JSFiddle - React, Tailwind, and code Playground

HTML

<html>
  <body>
    <div class="container" id="container">
      <div class="blockA" id="blockA">
      
      </div>
      <div class="blockB" id="blockB">
      
      </div>
    </div>
    <div id="output" class="ouput">
    </div>
  </body>
</html>

CSS

.container {
  display: flex;
  width: 100%;
  justify-content: space-between;
}

.blockA, .blockB {
  width: 100px;
  height: 100px;
  background: red;
}

JavaScript

let container = document.getElementById("container");
let blockA = document.getElementById("blockA");
let blockB = document.getElementById("blockB");
let output = document.getElementById("output")
window.addEventListener('resize', function(event) {
    
		let diff = container.offsetWidth - blockA.offsetWidth - blockB.offsetWidth
    output.innerHTML = "Distance between two blocks: " + diff + "px";
}, true);