedPI;
}
textInput.oninput = function compare() {
  const textInput = document.getElementById("textInput").value;
  const compareResult = document.getElementById("compareResult");
  if (showPI() === getInput()) {
    compareResult.innerHTML = " <b>correct</b>";
  } else {
    compareResult.innerHTML = " <b>incorrect</b>";
  }
}
const button = document.querySelector("#showPi");
button.addEventListener('click', showPI);

* {
  margin: 0;
  padding: 0;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

body {
  display: flex;
  justify-content: center;
  margin-top: 50px;
}

h1 {
  font-size: 24px;
}

label {
  display: block;
  padding-bottom: 5px;
  padding-top: 10px;
}

p {
  padding: 10px 0;
  word-wrap: break-word;
}

#wrapper {
  width: 493px;
  padding: 10px;
  background: rgb(255 246 230);
}

input {
  height: 26px;
  border: 1px solid #de8f02;
  border-radius: 3px;
  padding: 5px;
}

input:focus,
select:focus,
textarea:focus,
file:focus {
  outline: none;
}

input:hover,
input:focus {
  border: 2px solid #b97602 !important;
}

input::placeholder {
  color: rgb(175, 175, 175);
  opacity: 1;
}

#textInput {
  width: 380px;
}

#pFromInput {
  font-size: 24px;
}

#compareResult {
  font-size: 20px;
}

#numbersCount {
  border-bottom: 1px solid #ffdda2;
}

#showPi {
  background-color: #de8f02;
  border: none;
  color: #fff6e6;
  padding: 5px 5px;
  text-align: center;
  text-decoration: none;
  display: inline-block;
  font-size: 14px;
  border-radius: 4px;
  cursor: pointer;
}

#showPi:hover {
  background-color: #b97602;
  transition: 300ms;
}

<div id="wrapper">
  <h1>Memorizing &#960</h1>
  <label for="textInput">&#960 from your memory (with comma)</label>
  <input type="text" id="textInput" placeholder="3,">
  <p id="pFromInput"></p>
  <p id="numbersCount"></p>
  <label for="trimmed">1÷1000 &#960 digits after comma</label>
  <input type="number" id="trimmed" min="1" max="1000">
  <button id="showPi">show Pi</button>
  <p id="pShowPi"></p>
  <p id="compareResult"></p>
</div>