JSFiddle - React, Tailwind, and code Playground

HTML

<p id="output"></p>
<button id="btnCollect">collect wood</button>

JavaScript

window.addEventListener("DOMContentLoaded", function(){
  var wood = +localStorage.getItem("woodSave");
  wood = wood ? wood : 0;

  var output = document.getElementById('output');
  var btn = document.getElementById("btnCollect");
    
  formatWood();

  btn.addEventListener("click", collectWood);
  
  function collectWood() {
  	wood +=1;
    
    // You didn't have this in this function before, but 
    // you want to update your localStorage value when
    // the wood value goes up.
    localStorage.setItem("woodSave", wood);
    
    // And, every time you click the button, you want the 
    // output to be not only updated, but also formatted:
    formatWood();
  }
  
  function formatWood(){

    // You don't want to update the wood variable to potentially
    // contain any string characters. You only want to add those
    // characters for formatting sake, so just add them to the 
    // output.
    console.log( wood);
 	  output.textContent = (wood >= 1000) ? (wood / 1000).toFixed(2) + "k":  wood;
  }  
  
});