How to change font-weight for every keypress in javascript

Question

I am new to coding and am working on a code for a variable font, that I created, so that every letter you type changes font weight (and gets heavier). I already have a code where all letters get heavier at once, and one where font-size changes but I can't figure out how to rewrite that code, so that not the font-size but the font-weight changes. Thank you in advance for your help!

let initWeight = 101;

document.getElementById("testarea").onkeypress = function(event) {
  myFunction(event.key)
};

function myFunction(letter) {
  initWeight += 50;
  const letterHTML = '<span style="font-size:' + initWeight + 'px">' + letter + '</span>'
  document.getElementById("result").innerHTML += letterHTML
}

  @import {
  https: //static.wixstatic.com/ufonts/5bda5f_04476cfbcbfe4a349a33e0080a539b44/woff2/file.woff2}
  @font-face {
    font-family:wf_04476cfbcbfe4a349a33e0080;
    src: url("https://static.wixstatic.com/ufonts/5bda5f_04476cfbcbfe4a349a33e0080a539b44/woff2/file.woff2") format("woff2");
    font-weight: 101 900
  }
  div {
    font-family: 'wf_04476cfbcbfe4a349a33e0080', sans-serif;
    font-weight: 101;
    font-size: 100px;
  }

<div class="myInput" id="testarea" contentEditable="true"> insert text </div>
<div id="result" contentEditable="true"></div>

Answer 1

Don't you just mean

'<span style="font-weight:' + initWeight + '">'

let initWeight = 100;

document.getElementById("testarea").onkeypress = function(event) {
  myFunction(event.key)
};

function myFunction(letter) {
  const letterHTML = '<span style="font-weight:' + initWeight + '">' + letter + '</span>'
  document.getElementById("result").innerHTML += letterHTML
  initWeight += 100;
}

  @import {
  https: //static.wixstatic.com/ufonts/5bda5f_04476cfbcbfe4a349a33e0080a539b44/woff2/file.woff2}
  @font-face {
    font-family:wf_04476cfbcbfe4a349a33e0080;
    src: url("https://static.wixstatic.com/ufonts/5bda5f_04476cfbcbfe4a349a33e0080a539b44/woff2/file.woff2") format("woff2");
    font-weight: 101 900
  }
  div {
    font-family: 'wf_04476cfbcbfe4a349a33e0080', sans-serif;
    font-weight: 101;
    font-size: 100px;
  }

<div class="myInput" id="testarea" contentEditable="true"> insert text </div>
<div id="result" contentEditable="true"></div>